CHAPTER XIV

CHAPTER XIVTHE EFFECT OF OTHER BODIES ON THE SUNIf solar activity is really an important factor in causing climatic changes, it behooves us to subject the sun to the same kind of inquiry to which we have subjected the earth. We have inquired into the nature of the changes through which the earth's crust, the oceans, and the atmosphere have influenced the climate of geological times. It has not been necessary, however, to study the origin of the earth, nor to trace its earlier stages. Our study of the geological record begins only when the earth had attained practically its present mass, essentially its present shape, and a climate so similar to that of today that life as we know it was possible. In other words, the earth had passed the stages of infancy, childhood, youth, and early maturity, and had reached full maturity. As it still seems to be indefinitely far from old age, we infer that during geological times its relative changes have been no greater than those which a man experiences between the ages of perhaps twenty-five and forty.Similar reasoning applies with equal or greater force to the sun. Because of its vast size it presumably passes through its stages of development much more slowly than the earth. In the first chapter of this book we saw that the earth's relative uniformity of climate for hundreds of millions of years seems to imply a similar uniformity in solar activity. This accords with a recent tendency amongastronomers who are more and more recognizing that the stars and the solar system possess an extraordinary degree of conservatism. Changes that once were supposed to take place in thousands of years are now thought to have required millions. Hence in this chapter we shall assume that throughout geological times the condition of the sun has been almost as at present. It may have been somewhat larger, or different in other ways, but it was essentially a hot, gaseous body such as we see today and it gave out essentially the same amount of energy. This assumption will affect the general validity of what follows only if it departs widely from the truth. With this assumption, then, let us inquire into the degree to which the sun's atmosphere has probably been disturbed throughout geological times.InEarth and Sun, as already explained, a detailed study has led to the conclusion that cyclonic storms are influenced by the electrical action of the sun. Such action appears to be most intense in sunspots, but apparently pertains also to other disturbed areas in the sun's atmosphere. A study of sunspots suggests that their true periodicity is almost if not exactly identical with that of the orbital revolution of Jupiter, 11.8 years. Other investigations show numerous remarkable coincidences between sunspots and the orbital revolution of the other planets, including especially Saturn and Mercury. This seems to indicate that there is some truth in the hypothesis that sunspots and other related disturbances of the solar atmosphere owe their periodicity to the varying effects of the planets as they approach and recede from the sun in their eccentric orbits and as they combine or oppose their effects according to their relative positions. This does not mean that the energy of the solar disturbances is supposed to come from the planets, but merelythat their variations act like the turning of a switch to determine when and how violently the internal forces of the sun shall throw the solar atmosphere into commotion. This hypothesis is by no means new, for in one form or another it has been advocated by Wolfer, Birkeland, E. W. Brown, Schuster, Arctowski, and others.The agency through which the planets influence the solar atmosphere is not yet clear. The suggested agencies are the direct pull of gravitation, the tidal effect of the planets, and an electro-magnetic effect. InEarth and Sunthe conclusion is reached that the first two are out of the question, a conclusion in which E. W. Brown acquiesces. Unless some unknown cause is appealed to, this leaves an electro-magnetic hypothesis as the only one which has a reasonable foundation. Schuster inclines to this view. The conclusions set forth inEarth and Sunas to the electrical nature of the sun's influence on the earth point somewhat in the same direction. Hence in this chapter we shall inquire what would happen to the sun, and hence to the earth, on their journey through space, if the solar atmosphere is actually subject to disturbance by the electrical or other effects of other heavenly bodies. It need hardly be pointed out that we are here venturing into highly speculative ground, and that the verity or falsity of the conclusions reached in this chapter has nothing to do with the validity of the reasoning in previous chapters. Those chapters are based on the assumption that terrestrial causes of climatic changes are supplemented by solar disturbances which produce their effect partly through variations in temperature but also through variations in the intensity and paths of cyclonic storms. The present chapter seeks to shed some light on the possible causes and sequence of solar disturbances.Let us begin by scanning the available evidence as tosolar disturbances previous to the time when accurate sunspot records are available. Two rather slender bits of evidence point to cycles of solar activity lasting hundreds of years. One of these has already been discussed in Chapter VI, where the climatic stress of the fourteenth century was described. At that time sunspots are known to have been unusually numerous, and there were great climatic extremes. Lakes overflowed in Central Asia; storms, droughts, floods, and cold winters were unusually severe in Europe; the Caspian Sea rose with great rapidity; the trees of California grew with a vigor unknown for centuries; the most terrible of recorded famines occurred in England and India; the Eskimos were probably driven south by increasing snowiness in Greenland; and the Mayas of Yucatan appear to have made their last weak attempt at a revival of civilization under the stimulus of greater storminess and less constant rainfall.The second bit of evidence is found in recent exhaustive studies of periodicities by Turner[115]and other astronomers. They have sought every possible natural occurrence for which a numerical record is available for a long period. The most valuable records appear to be those of tree growth, Nile floods, Chinese earthquakes, and sunspots. Turner reaches the conclusion that all four types of phenomena show the same periodicity, namely, cycles with an average length of about 260 to 280 years. He suggests that if this is true, the cycles in tree growth and in floods, both of which are climatic, are probably due to a non-terrestrial cause. The fact that the sunspotsshow similar cycles suggests that the sun's variations are the cause.These two bits of evidence are far too slight to form the foundation of any theory as to changes in solar activity in the geological past. Nevertheless it may be helpful to set forth certain possibilities as a stimulus to further research. For example, it has been suggested that meteoric bodies may have fallen into the sun and caused it suddenly to flare up, as it were. This is not impossible, although it does not appear to have taken place since men became advanced enough to make careful observations. Moreover, the meteorites which now fall on the earth are extremely small, the average size being computed as no larger than a grain of wheat. The largest ever found on the earth's surface, at Bacubirito in Mexico, weighs only about fifty tons, while within the rocks the evidences of meteorites are extremely scanty and insignificant. If meteorites had fallen into the sun often enough and of sufficient size to cause glacial fluctuations and historic pulsations of climate, it seems highly probable that the earth would show much more evidence of having been similarly disturbed. And even if the sun should be bombarded by large meteors the result would probably not be sudden cold periods, which are the most notable phenomena of the earth's climatic history, but sudden warm periods followed by slow cooling. Nevertheless, the disturbance of the sun by collision with meteoric matter can by no means be excluded as a possible cause of climatic variations.Allied to the preceding hypothesis is Shapley's[116]nebular hypothesis. At frequent intervals, averaging aboutonce a year during the last thirty years, astronomers have discovered what are known as novæ. These are stars which were previously faint or even invisible, but which flash suddenly into brilliancy. Often their light-giving power rises seven or eight magnitudes—a thousand-fold. In addition to the spectacular novæ there are numerous irregular variables whose brilliancy changes in every ratio from a few per cent up to several magnitudes. Most of them are located in the vicinity of nebulæ, as is also the case with novæ. This, as well as other facts, makes it probable that all these stars are "friction variables," as Shapley calls them. Apparently as they pass through the nebulæ they come in contact with its highly diffuse matter and thereby become bright much as the earth would become bright if its atmosphere were filled with millions of almost infinitesimally small meteorites. A star may also lose brilliancy if nebulous matter intervenes between it and the observer. If our sun has been subjected to any of these changes some sort of climatic effect must have been produced.In a personal communication Shapley amplifies the nebular climatic hypothesis as follows:Within 700 light years of the sun in many directions (Taurus, Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebulosity, some bright, most of them dark. The probability that stars moving in the general region of such clouds will encounter this material is very high, for the clouds fill enormous volumes of space,—e.g., probably more than a hundred thousand cubic light years in the Orion region, and are presumably composed of rarefied gases or of dust particles. Probably throughout all our part of space such nebulosity exists (it is all around us, we are sure), but only in certain regions is it dense enough to affect conspicuously the stars involved in it. If a star moving at high velocity should collide with a dense part of such a nebulouscloud, we should probably have a typical nova. If the relative velocity of nebulous material and star were low or moderate, or if the material were rare, we should not expect a conspicuous effect on the star's light.In the nebulous region of Orion, which is probably of unusually high density, there are about 100 known stars, varying between 20% and 80% of their total light—all of them irregularly—some slowly, some suddenly. Apparently they are "friction variables." Some of the variables suddenly lose 40% of their light as if blanketed by nebulous matter. In the Trifid Nebula there are variables like those of Orion, in Messier 8 also, and probably many of the 100 or so around the Rho Ophiuchi region belong to this kind.I believe that our sun could not have been a typical nova, at least not since the Archeozoic, that is for perhaps a billion years. I believe we have in geological climates final proof of this, because an increase in the amount of solar radiation by 1000 times as in the typical nova, would certainly punctuate emphatically the life cycle on the earth, even if the cause of the nova would not at the same time eliminate the smaller planets. But the sun may have been one of these miniature novæ or friction variables; and I believe it very probable that its wanderings through this part of space could not long leave its mean temperature unaffected to the amount of a few per cent.One reason we have not had this proposal insisted upon before is that the data back of it are mostly new—the Orion variables have been only recently discovered and studied, the distribution and content of the dark nebulæ are hardly as yet generally known.This interesting hypothesis cannot be hastily dismissed. If the sun should pass through a nebula it seems inevitable that there would be at least slight climatic effects and perhaps catastrophic effects through the action of the gaseous matter not only on the sun but on the earth's own atmosphere. As an explanation of thegeneral climatic conditions of the past, however, Shapley points out that the hypothesis has the objection of being vague, and that nebulosity should not be regarded as more than "a possible factor." One of the chief difficulties seems to be the enormously wide distribution of as yet undiscovered nebulous matter which must be assumed if any large share of the earth's repeated climatic changes is to be ascribed to such matter. If such matter is actually abundant in space, it is hard to see how any but the nearest stars would be visible. Another objection is that there is no known nebulosity near at hand with which to connect the climatic vicissitudes of the last glacial period. Moreover, the known nebulæ are so much less numerous than stars that the chances that the sun will encounter one of them are extremely slight. This, however, is not an objection, for Shapley points out that during geological times the sun can never have varied as much as do the novæ, or even as most of the friction variables. Thus the hypothesis stands as one that is worth investigating, but that cannot be finally rejected or accepted until it is made more definite and until more information is available.Another suggested cause of solar variations is the relatively sudden contraction of the sun such as that which sometimes occurs on the earth when continents are uplifted and mountains upheaved. It seems improbable that this could have occurred in a gaseous body like the sun. Lacking, as it does, any solid crust which resists a change of form, the sun probably shrinks steadily. Hence any climatic effects thus produced must be extremely gradual and must tend steadily in one direction for millions of years.Still another suggestion is that the tidal action of the stars and other bodies which may chance to approachthe sun's path may cause disturbances of the solar atmosphere. The vast kaleidoscope of space is never quiet. The sun, the stars, and all the other heavenly bodies are moving, often with enormous speed. Hence the effect of gravitation upon the sun must vary constantly and irregularly, as befits the geological requirements. In the case of the planets, however, the tidal effect does not seem competent to produce the movements of the solar atmosphere which appear to be concerned in the inception of sunspots. Moreover, there is only the most remote probability that a star and the sun will approach near enough to one another to produce a pronounced gravitational disturbance in the solar atmosphere. For instance, if it be assumed that changes in Jupiter's tidal effect on the sun are the main factor in regulating the present difference between sunspot maxima and sunspot minima, the chances that a star or some non-luminous body of similar mass will approach near enough to stimulate solar activity and thereby bring on glaciation are only one in twelve billion years, as will be explained below. This seems to make a gravitational hypothesis impossible.Another possible cause of solar disturbances is that the stars in their flight through space may exert an electrical influence which upsets the equilibrium of the solar atmosphere. At first thought this seems even more impossible than a gravitational effect. Electrostatic effects, however, differ greatly from those of tides. They vary as the diameter of a body instead of as its mass; their differentials also vary inversely as the square of the distance instead of as the cube. Electrostatic effects also increase as the fourth power of the temperature or at least would do so if they followed the law of black bodies; they are stimulated by the approach of one bodyto another; and they are cumulative, for if ions arrive from space they must accumulate until the body to which they have come begins to discharge them. Hence, on the basis of assumptions such as those used in the preceding paragraph, the chances of an electrical disturbance of the solar atmosphere sufficient to cause glaciation on the earth may be as high as one in twenty or thirty million years. This seems to put an electrical hypothesis within the bounds of possibility. Further than that we cannot now go. There may be other hypotheses which fit the facts much better, but none seems yet to have been suggested.In the rest of this chapter the tidal and electrical hypotheses of stellar action on the sun will be taken up in detail. The tidal hypothesis is considered because in discussions of the effect of the planets it has hitherto held almost the entire field. The electrical hypothesis will be considered because it appears to be the best yet suggested, although it still seems doubtful whether electrical effects can be of appreciable importance over such vast distances as are inevitably involved. The discussion of both hypotheses will necessarily be somewhat technical, and will appeal to the astronomer more than to the layman. It does not form a necessary part of this book, for it has no bearing on our main thesis of the effect of the sun on the earth. It is given here because ultimately the question of changes in solar activity during geological times must be faced.In the astronomical portion of the following discussion we shall follow Jeans[117]in his admirable attempt at a mathematical analysis of the motions of the universe. Jeans divides the heavenly bodies into five main types. (1) Spiral nebulæ, which are thought by some astronomersto be systems like our own in the making, and by others to be independent universes lying at vast distances beyond the limits of our Galactic universe, as it is called from the Galaxy or Milky Way. (2) Nebulæ of a smaller type, called planetary. These lie within the Galactic portion of the universe and seem to be early stages of what may some day be stars or solar systems. (3) Binary or multiple stars, which are extraordinarily numerous. In some parts of the heavens they form 50 or even 60 per cent of the stars and in the galaxy as a whole they seem to form "fully one third." (4) Star clusters. These consist of about a hundred groups of stars in each of which the stars move together in the same direction with approximately the same velocity. These, like the spiral nebulæ, are thought by some astronomers to lie outside the limits of the galaxy, but this is far from certain. (5) The solar system. According to Jeans this seems to be unique. It does not fit into the general mathematical theory by which he explains spiral nebulæ, planetary nebulæ, binary stars, and star clusters. It seems to demand a special explanation, such as is furnished by tidal disruption due to the passage of the sun close to another star.The part of Jeans' work which specially concerns us is his study of the probability that some other star will approach the sun closely enough to have an appreciable gravitative or electrical effect, and thus cause disturbances in the solar atmosphere. Of course both the star and the sun are moving, but to avoid circumlocution we shall speak of such mutual approaches simply as approaches of the sun. For our present purpose the most fundamental fact may be summed up in a quotation from Jeans in which he says that most stars "show evidence of having experienced considerable disturbance by other systems; there is no reason why our solar system shouldbe expected to have escaped the common fate." Jeans gives a careful calculation from which it is possible to derive some idea of the probability of any given degree of approach of the sun and some other star. Of course all such calculations must be based on certain assumptions. The assumptions made by Jeans are such as to make the probability of close approaches as great as possible. For example, he allows only 560 million years for the entire evolution of the sun, whereas some astronomers and geologists would put the figure ten or more times as high. Nevertheless, Jeans' assumptions at least show the order of magnitude which we may expect on the basis of reasonable astronomical conclusions.According to the planetary hypothesis of sunspots, the difference in the effect of Jupiter when it is nearest and farthest from the sun is the main factor in starting the sunspot cycle and hence the corresponding terrestrial cycle. The climatic difference between sunspot maxima and minima, as measured by temperature, apparently amounts to at least a twentieth and perhaps a tenth of the difference between the climate of the last glacial epoch and the present. We may suppose, then, that a body which introduced a gravitative or electrical factor twenty times as great as the difference in Jupiter's effect at its maximum and minimum distances from the sun would cause a glacial epoch if the effect lasted long enough. Of course the other planets combine their effects with that of Jupiter, but for the sake of simplicity we will leave the others out of account. The difference between Jupiter's maximum and minimum tidal effect on the sun amounts to 29 per cent of the planet's average effect. The corresponding difference, according to the electrical hypothesis, is about 19 per cent, for electrostatic action varies as the square of the distance instead of as the cube.Let us assume that a body exerting four times Jupiter's present tidal effect and placed at the average distance of Jupiter from the sun would disturb the sun's atmosphere twenty times as much as the present difference between sunspot maxima and minima, and thus, perhaps, cause a glacial period on the earth.On the basis of this assumption our first problem is to estimate the frequency with which a star, visible or dark, is likely to approach near enough to the sun to produce atidaleffect four times that of Jupiter. The number of visible stars is known or at least well estimated. As to dark stars, which have grown cool, Arrhenius believed that they are a hundred times as numerous as bright stars; few astronomers believe that there are less than three or four times as many. Dr. Shapley of the Harvard Observatory states that a new investigation of the matter suggests that eight or ten is probably a maximum figure. Let us assume that nine is correct. The average visible star, so far as measured, has a mass about twice that of the sun, or about 2100 times that of Jupiter. The distances of the stars have been measured in hundreds of cases and thus we can estimate how many stars, both visible and invisible, are on an average contained in a given volume of space. On this basis Jeans estimates that there is only one chance in thirty billion years that a visible star will approach within 2.8 times the distance of Neptune from the sun, that is, within about eight billion miles. If we include the invisible stars the chances become one in three billion years. In order to produce four times the tidal effect of Jupiter, however, the average star would have to approach within about four billion miles of the sun, and the chances of that are only one in twelve billion years. The disturbing starwould be only 40 per cent farther from the sun than Neptune, and would almost pass within the solar system.Even though Jeans holds that the frequency of the mutual approach of the sun and a star was probably much greater in the distant past than at present, the figures just given lend little support to the tidal hypothesis. In fact, they apparently throw it out of court. It will be remembered that Jeans has made assumptions which give as high a frequency of stellar encounters as is consistent with the astronomical facts. We have assumed nine dark stars for every bright one, which may be a liberal estimate. Also, although we have assumed that a disturbance of the sun's atmosphere sufficient to cause a glacial period would arise from a tidal effect only twenty times as great as the difference in Jupiter's effect when nearest the sun and farthest away, in our computations this has actually been reduced to thirteen. With all these favorable assumptions the chances of a stellar approach of the sort here described are now only one in twelve billion years. Yet within a hundred million years, according to many estimates of geological time, and almost certainly within a billion, there have been at least half a dozen glaciations.Our use of Jeans' data interposes another and equally insuperable difficulty to any tidal hypothesis. Four billion miles is a very short distance in the eyes of an astronomer. At that distance a star twice the size of the sun would attract the outer planets more strongly than the sun itself, and might capture them. If a star should come within four billion miles of the sun, its effect in distorting the orbits of all the planets would be great. If this had happened often enough to cause all the glaciations known to geologists, the planetary orbits would be strongly elliptical instead of almost circular. The considerationshere advanced militate so strongly against the tidal hypothesis of solar disturbances that it seems scarcely worth while to consider it further.Let us turn now to the electrical hypothesis. Here the conditions are fundamentally different from those of the tidal hypothesis. In the first place the electrostatic effect of a body has nothing to do with its mass, but depends on the area of its surface; that is, it varies as the square of the radius. Second, the emission of electrons varies exponentially. If hot glowing stars follow the same law as black bodies at lower temperatures, the emission of electrons, like the emission of other kinds of energy, varies as the fourth power of the absolute temperature. In other words, suppose there are two black bodies, otherwise alike, but one with a temperature of 27° C. or 300° on the absolute scale, and the other with 600° on the absolute scale. The temperature of one is twice as high as that of the other, but the electrostatic effect will be sixteen times as great.[118]Third, the number of electronsthat reach a given body varies inversely as the square of the distance, instead of as the cube which is the case with tide-making forces.In order to use these three principles in calculating the effect of the stars we must know the diameters, distances, temperature, and number of the stars. The distances and number may safely be taken as given by Jeans in the calculations already cited. As to the diameters, the measurements of the stars thus far made indicate that the average mass is about twice that of the sun. The average density, as deduced by Shapley[119]from the movements of double stars, is about one-eighth the solar density. This would give an average diameter about two and a half times that of the sun. For the dark stars, we shall assume for convenience that they are ten times as numerous as the bright ones. We shall also assume that their diameter is half that of the sun, for being cool they must be relatively dense, and that their temperature is the same as that which we shall assume for Jupiter.As to Jupiter we shall continue our former assumption that a body with four times the effectiveness of that planet, which here means with twice as great a radius, would disturb the sun enough to cause glaciation. It would produce about twenty times the electrostatic effectwhich now appears to be associated with the difference in Jupiter's effect at maximum and minimum. The temperature of Jupiter must also be taken into account. The planet is supposed to be hot because its density is low, being only about 1.25 that of water. Nevertheless, it is probably not luminous, for as Moulton[120]puts it, shadows upon it are black and its moons show no sign of illumination except from the sun. Hence a temperature of about 600°C., or approximately 900° on the absolute scale, seems to be the highest that can reasonably be assigned to the cold outer layer whence electrons are emitted. As to the temperature of the sun, we shall adopt the common estimate of about 6300°C. on the absolute scale. The other stars will be taken as averaging the same, although of course they vary greatly.When Jeans' method of calculating the probability of a mutual approach of the sun and a star is applied to the assumptions given above, the results are as shown in Table 5. On that basis the dark stars seem to be of negligible importance so far as the electrical hypothesis is concerned. Even though they may be ten times as numerous as the bright ones there appears to be only one chance in 130 billion years that one of them will approach the sun closely enough to cause the assumed disturbance of the solar atmosphere. On the other hand, if all the visible stars were the size of the sun, and as hot as that body, their electrical effect would be fourfold that of our assumed dark star because of their size, and 2401 times as great because of their temperature, or approximately 10,000 times as great. Under such conditions the theoretical chance of an approach that would cause glaciation is one in 130 million years. If the average visible star is somewhat cooler than the sun and has aradius about two and one-half times as great, as appears to be the fact, the chances rise to one in thirty-eight million years. A slight and wholly reasonable change in our assumptions would reduce this last figure to only five or ten million. For instance, the earth's mean temperature during the glacial period has been assumed as 10°C. lower than now, but the difference may have been only 6°. Again, the temperature of the outer atmosphere of Jupiter where the electrons are shot out may be only 500° or 700° absolute, instead of 900°. Or the diameter of the average star may be five or ten times that of the sun, instead of only two and one-half times as great. All this, however, may for the present be disregarded. The essential point is that even when the assumptions err on the side of conservatism, the results are of an order of magnitude which puts the electrical hypothesis within the bounds of possibility, whereas similar assumptions put the tidal hypothesis, with its single approach in twelve billion years, far beyond those limits.The figures for Betelgeuse in Table 5 are interesting. At a meeting of the American Association for the Advancement of Science in December, 1920, Michelson reported that by measurements of the interference of light coming from the two sides of that bright star in Orion, the observers at Mount Wilson had confirmed the recent estimates of three other authorities that the star's diameter is about 218 million miles, or 250 times that of the sun. If other stars so much surpass the estimates of only a decade or two ago, the average diameter of all the visible stars must be many times that of the sun. The low figure for Betelgeuse in section D of the table means that if all the stars were as large as Betelgeuse, several might often be near enough to cause profound disturbances of the solar atmosphere. Nevertheless, because of the lowtemperature of the giant red stars of the Betelgeuse type, the distance at which one of them would produce a given electrical effect is only about five times the distance at which our assumed average star would produce the same effect. This, to be sure, is on the assumption that theradiation of energy from incandescent bodies varies according to temperature in the same ratio as the radiation from black bodies. Even if this assumption departs somewhat from the truth, it still seems almost certain that the lower temperature of the red compared with the high temperature of the white stars must to a considerable degree reduce the difference in electrical effect which would otherwise arise from their size.TABLE 5THEORETICAL PROBABILITY OF STELLAR APPROACHES1Dark Stars2Sun3Average Star4BetelgeuseA. Approximate radius in miles.430,000860,0002,150,000218,000,000B. Assumed temperature above absolute zero.900° C.6300° C.5400° C.3150° C.C. Approximate theoretical distance at which star would cause solar disturbance great enough to cause glaciation (billions[121]of miles).1.21202203200D. Average interval between approaches close enough to cause glaciation if all stars were of given type. Years.130,000,000,000[122]130,000,00038,000,000700,000Thus far in our attempt to estimate the distance at which a star might disturb the sun enough to cause glaciation on the earth, we have considered only the star's size and temperature. No account has been taken of the degree to which its atmosphere is disturbed. Yet in the case of the sun this seems to be one of the most important factors. The magnetic field of sunspots is sometimes 50 or 100 times as strong as that of the sun in general. The strength of the magnetic field appears to depend on the strength of the electrical currents in the solar atmosphere. But the intensity of the sunspots and, by inference, of the electrical currents, may depend on the electrical action of Jupiter and the other planets. If we apply a similar line of reasoning to the stars, we are at once led to question whether the electrical activity of double stars may not be enormously greater than that of isolated stars like the sun.If this line of reasoning is correct, the atmosphere of every double star must be in a state of commotion vastly greater than that of the sun's atmosphere even when it is most disturbed. For example, suppose the sun were accompanied by a companion of equal size at a distance of one million miles, which would make it much like many known double stars. Suppose also that in accordance with the general laws of physics the electrical effect of the two suns upon one another is proportional to the fourthpower of the temperature, the square of the radius, and the inverse square of the distance. Then the effect of each sun upon the other would be sixty billion (6 × 1010) times as great as the present electrical effect of Jupiter upon the sun. Just what this would mean as to the net effect of a pair of such suns upon the electrical potential of other bodies at a distance we can only conjecture. The outstanding fact is that the electrical conditions of a double star must be radically different and vastly more intense than those of a single star like the sun.This conclusion carries weighty consequences. At present twenty or more stars are known to be located within about 100 trillion miles of the sun (five parsecs, as the astronomers say), or 16.5 light years. According to the assumptions employed in Table 5 an average single star would influence the sun enough to cause glaciation if it came within approximately 200 billion miles. If the star were double, however, it might have an electrical capacity enormously greater than that of the sun. Then it would be able to cause glaciation at a correspondingly great distance. Today Alpha Centauri, the nearest known star about twenty-five trillion miles, or 4.3 light years from the sun, and Sirius, the brightest star in the heavens, is about fifty trillion miles away, or 8.5 light years. If these stars were single and had a diameter three times that of the sun, and if they were of the same temperature as has been assumed for Betelgeuse, which is about fifty times as far away as Alpha Centauri, the relative effects of the three stars upon the sun would be, approximately, Betelgeuse 700, Alpha Centauri 250, Sirius 1. But Alpha Centauri is triple and Sirius double, and both are much hotter than Betelgeuse. Hence Alpha Centauri and even Sirius may be far more effective than Betelgeuse.The two main components of Alpha Centauri are separatedby an average distance of about 2,200,000,000 miles, or somewhat less than that of Neptune from the sun. A third and far fainter star, one of the faintest yet measured, revolves around them at a great distance. In mass and brightness the two main components are about like the sun, and we will assume that the same is true of their radius. Then, according to the assumptions made above, their effect in disturbing one another electrically would be about 10,000 times the total effect of Jupiter upon the sun, or 2500 times the effect that we have assumed to be necessary to produce a glacial period. We have already seen in Table 5 that, according to our assumptions, a single star like the sun would have to approach within 120 billion miles of the solar system, or within 2 per cent of a light year, in order to cause glaciation. By a similar process of reasoning it appears that if the mutual electrical excitation of the two main parts of Alpha Centauri, regardless of the third part, is proportional to the apparent excitation of the sun by Jupiter, Alpha Centauri would be 5000 times as effective as the sun. In other words, if it came within 8,500,000,000,000 miles of the sun, or 1.4 light years, it would so change the electrical conditions as to produce a glacial epoch. In that case Alpha Centauri is now so near that it introduces a disturbing effect equal to about one-sixth of the effect needed to cause glaciation on the earth. Sirius and perhaps others of the nearer and brighter or larger stars may also create appreciable disturbances in the electrical condition of the sun's atmosphere, and may have done so to a much greater degree in the past, or be destined to do so in the future. Thus an electrical hypothesis of solar disturbances seems to indicate that the position of the sun in respect to other stars may be a factor of great importance in determining the earth's climate.CHAPTER XVTHE SUN'S JOURNEY THROUGH SPACEHaving gained some idea of the nature of the electrical hypothesis of solar disturbances and of the possible effect of other bodies upon the sun's atmosphere, let us now compare the astronomical data with those of geology. Let us take up five chief points for which the geologist demands an explanation, and which any hypothesis must meet if it is to be permanently accepted. These are (1) the irregular intervals at which glacial periods occur; (2) the division of glacial periods into epochs separated sometimes by hundreds of thousands of years; (3) the length of glacial periods and epochs; (4) the occurrence of glacial stages and historic pulsations in the form of small climatic waves superposed upon the larger waves of glacial epochs; (5) the occurrence of climatic conditions much milder than those of today, not only in the middle portion of the great geological eras, but even in some of the recent inter-glacial epochs.1. The irregular duration of the interval from one glacial epoch to another corresponds with the irregular distribution of the stars. If glaciation is indirectly due to stellar influences, the epochs might fall close together, or might be far apart. If the average interval were ten million years, one interval might be thirty million or more and the next only one or two hundred thousand.According to Schuchert, the known periods of glacial or semi-glacial climate have been approximately as follows:LIST OF GLACIAL PERIODSArcheozoic.(¼ of geological time or perhaps much more)No known glacial periods.Proterozoic.(¼ of geological time)a. Oldest known glacial period near base of Proterozoic in Canada. Evidence widely distributed.b. Indian glacial period; time unknown.c. African glacial period; time unknown.d. Glaciation near end of Proterozoic in Australia, Norway, and China.Paleozoic.(¼ of geological time)a. Late Ordovician(?). Local in Arctic Norway.b. Silurian. Local in Alaska.c. Early Devonian. Local in South Africa.d. Early Permian. World-wide and very severe.Mesozoic and Cenozoic.(¼ of geological time)a-b. None definitely determined during Mesozoic, although there appears to have been periods of cooling (a) in the late Triassic, and (b) in the late Cretacic, with at least local glaciation in early Eocene.c. Severe glacial period during Pleistocene.This table suggests an interesting inquiry. During the last few decades there has been great interest in ancient glaciation and geologists have carefully examined rocks of all ages for signs of glacial deposits. In spite of the large parts of the earth which are covered with deposits belonging to the Mesozoic and Cenozoic, which form thelast quarter of geological time, the only signs of actual glaciation are those of the great Pleistocene period and a few local occurrences at the end of the Mesozoic or beginning of the Cenozoic. Late in the Triassic and early in the Jurassic, the climate appears to have been rigorous, although no tillites have been found to demonstrate glaciation. In the preceding quarter, that is, the Paleozoic, the Permian glaciation was more severe than that of the Pleistocene, and the Devonian than that of the Eocene, while the Ordovician evidences of low temperature are stronger than those at the end of the Triassic. In view of the fact that rocks of Paleozoic age cover much smaller areas than do those of later age, the three Paleozoic glaciations seem to indicate a relative frequency of glaciation. Going back to the Proterozoic, it is astonishing to find that evidence of two highly developed glacial periods, and possibly four, has been discovered. Since the Indian and the African glaciations of Proterozoic times are as yet undated, we cannot be sure that they are not of the same date as the others. Nevertheless, even two is a surprising number, for not only are most Proterozoic rocks so metamorphosed that possible evidences of glacial origin are destroyed, but rocks of that age occupy far smaller areas than either those of Paleozoic or, still more, Mesozoic and Cenozoic age. Thus the record of the last three-quarters of geological time suggests that if rocks of all ages were as abundant and as easily studied as those of the later periods, the frequency of glacial periods would be found to increase as one goes backward toward the beginnings of the earth's history. This is interesting, for Jeans holds that the chances that the stars would approach one another were probably greater in the past than at present. This conclusion is based on the assumption that our universeis like the spiral nebulæ in which the orbits of the various members are nearly circular during the younger stages. Jeans considers it certain that in such cases the orbits will gradually become larger and more elliptical because of the attraction of one body for another. Thus as time goes on the stars will be more widely distributed and the chances of approach will diminish. If this is correct, the agreement between astronomical theory and geological conclusions suggests that the two are at least not in opposition.The first quarter of geological time as well as the last three must be considered in this connection. During the Archeozoic, no evidence of glaciation has yet been discovered. This suggests that the geological facts disprove the astronomical theory. But our knowledge of early geological times is extremely limited, so limited that lack of evidence of glaciation in the Archeozoic may have no significance. Archeozoic rocks have been studied minutely over a very small percentage of the earth's land surface. Moreover, they are highly metamorphosed so that, even if glacial tills existed, it would be hard to recognize them. Third, according to both the nebular and the planetesimal hypotheses, it seems possible that during the earliest stages of geological history the earth's interior was somewhat warmer than now, and the surface may have been warmed more than at present by conduction, by lava flows, and by the fall of meteorites. If the earth during the Archeozoic period emitted enough heat to raise its surface temperature a few degrees, the heat would not prevent the development of low forms of life but might effectively prevent all glaciation. This does not mean that it would prevent changes of climate, but merely changes so extreme that their record would be preserved by means of ice. It will be most interestingto see whether future investigations in geology and astronomy indicate either a semi-uniform distribution of glacial periods throughout the past, or a more or less regular decrease in frequency from early times down to the present.2. The Pleistocene glacial period was divided into at least four epochs, while in the Permian at least one inter-glacial epoch seems certain, and in some places the alternation between glacial and non-glacial beds suggests no less than nine. In the other glaciations the evidence is not yet clear. The question of periodicity is so important that it overthrows most glacial hypotheses. Indeed, had their authors known the facts as established in recent years, most of the hypotheses would never have been advanced. The carbon dioxide hypothesis is the only one which was framed with geologically rapid climatic alternations in mind. It certainly explains the facts of periodicity better than does any of its predecessors, but even so it does not account for the intimate way in which variations of all degrees from those of the weather up to glacial epochs seem to grade into one another.According to our stellar hypothesis, occasional groups of glacial epochs would be expected to occur close together and to form long glacial periods. This is because many of the stars belong to groups or clusters in which the stars move in parallel paths. A good example is the cluster in the Hyades, where Boss has studied thirty-nine stars with special care.[123]The stars are grouped about a center about 130 light years from the sun. The stars themselves are scattered over an area about thirty light years in diameter. They average about the same distance apart as do those near the sun, but toward thecenter of the group they are somewhat closer together. The whole thirty-nine sweep forward in essentially parallel paths. Boss estimates that 800,000 years ago the cluster was only half as far from the sun as at present, but probably that was as near as it has been during recent geological times. All of the thirty-nine stars of this cluster, as Moulton[124]puts it, "are much greater in light-giving power than the sun. The luminosities of even the five smallest are from five to ten times that of the sun, while the largest are one hundred times greater in light-giving power than our own luminary. Their masses are probably much greater than that of the sun." If the sun were to pass through such a cluster, first one star and then another might come so near as to cause a profound disturbance in the sun's atmosphere.3. Another important point upon which a glacial hypothesis may come to grief is the length of the periods or rather of the epochs which compose the periods. During the last or Pleistocene glacial period the evidence in America and Europe indicates that the inter-glacial epochs varied in length and that the later ones were shorter than the earlier. Chamberlin and Salisbury, from a comparison of various authorities, estimate that the intervals from one glacial epoch to another form a declining series, which may be roughly expressed as follows: 16-8-4-2-1, where unity is the interval from the climax of the late Wisconsin, or last glacial epoch, to the present. Most authorities estimate the culmination of the late Wisconsin glaciation as twenty or thirty thousand years ago. Penck estimates the length of the last inter-glacial period as 60,000 years and the preceding one as 240,000.[125]R. T. Chamberlin, as already stated, finds thatthe consensus of opinion is that inter-glacial epochs have averaged five times as long as glacial epochs. The actual duration of the various glaciations probably did not vary in so great a ratio as did the intervals from one glaciation to another. The main point, however, is the irregularity of the various periods.The relation of the stellar electrical hypothesis to the length of glacial epochs may be estimated from column C, in Table 5. There we see that the distances at which a star might possibly disturb the sun enough to cause glaciation range all the way from 120 billion miles in the case of a small star like the sun, to 3200 billion in the case of Betelgeuse, while for double stars the figure may rise a hundred times higher. From this we can calculate how long it would take a star to pass from a point where its influence would first amount to a quarter of the assumed maximum to a similar point on the other side of the sun. In making these calculations we will assume that the relative rate at which the star and the sun approach each other is about twenty-two miles per second, or 700 million miles per year, which is the average rate of motion of all the known stars. According to the distances in Table 5 this gives a range from about 500 years up to about 10,000, which might rise to a million in the case of double stars. Of course the time might be relatively short if the sun and a rapidly moving star were approaching one another almost directly, or extremely long if the sun and the star were moving in almost the same direction and at somewhat similar rates,—a condition more common than the other. Here, as in so many other cases, the essential point is that the figures which we thus obtain seem to be of the right order of magnitude.4. Post-glacial climatic stages are so well known that in Europe they have definite names. Their sequence hasalready been discussed in Chapter XII. Fossils found in the peat bogs of Denmark and Scandinavia, for example, prove that since the final disappearance of the continental ice cap at the close of the Wisconsin there has been at least one period when the climate of Europe was distinctly milder than now. Directly overlying the sheets of glacial drift laid down by the ice there is a flora corresponding to that of the present tundras. Next come remains of a forest vegetation dominated by birches and poplars, showing that the climate was growing a little warmer. Third, there follow evidences of a still more favorable climate in the form of a forest dominated by pines; fourth, one where oak predominates; and fifth, a flora similar to that of the Black Forest of Germany, indicating that in Scandinavia the temperature was then decidedly higher than today. This fifth flora has retreated southward once more, having been driven back to its present latitude by a slight recurrence of a cool stormy climate.[126]In central Asia evidence of post-glacial stages is found not only in five distinct moraines but in a corresponding series of elevated strands surrounding salt lakes and of river terraces in non-glaciated arid regions.[127]In historic as well as prehistoric times, as we have already seen, there have been climatic fluctuations. For instance, the twelfth or thirteenth century B. C. appears to have been almost as mild as now, as does the seventh century B. C. On the other hand about 1000 B. C., at the time of Christ, and in the fourteenth century there were times of relative severity. Thus it appears that both ona large and on a small scale pulsations of climate are the rule. Any hypothesis of climatic changes must satisfy the periods of these pulsations. These conditions furnish a problem which makes difficulty for almost all hypotheses of climatic change. According to the present hypothesis, earth movements such as are discussed in Chapter XII may coöperate with two astronomical factors. One is the constant change in the positions of the stars, a change which we have already called kaleidoscopic, and the other is the fact that a large proportion of the stars are double or multiple. When one star in a group approaches the sun closely enough to cause a great solar disturbance, numerous others may approach or recede and have a minor effect. Thus, whenever the sun is near groups of stars we should expect that the earth would show many minor climatic pulsations and stages which might or might not be connected with glaciation. The historic pulsations shown in the curve of tree growth in California, Fig. 4, are the sort of changes that would be expected if movements of the stars have an effect on the solar atmosphere.Not only are fully a third of all the visible stars double, as we have already seen, but at least a tenth of these are known to be triple or multiple. In many of the double stars the two bodies are close together and revolve so rapidly that whatever periodicity they might create in the sun's atmosphere would be very short. In the triplets, however, the third star is ordinarily at least ten times as far from the other two as they are from each other, and its period of rotation sometimes runs into hundreds or thousands of years. An actual multiple star in the constellation Polaris will serve as an example. The main star is believed by Jeans to consist of two parts which are almost in contact and whirl around each other withextraordinary speed in four days. If this is true they must keep each other's atmospheres in a state of intense commotion. Much farther away a third star revolves around this pair in twelve years. At a much greater distance a fourth star revolves around the common center of gravity of itself and the other three in a period which may be 20,000 years. Still more complicated cases probably exist. Suppose such a system were to traverse a path where it would exert a perceptible influence on the sun for thirty or forty thousand years. The varying movements of its members would produce an intricate series of cycles which might show all sorts of major and minor variations in length and intensity. Thus the varied and irregular stages of glaciation and the pulsations of historic times might be accounted for on the hypothesis of the proximity of the sun to a multiple star, as well as on that of the less pronounced approach and recession of a number of stars. In addition to all this, an almost infinitely complex series of climatic changes of long and short duration might arise if the sun passed through a nebula.5. We have seen in Chapter VIII that the contrast between the somewhat severe climate of the present and the generally mild climate of the past is one of the great geological problems. The glacial period is not a thing of the distant past. Geologists generally recognize that it is still with us. Greenland and Antarctica are both shrouded in ice sheets in latitudes where fossil floras prove that at other periods the climate was as mild as in England or even New Zealand. The present glaciated regions, be it noted, are on the polar borders of the world's two most stormy oceanic areas, just where ice would be expected to last longest according to the solar cyclonic hypothesis. In contrast with the semi-glacialconditions of the present, the last inter-glacial epoch was so mild that not only men but elephants and hippopotamuses flourished in central Europe, while at earlier times in the middle of long eras, such as the Paleozoic and Mesozoic, corals, cycads, and tree ferns flourished within the Arctic circle.If the electro-stellar hypothesis of solar disturbances proves well founded, it may explain these peculiarities. Periods of mild climate would represent a return of the sun and the earth to their normal conditions of quiet. At such times the atmosphere of the sun is assumed to be little disturbed by sunspots, faculæ, prominences, and other allied evidences of movements; and the rice-grain structure is perhaps the most prominent of the solar markings. The earth at such times is supposed to be correspondingly free from cyclonic storms. Its winds are then largely of the purely planetary type, such as trade winds and westerlies. Its rainfall also is largely planetary rather than cyclonic. It falls in places such as the heat equator where the air rises under the influence of heat, or on the windward slopes of mountains, or in regions where warm winds blow from the ocean over cold lands.According to the electro-stellar hypothesis, the conditions which prevailed during hundreds of millions of years of mild climate mean merely that the solar system was then in parts of the heavens where stars—especially double stars—were rare or small, and electrical disturbances correspondingly weak. Today, on the other hand, the sun is fairly near a number of stars, many of which are large doubles. Hence it is supposed to be disturbed, although not so much as at the height of the last glacial epoch.After the preceding parts of this book had beenwritten, the assistance of Dr. Schlesinger made it possible to test the electro-stellar hypothesis by comparing actual astronomical dates with the dates of climatic or solar phenomena. In order to make this possible, Dr. Schlesinger and his assistants have prepared Table 6, giving the position, magnitude, and motions of the thirty-eight nearest stars, and especially the date at which each was nearest the sun. In column 10 where the dates are given, a minus sign indicates the past and a plus sign the future. Dr. Shapley has kindly added column 12, giving the absolute magnitudes of the stars, that of the sun being 4.8, and column 13, showing their luminosity or absolute radiation, that of the sun being unity. Finally, column 14 shows the effective radiation received by the sun from each star when the star is at a minimum distance. Unity in this case is the effect of a star like the sun at a distance of one light year.It is well known that radiation of all kinds, including light, heat, and electrical emissions, varies in direct proportion to the exposed surface, that is, as the square of the radius of a sphere, and inversely as the square of the distance. From black bodies, as we have seen, the total radiation varies as the fourth power of the absolute temperature. It is not certain that either light or electrical emissions from incandescent bodies vary in quite this same proportion, nor is it yet certain whether luminous and electrical emissions vary exactly together. Nevertheless they are closely related. Since the light coming from each star is accurately measured, while no information is available as to electrical emissions, we have followed Dr. Shapley's suggestion and used the luminosity of the stars as the best available measure of total radiation. This is presumably an approximate measure of electrical activity, provided some allowance be made for disturbances by outside bodies such as companion stars. Hence the inclusion of column 14.TABLE 6THIRTY-EIGHT STARS HAVING LARGEST KNOWN PARALLAXES(1)Right Ascension α 1900(2)Declination δ 1900(3)Visual Mag. m(4)Spectrum(5)Proper Motion(6)Radial Velocitykm. per sec.(7)Present Parallax π(8)Maximum Parallax(9)Minimum Distance Light Yrs.(10)Time ofMinimumDistance(11)Magnitudeat Min. Dist.(12)AbsoluteMagnitude(13)Luminosity(14)Effectiveradiationat minimumdistancefrom sunGroombr. 340h12m.7+43°27'8.1Ma2".89+ 3".28".2811.6- 40008.110.30.00630.000051[128]η Cassiop.43 .0+57 173.6F81 .24+ 10.18.1917.1- 470003.54.90.910.00311043 .9+4 5512.3F03 .01......24...................14.20.00017........[128]κ Tucanæ1 12 .4-69 245.0F8.39+ 12.16.2314.2-2640004.26.00.330.001610τ Ceti39 .4-16 283.6K01 .92- 16.32.378.8+ 460003.36.10.300.003840δ2Eridani3 15 .9-43 274.3G53 .16+ 87.16.2214.8- 330003.65.30.630.002960[128]ε Eridani28 .2- 9 483.8K0.97+ 16.31.467.1-1060003.06.30.250.004970[128]40(0)2Eridani4 10 .7- 7 494.5G54 .08- 42.21.2314.2+ 190004.36.10.300.001470Cordoba Z. 2435 7 .7-44 599.2K28 .75+242.32.684.8- 100007.611.70.00170.000074Weisse 59226 .4- 3 428.8K22 .22......17...................9.90.009........[128]α Can. Maj. (Sirius)6 40 .7-16 35-1.6A01 .32- 8.37.418.0+ 65000-1.81.227.500.429000[128]α Can. Min. (Procyon)7 34 .1+ 5 290.5F51 .24- 4.31.3210.2+ 340000.53.05.250.051300[128]Fedorenko 1457-89 7 .6+53 77.9Ma1 .68+ 10.16.1620.4- 240007.98.90.0230.000055Groombr. 161810 5 .3+49 586.8K5p1 .45- 30.18.2314.2+ 690006.38.10.0480.000238Weisse 23414 .2+20 229.0....49......19...................10.40.0057........Lalande 2118557 .9+36 387.6Mb4 .78- 87.41.764.3+ 200006.210.70.00440.000238Lalande 2125811 0 .5+44 28.5K54 .52+ 65.19.2214.8- 200008.29.90.0090.00004112 .0-57 212.0...2 .69......34...................14.70.00011........Lalande 2537213 40 .7+15 268.5K52 .30......19...................9.90.009........[128]α Centauri14 32 .8-60 250.2G3 .68+ 22.761.033.2- 28000-0.54.61.200.117500[128]ξ Bootes14 46 .8+19 314.6K5p.17+ 4.17.2214.8-5980004.05.80.400.001815[128]Lalande 2717351 .6-20 585.8Kp1 .96+ 20.18.1917.1- 360005.67.10.120.000412Weisse 125916 41 .4+33 418.4....37......18...................9.70.011........Lacaille 719417 11 .5-46 325.7K.97......19...................7.10.12........[128]β 41612 .1-34 535.9K51 .19- 4.17.1719.2+ 210005.77.10.120.000329Argel-0.17415-637 .0+68 269.1K1 .33......22...................10.80.004........Barnard's star52 .9+ 4 259.7Mb10 .30- 80.53.704.7+ 100009.113.30.00250.000114[128]70p Ophiuchi18 0 .4+ 2 314.3K1 .13......19...................5.70.44........[128]Σ 239841 .7+59 298.8K2 .31......29...................11.10.0030........σ Draconis19 32 .5+69 294.8G51 .84+ 26.20.2314.2- 490004.56.30.250.001238[128]α Aquilæ (Altair)45 .9+ 8 361.2A5.66- 33.21.516.4+117000-0.72.86.300.153600[128]61 Cygni21 2 .4+38 155.6K55 .20- 64.30.388.6+ 190005.18.00.0530.000715Lacaille 876011 .4-39 156.6G3 .53+ 13.25.2612.6- 110006.68.60.0300.000189ε Indi55 .7-57 124.8K54 .70- 39.28.3110.5+ 170004.67.00.130.001230[128]Krüger 6022 24 .4+57 129.2....87......26...................11.30.0025.......Lacaille 935259 .4-36 267.1K6 .90+ 12.29.2911.2- 30007.19.40.0140.000111Lalande 4665023 44 .0+ 1 528.7Ma1 .39......17...................9.90.009.......C. G. A. 3241659 .5-37 518.2G6 .05+ 26.22.2214.8- 70008.29.90.0090.000041On the basis of column 14 and of the movements and distances of the stars as given in the other columns Fig. 10 has been prepared. This gives an estimate of the approximate electrical energy received by the sun from the nearest stars for 70,000 years before and after the present. It is based on the twenty-six stars for which complete data are available in Table 6. The inclusion of the other twelve would not alter the form of the curve, for even the largest of them would not change any part by more than about half of 1 per cent, if as much. Nor would the curve be visibly altered by the omission of all except four of the twenty-six stars actually used. The four that are important, and their relative luminosity when nearest the sun, are Sirius 429,000, Altair 153,000, Alpha Centauri 117,500, and Procyon 51,300. The figure for the next star is only 4970, while for this star combined with the other twenty-one that are unimportant it is only 24,850.Figure 10 is not carried more than 70,000 years into the past or into the future because the stars near the sun at more remote times are not included among the thirty-eight having the largest known parallaxes. That is, they have either moved away or are not yet near enough to be included. Indeed, as Dr. Schlesinger strongly emphasizes, there may be swiftly moving, bright or gigantic stars which are now quite far away, but whose inclusion would alter Fig. 10 even within the limits of the 140,000 years there shown. It is almost certain, however, that the most that these would do would be to raise, but not obliterate, the minima on either side of the main maximum.In preparing Fig. 10 it has been necessary to makeallowance for double stars. Passing by the twenty-two unimportant stars, it appears that the companion of Sirius is eight or ten magnitudes smaller than that star, while the companions of Procyon and Altair are five or more magnitudes smaller than their bright comrades. This means that the luminosity of the faint components is at most only 1 per cent of that of their bright companions and in the case of Sirius not a hundredth of 1 per cent. Hence their inclusion would have no visible effect on Fig. 10. In Alpha Centauri, on the other hand, the two components are of almost the same magnitude. For this reason the effective radiation of that star as given in column 14 is doubled in Fig. 10, while for another reason it is raised still more. The other reason is that if our inferences as to the electrical effect of the sun on the earth and of the planets on the sun are correct, double stars, as we have seen, must be much more effective electrically than single stars. By the same reasoning two bright stars close together must excite one another much more than a bright star and a very faint one, even if the distances in both cases are the same. So, too, other things being equal, a triple star must be more excited electrically than a double star. Hence in preparing Fig. 10 all double stars receive double weight and each part of Alpha Centauri receives an additional 50 per cent because both parts are bright and because they have a third companion to help in exciting them.Fig. 10

If solar activity is really an important factor in causing climatic changes, it behooves us to subject the sun to the same kind of inquiry to which we have subjected the earth. We have inquired into the nature of the changes through which the earth's crust, the oceans, and the atmosphere have influenced the climate of geological times. It has not been necessary, however, to study the origin of the earth, nor to trace its earlier stages. Our study of the geological record begins only when the earth had attained practically its present mass, essentially its present shape, and a climate so similar to that of today that life as we know it was possible. In other words, the earth had passed the stages of infancy, childhood, youth, and early maturity, and had reached full maturity. As it still seems to be indefinitely far from old age, we infer that during geological times its relative changes have been no greater than those which a man experiences between the ages of perhaps twenty-five and forty.

Similar reasoning applies with equal or greater force to the sun. Because of its vast size it presumably passes through its stages of development much more slowly than the earth. In the first chapter of this book we saw that the earth's relative uniformity of climate for hundreds of millions of years seems to imply a similar uniformity in solar activity. This accords with a recent tendency amongastronomers who are more and more recognizing that the stars and the solar system possess an extraordinary degree of conservatism. Changes that once were supposed to take place in thousands of years are now thought to have required millions. Hence in this chapter we shall assume that throughout geological times the condition of the sun has been almost as at present. It may have been somewhat larger, or different in other ways, but it was essentially a hot, gaseous body such as we see today and it gave out essentially the same amount of energy. This assumption will affect the general validity of what follows only if it departs widely from the truth. With this assumption, then, let us inquire into the degree to which the sun's atmosphere has probably been disturbed throughout geological times.

InEarth and Sun, as already explained, a detailed study has led to the conclusion that cyclonic storms are influenced by the electrical action of the sun. Such action appears to be most intense in sunspots, but apparently pertains also to other disturbed areas in the sun's atmosphere. A study of sunspots suggests that their true periodicity is almost if not exactly identical with that of the orbital revolution of Jupiter, 11.8 years. Other investigations show numerous remarkable coincidences between sunspots and the orbital revolution of the other planets, including especially Saturn and Mercury. This seems to indicate that there is some truth in the hypothesis that sunspots and other related disturbances of the solar atmosphere owe their periodicity to the varying effects of the planets as they approach and recede from the sun in their eccentric orbits and as they combine or oppose their effects according to their relative positions. This does not mean that the energy of the solar disturbances is supposed to come from the planets, but merelythat their variations act like the turning of a switch to determine when and how violently the internal forces of the sun shall throw the solar atmosphere into commotion. This hypothesis is by no means new, for in one form or another it has been advocated by Wolfer, Birkeland, E. W. Brown, Schuster, Arctowski, and others.

The agency through which the planets influence the solar atmosphere is not yet clear. The suggested agencies are the direct pull of gravitation, the tidal effect of the planets, and an electro-magnetic effect. InEarth and Sunthe conclusion is reached that the first two are out of the question, a conclusion in which E. W. Brown acquiesces. Unless some unknown cause is appealed to, this leaves an electro-magnetic hypothesis as the only one which has a reasonable foundation. Schuster inclines to this view. The conclusions set forth inEarth and Sunas to the electrical nature of the sun's influence on the earth point somewhat in the same direction. Hence in this chapter we shall inquire what would happen to the sun, and hence to the earth, on their journey through space, if the solar atmosphere is actually subject to disturbance by the electrical or other effects of other heavenly bodies. It need hardly be pointed out that we are here venturing into highly speculative ground, and that the verity or falsity of the conclusions reached in this chapter has nothing to do with the validity of the reasoning in previous chapters. Those chapters are based on the assumption that terrestrial causes of climatic changes are supplemented by solar disturbances which produce their effect partly through variations in temperature but also through variations in the intensity and paths of cyclonic storms. The present chapter seeks to shed some light on the possible causes and sequence of solar disturbances.

Let us begin by scanning the available evidence as tosolar disturbances previous to the time when accurate sunspot records are available. Two rather slender bits of evidence point to cycles of solar activity lasting hundreds of years. One of these has already been discussed in Chapter VI, where the climatic stress of the fourteenth century was described. At that time sunspots are known to have been unusually numerous, and there were great climatic extremes. Lakes overflowed in Central Asia; storms, droughts, floods, and cold winters were unusually severe in Europe; the Caspian Sea rose with great rapidity; the trees of California grew with a vigor unknown for centuries; the most terrible of recorded famines occurred in England and India; the Eskimos were probably driven south by increasing snowiness in Greenland; and the Mayas of Yucatan appear to have made their last weak attempt at a revival of civilization under the stimulus of greater storminess and less constant rainfall.

The second bit of evidence is found in recent exhaustive studies of periodicities by Turner[115]and other astronomers. They have sought every possible natural occurrence for which a numerical record is available for a long period. The most valuable records appear to be those of tree growth, Nile floods, Chinese earthquakes, and sunspots. Turner reaches the conclusion that all four types of phenomena show the same periodicity, namely, cycles with an average length of about 260 to 280 years. He suggests that if this is true, the cycles in tree growth and in floods, both of which are climatic, are probably due to a non-terrestrial cause. The fact that the sunspotsshow similar cycles suggests that the sun's variations are the cause.

These two bits of evidence are far too slight to form the foundation of any theory as to changes in solar activity in the geological past. Nevertheless it may be helpful to set forth certain possibilities as a stimulus to further research. For example, it has been suggested that meteoric bodies may have fallen into the sun and caused it suddenly to flare up, as it were. This is not impossible, although it does not appear to have taken place since men became advanced enough to make careful observations. Moreover, the meteorites which now fall on the earth are extremely small, the average size being computed as no larger than a grain of wheat. The largest ever found on the earth's surface, at Bacubirito in Mexico, weighs only about fifty tons, while within the rocks the evidences of meteorites are extremely scanty and insignificant. If meteorites had fallen into the sun often enough and of sufficient size to cause glacial fluctuations and historic pulsations of climate, it seems highly probable that the earth would show much more evidence of having been similarly disturbed. And even if the sun should be bombarded by large meteors the result would probably not be sudden cold periods, which are the most notable phenomena of the earth's climatic history, but sudden warm periods followed by slow cooling. Nevertheless, the disturbance of the sun by collision with meteoric matter can by no means be excluded as a possible cause of climatic variations.

Allied to the preceding hypothesis is Shapley's[116]nebular hypothesis. At frequent intervals, averaging aboutonce a year during the last thirty years, astronomers have discovered what are known as novæ. These are stars which were previously faint or even invisible, but which flash suddenly into brilliancy. Often their light-giving power rises seven or eight magnitudes—a thousand-fold. In addition to the spectacular novæ there are numerous irregular variables whose brilliancy changes in every ratio from a few per cent up to several magnitudes. Most of them are located in the vicinity of nebulæ, as is also the case with novæ. This, as well as other facts, makes it probable that all these stars are "friction variables," as Shapley calls them. Apparently as they pass through the nebulæ they come in contact with its highly diffuse matter and thereby become bright much as the earth would become bright if its atmosphere were filled with millions of almost infinitesimally small meteorites. A star may also lose brilliancy if nebulous matter intervenes between it and the observer. If our sun has been subjected to any of these changes some sort of climatic effect must have been produced.

In a personal communication Shapley amplifies the nebular climatic hypothesis as follows:

Within 700 light years of the sun in many directions (Taurus, Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebulosity, some bright, most of them dark. The probability that stars moving in the general region of such clouds will encounter this material is very high, for the clouds fill enormous volumes of space,—e.g., probably more than a hundred thousand cubic light years in the Orion region, and are presumably composed of rarefied gases or of dust particles. Probably throughout all our part of space such nebulosity exists (it is all around us, we are sure), but only in certain regions is it dense enough to affect conspicuously the stars involved in it. If a star moving at high velocity should collide with a dense part of such a nebulouscloud, we should probably have a typical nova. If the relative velocity of nebulous material and star were low or moderate, or if the material were rare, we should not expect a conspicuous effect on the star's light.In the nebulous region of Orion, which is probably of unusually high density, there are about 100 known stars, varying between 20% and 80% of their total light—all of them irregularly—some slowly, some suddenly. Apparently they are "friction variables." Some of the variables suddenly lose 40% of their light as if blanketed by nebulous matter. In the Trifid Nebula there are variables like those of Orion, in Messier 8 also, and probably many of the 100 or so around the Rho Ophiuchi region belong to this kind.I believe that our sun could not have been a typical nova, at least not since the Archeozoic, that is for perhaps a billion years. I believe we have in geological climates final proof of this, because an increase in the amount of solar radiation by 1000 times as in the typical nova, would certainly punctuate emphatically the life cycle on the earth, even if the cause of the nova would not at the same time eliminate the smaller planets. But the sun may have been one of these miniature novæ or friction variables; and I believe it very probable that its wanderings through this part of space could not long leave its mean temperature unaffected to the amount of a few per cent.One reason we have not had this proposal insisted upon before is that the data back of it are mostly new—the Orion variables have been only recently discovered and studied, the distribution and content of the dark nebulæ are hardly as yet generally known.

Within 700 light years of the sun in many directions (Taurus, Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebulosity, some bright, most of them dark. The probability that stars moving in the general region of such clouds will encounter this material is very high, for the clouds fill enormous volumes of space,—e.g., probably more than a hundred thousand cubic light years in the Orion region, and are presumably composed of rarefied gases or of dust particles. Probably throughout all our part of space such nebulosity exists (it is all around us, we are sure), but only in certain regions is it dense enough to affect conspicuously the stars involved in it. If a star moving at high velocity should collide with a dense part of such a nebulouscloud, we should probably have a typical nova. If the relative velocity of nebulous material and star were low or moderate, or if the material were rare, we should not expect a conspicuous effect on the star's light.

In the nebulous region of Orion, which is probably of unusually high density, there are about 100 known stars, varying between 20% and 80% of their total light—all of them irregularly—some slowly, some suddenly. Apparently they are "friction variables." Some of the variables suddenly lose 40% of their light as if blanketed by nebulous matter. In the Trifid Nebula there are variables like those of Orion, in Messier 8 also, and probably many of the 100 or so around the Rho Ophiuchi region belong to this kind.

I believe that our sun could not have been a typical nova, at least not since the Archeozoic, that is for perhaps a billion years. I believe we have in geological climates final proof of this, because an increase in the amount of solar radiation by 1000 times as in the typical nova, would certainly punctuate emphatically the life cycle on the earth, even if the cause of the nova would not at the same time eliminate the smaller planets. But the sun may have been one of these miniature novæ or friction variables; and I believe it very probable that its wanderings through this part of space could not long leave its mean temperature unaffected to the amount of a few per cent.

One reason we have not had this proposal insisted upon before is that the data back of it are mostly new—the Orion variables have been only recently discovered and studied, the distribution and content of the dark nebulæ are hardly as yet generally known.

This interesting hypothesis cannot be hastily dismissed. If the sun should pass through a nebula it seems inevitable that there would be at least slight climatic effects and perhaps catastrophic effects through the action of the gaseous matter not only on the sun but on the earth's own atmosphere. As an explanation of thegeneral climatic conditions of the past, however, Shapley points out that the hypothesis has the objection of being vague, and that nebulosity should not be regarded as more than "a possible factor." One of the chief difficulties seems to be the enormously wide distribution of as yet undiscovered nebulous matter which must be assumed if any large share of the earth's repeated climatic changes is to be ascribed to such matter. If such matter is actually abundant in space, it is hard to see how any but the nearest stars would be visible. Another objection is that there is no known nebulosity near at hand with which to connect the climatic vicissitudes of the last glacial period. Moreover, the known nebulæ are so much less numerous than stars that the chances that the sun will encounter one of them are extremely slight. This, however, is not an objection, for Shapley points out that during geological times the sun can never have varied as much as do the novæ, or even as most of the friction variables. Thus the hypothesis stands as one that is worth investigating, but that cannot be finally rejected or accepted until it is made more definite and until more information is available.

Another suggested cause of solar variations is the relatively sudden contraction of the sun such as that which sometimes occurs on the earth when continents are uplifted and mountains upheaved. It seems improbable that this could have occurred in a gaseous body like the sun. Lacking, as it does, any solid crust which resists a change of form, the sun probably shrinks steadily. Hence any climatic effects thus produced must be extremely gradual and must tend steadily in one direction for millions of years.

Still another suggestion is that the tidal action of the stars and other bodies which may chance to approachthe sun's path may cause disturbances of the solar atmosphere. The vast kaleidoscope of space is never quiet. The sun, the stars, and all the other heavenly bodies are moving, often with enormous speed. Hence the effect of gravitation upon the sun must vary constantly and irregularly, as befits the geological requirements. In the case of the planets, however, the tidal effect does not seem competent to produce the movements of the solar atmosphere which appear to be concerned in the inception of sunspots. Moreover, there is only the most remote probability that a star and the sun will approach near enough to one another to produce a pronounced gravitational disturbance in the solar atmosphere. For instance, if it be assumed that changes in Jupiter's tidal effect on the sun are the main factor in regulating the present difference between sunspot maxima and sunspot minima, the chances that a star or some non-luminous body of similar mass will approach near enough to stimulate solar activity and thereby bring on glaciation are only one in twelve billion years, as will be explained below. This seems to make a gravitational hypothesis impossible.

Another possible cause of solar disturbances is that the stars in their flight through space may exert an electrical influence which upsets the equilibrium of the solar atmosphere. At first thought this seems even more impossible than a gravitational effect. Electrostatic effects, however, differ greatly from those of tides. They vary as the diameter of a body instead of as its mass; their differentials also vary inversely as the square of the distance instead of as the cube. Electrostatic effects also increase as the fourth power of the temperature or at least would do so if they followed the law of black bodies; they are stimulated by the approach of one bodyto another; and they are cumulative, for if ions arrive from space they must accumulate until the body to which they have come begins to discharge them. Hence, on the basis of assumptions such as those used in the preceding paragraph, the chances of an electrical disturbance of the solar atmosphere sufficient to cause glaciation on the earth may be as high as one in twenty or thirty million years. This seems to put an electrical hypothesis within the bounds of possibility. Further than that we cannot now go. There may be other hypotheses which fit the facts much better, but none seems yet to have been suggested.

In the rest of this chapter the tidal and electrical hypotheses of stellar action on the sun will be taken up in detail. The tidal hypothesis is considered because in discussions of the effect of the planets it has hitherto held almost the entire field. The electrical hypothesis will be considered because it appears to be the best yet suggested, although it still seems doubtful whether electrical effects can be of appreciable importance over such vast distances as are inevitably involved. The discussion of both hypotheses will necessarily be somewhat technical, and will appeal to the astronomer more than to the layman. It does not form a necessary part of this book, for it has no bearing on our main thesis of the effect of the sun on the earth. It is given here because ultimately the question of changes in solar activity during geological times must be faced.

In the astronomical portion of the following discussion we shall follow Jeans[117]in his admirable attempt at a mathematical analysis of the motions of the universe. Jeans divides the heavenly bodies into five main types. (1) Spiral nebulæ, which are thought by some astronomersto be systems like our own in the making, and by others to be independent universes lying at vast distances beyond the limits of our Galactic universe, as it is called from the Galaxy or Milky Way. (2) Nebulæ of a smaller type, called planetary. These lie within the Galactic portion of the universe and seem to be early stages of what may some day be stars or solar systems. (3) Binary or multiple stars, which are extraordinarily numerous. In some parts of the heavens they form 50 or even 60 per cent of the stars and in the galaxy as a whole they seem to form "fully one third." (4) Star clusters. These consist of about a hundred groups of stars in each of which the stars move together in the same direction with approximately the same velocity. These, like the spiral nebulæ, are thought by some astronomers to lie outside the limits of the galaxy, but this is far from certain. (5) The solar system. According to Jeans this seems to be unique. It does not fit into the general mathematical theory by which he explains spiral nebulæ, planetary nebulæ, binary stars, and star clusters. It seems to demand a special explanation, such as is furnished by tidal disruption due to the passage of the sun close to another star.

The part of Jeans' work which specially concerns us is his study of the probability that some other star will approach the sun closely enough to have an appreciable gravitative or electrical effect, and thus cause disturbances in the solar atmosphere. Of course both the star and the sun are moving, but to avoid circumlocution we shall speak of such mutual approaches simply as approaches of the sun. For our present purpose the most fundamental fact may be summed up in a quotation from Jeans in which he says that most stars "show evidence of having experienced considerable disturbance by other systems; there is no reason why our solar system shouldbe expected to have escaped the common fate." Jeans gives a careful calculation from which it is possible to derive some idea of the probability of any given degree of approach of the sun and some other star. Of course all such calculations must be based on certain assumptions. The assumptions made by Jeans are such as to make the probability of close approaches as great as possible. For example, he allows only 560 million years for the entire evolution of the sun, whereas some astronomers and geologists would put the figure ten or more times as high. Nevertheless, Jeans' assumptions at least show the order of magnitude which we may expect on the basis of reasonable astronomical conclusions.

According to the planetary hypothesis of sunspots, the difference in the effect of Jupiter when it is nearest and farthest from the sun is the main factor in starting the sunspot cycle and hence the corresponding terrestrial cycle. The climatic difference between sunspot maxima and minima, as measured by temperature, apparently amounts to at least a twentieth and perhaps a tenth of the difference between the climate of the last glacial epoch and the present. We may suppose, then, that a body which introduced a gravitative or electrical factor twenty times as great as the difference in Jupiter's effect at its maximum and minimum distances from the sun would cause a glacial epoch if the effect lasted long enough. Of course the other planets combine their effects with that of Jupiter, but for the sake of simplicity we will leave the others out of account. The difference between Jupiter's maximum and minimum tidal effect on the sun amounts to 29 per cent of the planet's average effect. The corresponding difference, according to the electrical hypothesis, is about 19 per cent, for electrostatic action varies as the square of the distance instead of as the cube.Let us assume that a body exerting four times Jupiter's present tidal effect and placed at the average distance of Jupiter from the sun would disturb the sun's atmosphere twenty times as much as the present difference between sunspot maxima and minima, and thus, perhaps, cause a glacial period on the earth.

On the basis of this assumption our first problem is to estimate the frequency with which a star, visible or dark, is likely to approach near enough to the sun to produce atidaleffect four times that of Jupiter. The number of visible stars is known or at least well estimated. As to dark stars, which have grown cool, Arrhenius believed that they are a hundred times as numerous as bright stars; few astronomers believe that there are less than three or four times as many. Dr. Shapley of the Harvard Observatory states that a new investigation of the matter suggests that eight or ten is probably a maximum figure. Let us assume that nine is correct. The average visible star, so far as measured, has a mass about twice that of the sun, or about 2100 times that of Jupiter. The distances of the stars have been measured in hundreds of cases and thus we can estimate how many stars, both visible and invisible, are on an average contained in a given volume of space. On this basis Jeans estimates that there is only one chance in thirty billion years that a visible star will approach within 2.8 times the distance of Neptune from the sun, that is, within about eight billion miles. If we include the invisible stars the chances become one in three billion years. In order to produce four times the tidal effect of Jupiter, however, the average star would have to approach within about four billion miles of the sun, and the chances of that are only one in twelve billion years. The disturbing starwould be only 40 per cent farther from the sun than Neptune, and would almost pass within the solar system.

Even though Jeans holds that the frequency of the mutual approach of the sun and a star was probably much greater in the distant past than at present, the figures just given lend little support to the tidal hypothesis. In fact, they apparently throw it out of court. It will be remembered that Jeans has made assumptions which give as high a frequency of stellar encounters as is consistent with the astronomical facts. We have assumed nine dark stars for every bright one, which may be a liberal estimate. Also, although we have assumed that a disturbance of the sun's atmosphere sufficient to cause a glacial period would arise from a tidal effect only twenty times as great as the difference in Jupiter's effect when nearest the sun and farthest away, in our computations this has actually been reduced to thirteen. With all these favorable assumptions the chances of a stellar approach of the sort here described are now only one in twelve billion years. Yet within a hundred million years, according to many estimates of geological time, and almost certainly within a billion, there have been at least half a dozen glaciations.

Our use of Jeans' data interposes another and equally insuperable difficulty to any tidal hypothesis. Four billion miles is a very short distance in the eyes of an astronomer. At that distance a star twice the size of the sun would attract the outer planets more strongly than the sun itself, and might capture them. If a star should come within four billion miles of the sun, its effect in distorting the orbits of all the planets would be great. If this had happened often enough to cause all the glaciations known to geologists, the planetary orbits would be strongly elliptical instead of almost circular. The considerationshere advanced militate so strongly against the tidal hypothesis of solar disturbances that it seems scarcely worth while to consider it further.

Let us turn now to the electrical hypothesis. Here the conditions are fundamentally different from those of the tidal hypothesis. In the first place the electrostatic effect of a body has nothing to do with its mass, but depends on the area of its surface; that is, it varies as the square of the radius. Second, the emission of electrons varies exponentially. If hot glowing stars follow the same law as black bodies at lower temperatures, the emission of electrons, like the emission of other kinds of energy, varies as the fourth power of the absolute temperature. In other words, suppose there are two black bodies, otherwise alike, but one with a temperature of 27° C. or 300° on the absolute scale, and the other with 600° on the absolute scale. The temperature of one is twice as high as that of the other, but the electrostatic effect will be sixteen times as great.[118]Third, the number of electronsthat reach a given body varies inversely as the square of the distance, instead of as the cube which is the case with tide-making forces.

In order to use these three principles in calculating the effect of the stars we must know the diameters, distances, temperature, and number of the stars. The distances and number may safely be taken as given by Jeans in the calculations already cited. As to the diameters, the measurements of the stars thus far made indicate that the average mass is about twice that of the sun. The average density, as deduced by Shapley[119]from the movements of double stars, is about one-eighth the solar density. This would give an average diameter about two and a half times that of the sun. For the dark stars, we shall assume for convenience that they are ten times as numerous as the bright ones. We shall also assume that their diameter is half that of the sun, for being cool they must be relatively dense, and that their temperature is the same as that which we shall assume for Jupiter.

As to Jupiter we shall continue our former assumption that a body with four times the effectiveness of that planet, which here means with twice as great a radius, would disturb the sun enough to cause glaciation. It would produce about twenty times the electrostatic effectwhich now appears to be associated with the difference in Jupiter's effect at maximum and minimum. The temperature of Jupiter must also be taken into account. The planet is supposed to be hot because its density is low, being only about 1.25 that of water. Nevertheless, it is probably not luminous, for as Moulton[120]puts it, shadows upon it are black and its moons show no sign of illumination except from the sun. Hence a temperature of about 600°C., or approximately 900° on the absolute scale, seems to be the highest that can reasonably be assigned to the cold outer layer whence electrons are emitted. As to the temperature of the sun, we shall adopt the common estimate of about 6300°C. on the absolute scale. The other stars will be taken as averaging the same, although of course they vary greatly.

When Jeans' method of calculating the probability of a mutual approach of the sun and a star is applied to the assumptions given above, the results are as shown in Table 5. On that basis the dark stars seem to be of negligible importance so far as the electrical hypothesis is concerned. Even though they may be ten times as numerous as the bright ones there appears to be only one chance in 130 billion years that one of them will approach the sun closely enough to cause the assumed disturbance of the solar atmosphere. On the other hand, if all the visible stars were the size of the sun, and as hot as that body, their electrical effect would be fourfold that of our assumed dark star because of their size, and 2401 times as great because of their temperature, or approximately 10,000 times as great. Under such conditions the theoretical chance of an approach that would cause glaciation is one in 130 million years. If the average visible star is somewhat cooler than the sun and has aradius about two and one-half times as great, as appears to be the fact, the chances rise to one in thirty-eight million years. A slight and wholly reasonable change in our assumptions would reduce this last figure to only five or ten million. For instance, the earth's mean temperature during the glacial period has been assumed as 10°C. lower than now, but the difference may have been only 6°. Again, the temperature of the outer atmosphere of Jupiter where the electrons are shot out may be only 500° or 700° absolute, instead of 900°. Or the diameter of the average star may be five or ten times that of the sun, instead of only two and one-half times as great. All this, however, may for the present be disregarded. The essential point is that even when the assumptions err on the side of conservatism, the results are of an order of magnitude which puts the electrical hypothesis within the bounds of possibility, whereas similar assumptions put the tidal hypothesis, with its single approach in twelve billion years, far beyond those limits.

The figures for Betelgeuse in Table 5 are interesting. At a meeting of the American Association for the Advancement of Science in December, 1920, Michelson reported that by measurements of the interference of light coming from the two sides of that bright star in Orion, the observers at Mount Wilson had confirmed the recent estimates of three other authorities that the star's diameter is about 218 million miles, or 250 times that of the sun. If other stars so much surpass the estimates of only a decade or two ago, the average diameter of all the visible stars must be many times that of the sun. The low figure for Betelgeuse in section D of the table means that if all the stars were as large as Betelgeuse, several might often be near enough to cause profound disturbances of the solar atmosphere. Nevertheless, because of the lowtemperature of the giant red stars of the Betelgeuse type, the distance at which one of them would produce a given electrical effect is only about five times the distance at which our assumed average star would produce the same effect. This, to be sure, is on the assumption that theradiation of energy from incandescent bodies varies according to temperature in the same ratio as the radiation from black bodies. Even if this assumption departs somewhat from the truth, it still seems almost certain that the lower temperature of the red compared with the high temperature of the white stars must to a considerable degree reduce the difference in electrical effect which would otherwise arise from their size.

Thus far in our attempt to estimate the distance at which a star might disturb the sun enough to cause glaciation on the earth, we have considered only the star's size and temperature. No account has been taken of the degree to which its atmosphere is disturbed. Yet in the case of the sun this seems to be one of the most important factors. The magnetic field of sunspots is sometimes 50 or 100 times as strong as that of the sun in general. The strength of the magnetic field appears to depend on the strength of the electrical currents in the solar atmosphere. But the intensity of the sunspots and, by inference, of the electrical currents, may depend on the electrical action of Jupiter and the other planets. If we apply a similar line of reasoning to the stars, we are at once led to question whether the electrical activity of double stars may not be enormously greater than that of isolated stars like the sun.

If this line of reasoning is correct, the atmosphere of every double star must be in a state of commotion vastly greater than that of the sun's atmosphere even when it is most disturbed. For example, suppose the sun were accompanied by a companion of equal size at a distance of one million miles, which would make it much like many known double stars. Suppose also that in accordance with the general laws of physics the electrical effect of the two suns upon one another is proportional to the fourthpower of the temperature, the square of the radius, and the inverse square of the distance. Then the effect of each sun upon the other would be sixty billion (6 × 1010) times as great as the present electrical effect of Jupiter upon the sun. Just what this would mean as to the net effect of a pair of such suns upon the electrical potential of other bodies at a distance we can only conjecture. The outstanding fact is that the electrical conditions of a double star must be radically different and vastly more intense than those of a single star like the sun.

This conclusion carries weighty consequences. At present twenty or more stars are known to be located within about 100 trillion miles of the sun (five parsecs, as the astronomers say), or 16.5 light years. According to the assumptions employed in Table 5 an average single star would influence the sun enough to cause glaciation if it came within approximately 200 billion miles. If the star were double, however, it might have an electrical capacity enormously greater than that of the sun. Then it would be able to cause glaciation at a correspondingly great distance. Today Alpha Centauri, the nearest known star about twenty-five trillion miles, or 4.3 light years from the sun, and Sirius, the brightest star in the heavens, is about fifty trillion miles away, or 8.5 light years. If these stars were single and had a diameter three times that of the sun, and if they were of the same temperature as has been assumed for Betelgeuse, which is about fifty times as far away as Alpha Centauri, the relative effects of the three stars upon the sun would be, approximately, Betelgeuse 700, Alpha Centauri 250, Sirius 1. But Alpha Centauri is triple and Sirius double, and both are much hotter than Betelgeuse. Hence Alpha Centauri and even Sirius may be far more effective than Betelgeuse.

The two main components of Alpha Centauri are separatedby an average distance of about 2,200,000,000 miles, or somewhat less than that of Neptune from the sun. A third and far fainter star, one of the faintest yet measured, revolves around them at a great distance. In mass and brightness the two main components are about like the sun, and we will assume that the same is true of their radius. Then, according to the assumptions made above, their effect in disturbing one another electrically would be about 10,000 times the total effect of Jupiter upon the sun, or 2500 times the effect that we have assumed to be necessary to produce a glacial period. We have already seen in Table 5 that, according to our assumptions, a single star like the sun would have to approach within 120 billion miles of the solar system, or within 2 per cent of a light year, in order to cause glaciation. By a similar process of reasoning it appears that if the mutual electrical excitation of the two main parts of Alpha Centauri, regardless of the third part, is proportional to the apparent excitation of the sun by Jupiter, Alpha Centauri would be 5000 times as effective as the sun. In other words, if it came within 8,500,000,000,000 miles of the sun, or 1.4 light years, it would so change the electrical conditions as to produce a glacial epoch. In that case Alpha Centauri is now so near that it introduces a disturbing effect equal to about one-sixth of the effect needed to cause glaciation on the earth. Sirius and perhaps others of the nearer and brighter or larger stars may also create appreciable disturbances in the electrical condition of the sun's atmosphere, and may have done so to a much greater degree in the past, or be destined to do so in the future. Thus an electrical hypothesis of solar disturbances seems to indicate that the position of the sun in respect to other stars may be a factor of great importance in determining the earth's climate.

Having gained some idea of the nature of the electrical hypothesis of solar disturbances and of the possible effect of other bodies upon the sun's atmosphere, let us now compare the astronomical data with those of geology. Let us take up five chief points for which the geologist demands an explanation, and which any hypothesis must meet if it is to be permanently accepted. These are (1) the irregular intervals at which glacial periods occur; (2) the division of glacial periods into epochs separated sometimes by hundreds of thousands of years; (3) the length of glacial periods and epochs; (4) the occurrence of glacial stages and historic pulsations in the form of small climatic waves superposed upon the larger waves of glacial epochs; (5) the occurrence of climatic conditions much milder than those of today, not only in the middle portion of the great geological eras, but even in some of the recent inter-glacial epochs.

1. The irregular duration of the interval from one glacial epoch to another corresponds with the irregular distribution of the stars. If glaciation is indirectly due to stellar influences, the epochs might fall close together, or might be far apart. If the average interval were ten million years, one interval might be thirty million or more and the next only one or two hundred thousand.According to Schuchert, the known periods of glacial or semi-glacial climate have been approximately as follows:

This table suggests an interesting inquiry. During the last few decades there has been great interest in ancient glaciation and geologists have carefully examined rocks of all ages for signs of glacial deposits. In spite of the large parts of the earth which are covered with deposits belonging to the Mesozoic and Cenozoic, which form thelast quarter of geological time, the only signs of actual glaciation are those of the great Pleistocene period and a few local occurrences at the end of the Mesozoic or beginning of the Cenozoic. Late in the Triassic and early in the Jurassic, the climate appears to have been rigorous, although no tillites have been found to demonstrate glaciation. In the preceding quarter, that is, the Paleozoic, the Permian glaciation was more severe than that of the Pleistocene, and the Devonian than that of the Eocene, while the Ordovician evidences of low temperature are stronger than those at the end of the Triassic. In view of the fact that rocks of Paleozoic age cover much smaller areas than do those of later age, the three Paleozoic glaciations seem to indicate a relative frequency of glaciation. Going back to the Proterozoic, it is astonishing to find that evidence of two highly developed glacial periods, and possibly four, has been discovered. Since the Indian and the African glaciations of Proterozoic times are as yet undated, we cannot be sure that they are not of the same date as the others. Nevertheless, even two is a surprising number, for not only are most Proterozoic rocks so metamorphosed that possible evidences of glacial origin are destroyed, but rocks of that age occupy far smaller areas than either those of Paleozoic or, still more, Mesozoic and Cenozoic age. Thus the record of the last three-quarters of geological time suggests that if rocks of all ages were as abundant and as easily studied as those of the later periods, the frequency of glacial periods would be found to increase as one goes backward toward the beginnings of the earth's history. This is interesting, for Jeans holds that the chances that the stars would approach one another were probably greater in the past than at present. This conclusion is based on the assumption that our universeis like the spiral nebulæ in which the orbits of the various members are nearly circular during the younger stages. Jeans considers it certain that in such cases the orbits will gradually become larger and more elliptical because of the attraction of one body for another. Thus as time goes on the stars will be more widely distributed and the chances of approach will diminish. If this is correct, the agreement between astronomical theory and geological conclusions suggests that the two are at least not in opposition.

The first quarter of geological time as well as the last three must be considered in this connection. During the Archeozoic, no evidence of glaciation has yet been discovered. This suggests that the geological facts disprove the astronomical theory. But our knowledge of early geological times is extremely limited, so limited that lack of evidence of glaciation in the Archeozoic may have no significance. Archeozoic rocks have been studied minutely over a very small percentage of the earth's land surface. Moreover, they are highly metamorphosed so that, even if glacial tills existed, it would be hard to recognize them. Third, according to both the nebular and the planetesimal hypotheses, it seems possible that during the earliest stages of geological history the earth's interior was somewhat warmer than now, and the surface may have been warmed more than at present by conduction, by lava flows, and by the fall of meteorites. If the earth during the Archeozoic period emitted enough heat to raise its surface temperature a few degrees, the heat would not prevent the development of low forms of life but might effectively prevent all glaciation. This does not mean that it would prevent changes of climate, but merely changes so extreme that their record would be preserved by means of ice. It will be most interestingto see whether future investigations in geology and astronomy indicate either a semi-uniform distribution of glacial periods throughout the past, or a more or less regular decrease in frequency from early times down to the present.

2. The Pleistocene glacial period was divided into at least four epochs, while in the Permian at least one inter-glacial epoch seems certain, and in some places the alternation between glacial and non-glacial beds suggests no less than nine. In the other glaciations the evidence is not yet clear. The question of periodicity is so important that it overthrows most glacial hypotheses. Indeed, had their authors known the facts as established in recent years, most of the hypotheses would never have been advanced. The carbon dioxide hypothesis is the only one which was framed with geologically rapid climatic alternations in mind. It certainly explains the facts of periodicity better than does any of its predecessors, but even so it does not account for the intimate way in which variations of all degrees from those of the weather up to glacial epochs seem to grade into one another.

According to our stellar hypothesis, occasional groups of glacial epochs would be expected to occur close together and to form long glacial periods. This is because many of the stars belong to groups or clusters in which the stars move in parallel paths. A good example is the cluster in the Hyades, where Boss has studied thirty-nine stars with special care.[123]The stars are grouped about a center about 130 light years from the sun. The stars themselves are scattered over an area about thirty light years in diameter. They average about the same distance apart as do those near the sun, but toward thecenter of the group they are somewhat closer together. The whole thirty-nine sweep forward in essentially parallel paths. Boss estimates that 800,000 years ago the cluster was only half as far from the sun as at present, but probably that was as near as it has been during recent geological times. All of the thirty-nine stars of this cluster, as Moulton[124]puts it, "are much greater in light-giving power than the sun. The luminosities of even the five smallest are from five to ten times that of the sun, while the largest are one hundred times greater in light-giving power than our own luminary. Their masses are probably much greater than that of the sun." If the sun were to pass through such a cluster, first one star and then another might come so near as to cause a profound disturbance in the sun's atmosphere.

3. Another important point upon which a glacial hypothesis may come to grief is the length of the periods or rather of the epochs which compose the periods. During the last or Pleistocene glacial period the evidence in America and Europe indicates that the inter-glacial epochs varied in length and that the later ones were shorter than the earlier. Chamberlin and Salisbury, from a comparison of various authorities, estimate that the intervals from one glacial epoch to another form a declining series, which may be roughly expressed as follows: 16-8-4-2-1, where unity is the interval from the climax of the late Wisconsin, or last glacial epoch, to the present. Most authorities estimate the culmination of the late Wisconsin glaciation as twenty or thirty thousand years ago. Penck estimates the length of the last inter-glacial period as 60,000 years and the preceding one as 240,000.[125]R. T. Chamberlin, as already stated, finds thatthe consensus of opinion is that inter-glacial epochs have averaged five times as long as glacial epochs. The actual duration of the various glaciations probably did not vary in so great a ratio as did the intervals from one glaciation to another. The main point, however, is the irregularity of the various periods.

The relation of the stellar electrical hypothesis to the length of glacial epochs may be estimated from column C, in Table 5. There we see that the distances at which a star might possibly disturb the sun enough to cause glaciation range all the way from 120 billion miles in the case of a small star like the sun, to 3200 billion in the case of Betelgeuse, while for double stars the figure may rise a hundred times higher. From this we can calculate how long it would take a star to pass from a point where its influence would first amount to a quarter of the assumed maximum to a similar point on the other side of the sun. In making these calculations we will assume that the relative rate at which the star and the sun approach each other is about twenty-two miles per second, or 700 million miles per year, which is the average rate of motion of all the known stars. According to the distances in Table 5 this gives a range from about 500 years up to about 10,000, which might rise to a million in the case of double stars. Of course the time might be relatively short if the sun and a rapidly moving star were approaching one another almost directly, or extremely long if the sun and the star were moving in almost the same direction and at somewhat similar rates,—a condition more common than the other. Here, as in so many other cases, the essential point is that the figures which we thus obtain seem to be of the right order of magnitude.

4. Post-glacial climatic stages are so well known that in Europe they have definite names. Their sequence hasalready been discussed in Chapter XII. Fossils found in the peat bogs of Denmark and Scandinavia, for example, prove that since the final disappearance of the continental ice cap at the close of the Wisconsin there has been at least one period when the climate of Europe was distinctly milder than now. Directly overlying the sheets of glacial drift laid down by the ice there is a flora corresponding to that of the present tundras. Next come remains of a forest vegetation dominated by birches and poplars, showing that the climate was growing a little warmer. Third, there follow evidences of a still more favorable climate in the form of a forest dominated by pines; fourth, one where oak predominates; and fifth, a flora similar to that of the Black Forest of Germany, indicating that in Scandinavia the temperature was then decidedly higher than today. This fifth flora has retreated southward once more, having been driven back to its present latitude by a slight recurrence of a cool stormy climate.[126]In central Asia evidence of post-glacial stages is found not only in five distinct moraines but in a corresponding series of elevated strands surrounding salt lakes and of river terraces in non-glaciated arid regions.[127]

In historic as well as prehistoric times, as we have already seen, there have been climatic fluctuations. For instance, the twelfth or thirteenth century B. C. appears to have been almost as mild as now, as does the seventh century B. C. On the other hand about 1000 B. C., at the time of Christ, and in the fourteenth century there were times of relative severity. Thus it appears that both ona large and on a small scale pulsations of climate are the rule. Any hypothesis of climatic changes must satisfy the periods of these pulsations. These conditions furnish a problem which makes difficulty for almost all hypotheses of climatic change. According to the present hypothesis, earth movements such as are discussed in Chapter XII may coöperate with two astronomical factors. One is the constant change in the positions of the stars, a change which we have already called kaleidoscopic, and the other is the fact that a large proportion of the stars are double or multiple. When one star in a group approaches the sun closely enough to cause a great solar disturbance, numerous others may approach or recede and have a minor effect. Thus, whenever the sun is near groups of stars we should expect that the earth would show many minor climatic pulsations and stages which might or might not be connected with glaciation. The historic pulsations shown in the curve of tree growth in California, Fig. 4, are the sort of changes that would be expected if movements of the stars have an effect on the solar atmosphere.

Not only are fully a third of all the visible stars double, as we have already seen, but at least a tenth of these are known to be triple or multiple. In many of the double stars the two bodies are close together and revolve so rapidly that whatever periodicity they might create in the sun's atmosphere would be very short. In the triplets, however, the third star is ordinarily at least ten times as far from the other two as they are from each other, and its period of rotation sometimes runs into hundreds or thousands of years. An actual multiple star in the constellation Polaris will serve as an example. The main star is believed by Jeans to consist of two parts which are almost in contact and whirl around each other withextraordinary speed in four days. If this is true they must keep each other's atmospheres in a state of intense commotion. Much farther away a third star revolves around this pair in twelve years. At a much greater distance a fourth star revolves around the common center of gravity of itself and the other three in a period which may be 20,000 years. Still more complicated cases probably exist. Suppose such a system were to traverse a path where it would exert a perceptible influence on the sun for thirty or forty thousand years. The varying movements of its members would produce an intricate series of cycles which might show all sorts of major and minor variations in length and intensity. Thus the varied and irregular stages of glaciation and the pulsations of historic times might be accounted for on the hypothesis of the proximity of the sun to a multiple star, as well as on that of the less pronounced approach and recession of a number of stars. In addition to all this, an almost infinitely complex series of climatic changes of long and short duration might arise if the sun passed through a nebula.

5. We have seen in Chapter VIII that the contrast between the somewhat severe climate of the present and the generally mild climate of the past is one of the great geological problems. The glacial period is not a thing of the distant past. Geologists generally recognize that it is still with us. Greenland and Antarctica are both shrouded in ice sheets in latitudes where fossil floras prove that at other periods the climate was as mild as in England or even New Zealand. The present glaciated regions, be it noted, are on the polar borders of the world's two most stormy oceanic areas, just where ice would be expected to last longest according to the solar cyclonic hypothesis. In contrast with the semi-glacialconditions of the present, the last inter-glacial epoch was so mild that not only men but elephants and hippopotamuses flourished in central Europe, while at earlier times in the middle of long eras, such as the Paleozoic and Mesozoic, corals, cycads, and tree ferns flourished within the Arctic circle.

If the electro-stellar hypothesis of solar disturbances proves well founded, it may explain these peculiarities. Periods of mild climate would represent a return of the sun and the earth to their normal conditions of quiet. At such times the atmosphere of the sun is assumed to be little disturbed by sunspots, faculæ, prominences, and other allied evidences of movements; and the rice-grain structure is perhaps the most prominent of the solar markings. The earth at such times is supposed to be correspondingly free from cyclonic storms. Its winds are then largely of the purely planetary type, such as trade winds and westerlies. Its rainfall also is largely planetary rather than cyclonic. It falls in places such as the heat equator where the air rises under the influence of heat, or on the windward slopes of mountains, or in regions where warm winds blow from the ocean over cold lands.

According to the electro-stellar hypothesis, the conditions which prevailed during hundreds of millions of years of mild climate mean merely that the solar system was then in parts of the heavens where stars—especially double stars—were rare or small, and electrical disturbances correspondingly weak. Today, on the other hand, the sun is fairly near a number of stars, many of which are large doubles. Hence it is supposed to be disturbed, although not so much as at the height of the last glacial epoch.

After the preceding parts of this book had beenwritten, the assistance of Dr. Schlesinger made it possible to test the electro-stellar hypothesis by comparing actual astronomical dates with the dates of climatic or solar phenomena. In order to make this possible, Dr. Schlesinger and his assistants have prepared Table 6, giving the position, magnitude, and motions of the thirty-eight nearest stars, and especially the date at which each was nearest the sun. In column 10 where the dates are given, a minus sign indicates the past and a plus sign the future. Dr. Shapley has kindly added column 12, giving the absolute magnitudes of the stars, that of the sun being 4.8, and column 13, showing their luminosity or absolute radiation, that of the sun being unity. Finally, column 14 shows the effective radiation received by the sun from each star when the star is at a minimum distance. Unity in this case is the effect of a star like the sun at a distance of one light year.

It is well known that radiation of all kinds, including light, heat, and electrical emissions, varies in direct proportion to the exposed surface, that is, as the square of the radius of a sphere, and inversely as the square of the distance. From black bodies, as we have seen, the total radiation varies as the fourth power of the absolute temperature. It is not certain that either light or electrical emissions from incandescent bodies vary in quite this same proportion, nor is it yet certain whether luminous and electrical emissions vary exactly together. Nevertheless they are closely related. Since the light coming from each star is accurately measured, while no information is available as to electrical emissions, we have followed Dr. Shapley's suggestion and used the luminosity of the stars as the best available measure of total radiation. This is presumably an approximate measure of electrical activity, provided some allowance be made for disturbances by outside bodies such as companion stars. Hence the inclusion of column 14.

On the basis of column 14 and of the movements and distances of the stars as given in the other columns Fig. 10 has been prepared. This gives an estimate of the approximate electrical energy received by the sun from the nearest stars for 70,000 years before and after the present. It is based on the twenty-six stars for which complete data are available in Table 6. The inclusion of the other twelve would not alter the form of the curve, for even the largest of them would not change any part by more than about half of 1 per cent, if as much. Nor would the curve be visibly altered by the omission of all except four of the twenty-six stars actually used. The four that are important, and their relative luminosity when nearest the sun, are Sirius 429,000, Altair 153,000, Alpha Centauri 117,500, and Procyon 51,300. The figure for the next star is only 4970, while for this star combined with the other twenty-one that are unimportant it is only 24,850.

Figure 10 is not carried more than 70,000 years into the past or into the future because the stars near the sun at more remote times are not included among the thirty-eight having the largest known parallaxes. That is, they have either moved away or are not yet near enough to be included. Indeed, as Dr. Schlesinger strongly emphasizes, there may be swiftly moving, bright or gigantic stars which are now quite far away, but whose inclusion would alter Fig. 10 even within the limits of the 140,000 years there shown. It is almost certain, however, that the most that these would do would be to raise, but not obliterate, the minima on either side of the main maximum.

In preparing Fig. 10 it has been necessary to makeallowance for double stars. Passing by the twenty-two unimportant stars, it appears that the companion of Sirius is eight or ten magnitudes smaller than that star, while the companions of Procyon and Altair are five or more magnitudes smaller than their bright comrades. This means that the luminosity of the faint components is at most only 1 per cent of that of their bright companions and in the case of Sirius not a hundredth of 1 per cent. Hence their inclusion would have no visible effect on Fig. 10. In Alpha Centauri, on the other hand, the two components are of almost the same magnitude. For this reason the effective radiation of that star as given in column 14 is doubled in Fig. 10, while for another reason it is raised still more. The other reason is that if our inferences as to the electrical effect of the sun on the earth and of the planets on the sun are correct, double stars, as we have seen, must be much more effective electrically than single stars. By the same reasoning two bright stars close together must excite one another much more than a bright star and a very faint one, even if the distances in both cases are the same. So, too, other things being equal, a triple star must be more excited electrically than a double star. Hence in preparing Fig. 10 all double stars receive double weight and each part of Alpha Centauri receives an additional 50 per cent because both parts are bright and because they have a third companion to help in exciting them.

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