CHAPTER V

“The lunar halo which by many persons is regarded as a remarkable and unexplained luminosity associated with the moon, is to meteorological students neither a mysterious nor an anomalous occurrence. It has been frequently observed and for many years thoroughly understood, and at the present time admits of an easy scientific explanation. It is an atmospheric exhibition due to the refraction and dispersion of the moon’s light through very minute ice crystals floating at great elevations above the earth, and it is explained by the science of meteorology, to which it properly belongs; for it is not of cosmical origin, and in no way pertains to astronomy, as most persons suppose, except as it depends on the moon, whose light passing through the atmosphere, produces the luminous halo, which as will be seen, is simply an optical illusion, originating, not in the vicinity of the moon—two hundred and forty thousand miles away—but just above the earth’s surface, and within the aqueous envelope that surrounds it on all sides.... A halo may form round the sun as well as the moon ... but a halo is more frequently noticed round the moon for the reason that we are too much dazzled by the sun’s light to distinguish faint colours surrounding its disc, and to see them it is necessary to look through smoked glass, or view the sun by reflection from the surface of still water, by which its brilliancy is very much reduced.”...

“A ‘corona’ is an appearance of faintly coloured rings often seen around the sun and moon whena light fleecy cloud passes over them, and should not be mistaken for a halo, which is much larger and more complicated in its structure. These two phenomena are frequently confounded by inexperienced observers.” With these remarks the present writer fully concurs.

Mr. Bartlett adds—

“As a halo is never seen except when the sky is hazy, it indicates that moisture is accumulating in the atmosphere which will form clouds, and usually result in a storm. But the popular notion that the number of bright stars visible within the circle indicates the number of days before the storm will occur, is without any foundation whatever, and the belief is almost too absurd to be refuted. In whatever part of the sky a lunar halo is seen, one or more bright stars are always sure to be noticed inside the luminous ring, and the number visible depends entirely upon the position of the moon. Moreover, when the sky within the circle is examined with even a small telescope, hundreds of stars are visible where only one, or perhaps two or three, are perceived with the naked eye.”

It is possible to have five Sundays in February (the year must of course be a “leap year”). This occurred in the year 1880, Sunday falling on February 1, 8, 15, 22, and 29. But this will not happen again till the year 1920. No century year (such as 1900, 2000, etc.) could possibly have five Sundays in February, and the Rev. Richard Campbell, who investigated this matter, findsthe following sequence of years in which five Sundays occur in February: 1604, 1632, 1660, 1688, 1728, 1756, 1784, 1824, 1852, 1880, 1920, 1948, 1976.[61]

In an article on “The Last Day and Year of the Century: Remarks on Time Reckoning,” inNature, September 10, 1896, Mr. W. T. Lynn, the eminent astronomer, says, “The late Astronomer Royal, Sir George Airy, once received a letter requesting him to settle a dispute which had arisen in some local debating society, as to which would be the first day of the next century. His reply was, ‘A very little consideration will suffice to show that the first day of the twentieth century will be January 1, 1901.’ Simple as the matter seems, the fact that it is occasionally brought into question shows that there is some little difficulty connected with it. Probably, however, this is in a great measure due to the circumstance that the actual figures are changed on January 1, 1900, the day preceding being December 31, 1899. A century is a very definite word for an interval respecting which there is no possible room for mistake or difference of opinion. But the date of its ending depends upon that of its beginning. Our double system of backward and forward reckoning leads to a good deal of inconvenience. Our reckoning supposes (what we know was not the case, but as an era the datedoes equally well) that Christ was born at the end ofB.C.1. At the end ofA.D.1, therefore, one year had elapsed from the event, at the end ofA.D.100, one century, and at the end of 1900, nineteen centuries.... It is clear, then, that the year, as we call it, is an ordinal number, and that 1900 years from the birth of Christ (reckoning as we do fromB.C.1) will not be completed until the end of December 31 in that year, the twentieth century beginning with January 1, 1901, that is (to be exact) at the previous midnight, when the day commences by civil reckoning.” With these remarks of Mr. Lynn I fully concur, and, so far as I know, all astronomers agree with him. As the discussion will probably again arise at the end of the twentieth century, I would like to put on record here what the scientific opinion was at the close of the nineteenth century.

Prof. E. Rutherford, the well-known authority on radium, suggests that possibly radium is a source of heat from within the earth. Traces of radium have been detected in many rocks and soils, and even in sea water. Calculation shows that the total amount distributed through the earth’s crust is enormously large, although relatively small “compared with the annual output of coal for the world.” The amount of radium necessary to compensate for the present loss of heat from the earth “corresponds to only five parts in one hundred millionmillions per unit mass,” and the “observations of Elster and Gertel show that the radio-activity observed in soils corresponds to the presence of about this proportion of radium.”[62]

The earth has 12 different motions. These are as follows:—

1. Rotation on its axis, having a period of 24 hours.

2. Revolution round the sun; period 365¼ days.

3. Precession; period of about 25,765 years.

4. Semi-lunar gravitation; period 28 days.

5. Nutation; period 18½ years.

6. Variation in obliquity of the ecliptic; about 47″ in 100 years.

7. Variation of eccentricity of orbit.

8. Change of line of apsides; period about 21,000 years.

9. Planetary perturbations.

10. Change of centre of gravity of whole solar system.

11. General motion of solar system in space.

12. Variation of latitude with several degrees of periodicity.[63]

“An amusing story has been told which affords a good illustration of the ignorance and popular notions regarding the tides prevailing even among persons of average intelligence. ‘Tell me,’ said a man to an eminent living Englishastronomer not long ago, ‘is it still considered probable that the tides are caused by the moon?’ The man of science replied that to the best of his belief it was, and then asked in turn whether the inquirer had any serious reason for questioning the relationship. ‘Well, I don’t know,’ was the answer; ‘sometimes when there is no moon there seems to be a tide all the same.’”![64]

With reference to the force of gravitation, on the earth and other bodies in the universe, Mr. William B. Taylor has well said, “With each revolving year new demonstrations of its absolute precision and of its universal domination serves only to fill the mind with added wonder and with added confidence in the stability and the supremacy of the power in which has been found no variableness neither shadow of turning, but which—the same yesterday, to-day and for ever—

“Lives through all life, extends through all extent,Spreads undivided, operates unspent.”[65]

With reference to the habitability of other planets, Tennyson has beautifully said—

“Venus near her! smiling downwards at this earthlier earth of ours,Closer on the sun, perhaps a world of never fading flowers.Hesper, whom the poets call’d the Bringer home of all good things;All good things may move in Hesper; perfect people, perfect kings.Hesper—Venus—were we native to that splendour, or in Mars,We should see the globe we groan in fairest of their evening stars.Could we dream of war and carnage, craft and madness, lust and spite,Roaring London, raving Paris, in that spot of peaceful light?Might we not in glancing heavenward on a star so silver fair,Yearn and clasp the hands, and murmur, ‘Would to God that we were there!’”

The ancient Greek writer, Diogenes Laertius, states that Anaximander (610-547B.C.) believed that the earth was a sphere. The Greek words are: μίσην τε τὴν γήν κεῖσθαι, κέντρυ τάξιν ἐπεχοῦσαν οὐσαν σφαιροειδῆ.[66]

With reference to the Aurora Borealis, the exact nature of which is not accurately known, “a good story used to be told some years ago of a candidate who, undergoing the torture of avivâ voceexamination, was unable to reply satisfactorily to any of the questions asked. ‘Come, sir,’ said the examiner, with the air of a man asking the simplest question, ‘explain to me the cause of the aurora borealis.’ ‘Sir,’ said the unhappy aspirant for physical honours, ‘I could have explained it perfectly yesterday, but nervousness has, I think, made me lose my memory.’ ‘This is very unfortunate,’ said the examiner; ‘you are the only man who could have explained this mystery, and you have forgotten it.’”[67]This was written in the year 1899, and probably thephenomenon of the aurora remains nearly as great a mystery to-day. In 1839, MM. Bravais and Lottin made observations on the aurora in Norway in about N. latitude 70°. Bravais found the height to be between 62 and 93 miles above the earth’s surface.

The cause of the so-called Glacial Epoch in the earth’s history has been much discussed. The Russian physicist, Rogovsky, has advanced the following theory—

“If we suppose that the temperature of the sun at the present time is still increasing, or at least has been increasing until now, the glacial epoch can be easily accounted for. Formerly the earth had a high temperature of its own, but received a lesser quantity of heat from the sun than now; on cooling gradually, the earth’s surface attained such a temperature as caused a great part of the surface of the northern and southern hemispheres to be covered with ice; but the sun’s radiation increasing, the glaciers melted, and the climatic conditions became as they are now. In a word, the temperature of the earth’s surface is a function of two quantities: one decreasing (the earth’s own heat), and the other increasing (the sun’s radiation), and consequently there may be a minimum, and this minimum was the glacial epoch, which, as shown by recent investigations, those of Luigi de Marchi (Report ofG. Schiaparelli, Meteorolog. Zeitschr., 30, 130-136, 1895), are not local, but general for the whole earth” (see also M. Neumahr,Erdegeschicht).[68]

Prof. Percival Lowell thinks that the life of geological palæozoic times was supported by the earth’s internal heat, which maintained the ocean at a comparatively warm temperature.[69]

The following passage in the Book of the Maccabees may possibly refer to an aurora—

“Now about this time Antiochus made his second inroad into Egypt. And itsobefell that throughout all the city, for the space of almost forty days, there appeared in the midst of the sky horsemen in swift motion, wearing robes inwrought with gold andcarryingspears, equipped in troops for battle; and drawing of swords; andon the other sidesquadrons of horse in array; and encounters and pursuits of both armies; and shaking of shields, and multitudes of lances, and casting of darts, and flashing of golden trappings, and girding on of all sorts of armour. Wherefore all men besought that the vision might have been given for food.”[70]

According to Laplace “the decrease of the mean heat of the earth during a period of 2000 years has not, taking the extremist limits, diminished as much as1⁄300th of a degree Fahrenheit.”[71]

From his researches on the cause of the Precession of the Equinoxes, Laplace concluded that “the motion of the earth’s axis is the same as if thewhole sea formed a solid mass adhering to its surface.”[72]

Laplace found that the major (or longer) axis of the earth’s orbit coincided with the line of Equinoxes in the year 4107B.C.The earth’s perigee then coincided with the autumnal equinox. The epoch at which the major axis was perpendicular to the line of equinoxes fell in the year 1250A.D.[73]

Leverrier has found the minimum eccentricity of the earth’s orbit round the sun to be 0·0047; so that the orbit will never become absolutely circular, as some have imagined.

Laplace says—

“Astronomy considered in its entirety is the finest monument of the human mind, the noblest essay of its intelligence. Seduced by the illusions of the senses and of self-pride, for a long time man considered himself as the centre of the movement of the stars; his vain-glory has been punished by the terrors which his own ideas have inspired. At last the efforts of several centuries brushed aside the veil which concealed the system of the world. We discover ourselves upon a planet, itself almost imperceptible in the vast extent of the solar system, which in its turn is only an insensible point in the immensity of space. The sublime results to which this discovery has led should suffice to console us for our extreme littleness, and the rank which it assigns to the earth. Let us treasure withsolicitude, let us add to as we may, this store of higher knowledge, the most exquisite treasure of thinking beings.”[74]

With reference to probable future changes in climate, the great physicist, Arrhenius, says—

“We often hear lamentation that the coal stored up in the earth is wasted by the present generation without any thought of the future, and we are terrified by the awful destruction of life and property which has followed the volcanic eruptions of our days. We may find a kind of consolation in the consideration that here, as in every other case, there is good mixed with evil. By the influence of the increasing percentage of carbonic acid in the atmosphere, we may hope to enjoy ages with more equable and better climates, especially as regards the colder regions of the earth, ages when the earth will bring forth much more abundant crops than at present, for the benefit of rapidly propagating mankind.”[75]

The night of July 1, 1908, was unusually bright. This was noticed in various parts of England and Ireland, and by the present writer in Dublin. Humboldt states that “at the time of the new moon at midnight in 1743, the phosphorescence was so intense that objects could be distinctly recognized at a distance of more than 600 feet.”[76]

An interesting proof of the earth’s rotation on its axis has recently been found.

“In a paper in theProceedingsof the Vienna Academy (June, 1908) by Herr Tumlirz, it is shown mathematically that if a liquid is flowing outwards between two horizontal discs, the lines of flow will be strictly straight only if the discs and vessel be at rest, and will assume certain curves if that vessel and the discs are in rotation, as, for example, due to the earth’s rotation. An experimental arrangement was set up with all precautions, and the stream lines were marked with coloured liquids and photographed. These were in general accord with the predictions of theory and the supposition that the earth is rotating about an axis.”[77]

In a book published in 1905 entitledThe Rational Almanac, by Moses B. Cotsworth, of York, the author states that (p. 397), “The explanation is apparent from the Great Pyramid’s Slope, which conclusively proves that when it was built the latitude of that region was 7°·1 more than at present. Egyptian Memphis now near Cairo was then in latitude 37°·1, where Asia Minor now ranges, whilst Syria would then be where the Caucasus regions now experience those rigorous winters formerly experienced in Syria.” But the reality of this comparatively great change of latitude in the position of the Great Pyramid can be easily disproved. Pytheas of Marseilles—who lived in the time of Alexander the Great, about 330B.C.—measured the latitude of Marseilles by means of a gnomon, and found it to be about 42° 56′½. As the present latitude of Marseilles is43° 17′ 50″, no great change in the latitude could have taken place in over 2000 years.[78]From this we may conclude that the latitude of the Great Pyramid hasnotchanged by 7°·1 since its construction. There is, it is true, a slow diminution going on in the obliquity of the ecliptic (or inclination of the earth’s axis), but modern observations show that this would not amount to as much as one degree in 6000 years. Eudemus of Rhodes—a disciple of Aristotle (who died in 322B.C.)—found the obliquity of the ecliptic to be 24°, which differs but little from its present value, 23° 27′. Al-Sufi in the tenth century measured the latitude of Schiraz in Persia, and found it 29° 36′. Its present latitude is 29° 36′ 30″,[79]so that evidently there has been no change in the latitude in 900 years.

The Moon

Thetotal area of the moon’s surface is about equal to that of North and South America. The actual surface visible at any one time is about equal to North America.

The famous lunar observer, Schröter, thought that the moon had an atmosphere, but estimated its height at only a little over a mile. Its density he supposed to be less than that of the vacuum in an air-pump. Recent investigations, however, seem to show that owing to its small mass and attractive force the moon could not retain an atmosphere like that of the earth.

Prof. N. S. Shaler, of Harvard (U.S.A.), finds from a study of the moon (from a geological point of view) with the 15-inch refractor of the Harvard Observatory, that our satellite has no atmosphere nor any form of organic life, and he believes that its surface “was brought to its present condition before the earth had even a solid crust.”[80]

There is a curious illusion with reference to themoon’s apparent diameter referred to by Proctor.[81]If, when the moon is absent in the winter months, we ask a person whether the moon’s diameter is greater or less than the distance between the stars δ and ε, and ε and ζ Orionis, the three well-known stars in the “belt of Orion,” the answer will probably be that the moon’s apparent diameter is about equal to each of these distances. But in reality the apparent distance between δ and ε Orionis (or between ε and ζ, which is about the same) is more than double the moon’s apparent diameter. This seems at first sight a startling statement; but its truth is, of course, beyond all doubt and is not open to argument. Proctor points out that if a person estimates the moon as a foot in diameter, as its apparent diameter is about half a degree, this would imply that the observer estimates the circumference of the star sphere as about 720 feet (360° × 2), and hence the radius (or the moon’s distance from the earth) about 115 feet. But in reality all such estimates have no scientific (that is, accurate) meaning. Some of the ancients, such as Aristotle, Cicero, and Heraclitus, seem to have estimated the moon’s apparent diameter at about a foot.[82]This shows that even great minds may make serious mistakes.

It has been stated by some writer that the moon as seen with the highest powers of the greatYerkes telescope (40 inches aperture) appears “just as it would be seen with the naked eye if it were suspended 60 miles over our heads.” But this statement is quite erroneous. The moon as seen with the naked eye or with a telescope shows us nearly a whole hemisphere of its surface. But if the eye were placed only 60 miles from the moon’s surface, we should see only a small portion of its surface. In fact, it is a curious paradox that the nearer the eye is to a sphere the less we see of its surface! The truth of this will be evident from the fact that on a level plain an eye placed at a height, say 5 feet, sees a very small portion indeed of the earth’s surface, and the higher we ascend the more of the surface we see. I find that at a distance of 60 miles from the moon’s surface we should only see a small portion of its visible hemisphere (about1⁄90th). The lunar features would also appear under a different aspect. The view would be more of a landscape than that seen in any telescope. This view of the matter is not new. It has been previously pointed out, especially by M. Flammarion and Mr. Whitmell, but its truth is not, I think, generally recognized. Prof. Newcomb doubts whether with any telescope the moon has ever been seen so well as it would be if brought within 500 miles of the earth.

A relief map of the moon 19 feet in diameter was added, in 1898, to the Field ColumbianMuseum (U.S.A.). It was prepared with great care from the lunar charts of Beer and Mädler, and Dr. Schmidt of the Athens Observatory, and it shows the lunar features very accurately. Its construction took five years.

On a photograph of a part of the moon’s surface near the crater Eratosthenes, Prof. William H. Pickering finds markings which very much resemble the so-called “canals” of Mars. The photograph was taken in Jamaica, and a copy of it is given in Prof. Pickering’s book on the Moon, and inPopular Astronomy, February, 1904.

Experiments made in America by Messrs. Stebbins and F. C. Brown, by means of selenium cells, show that the light of the full moon is about nine times that of the half moon;[83]and that “the moon is brighter between the first quarter and full than in the corresponding phase after full moon.” They also find that the light of the full moon is equal to “0·23 candle power,”[83]that is, according to the method of measurement used in America, its light is equal to 0·23 of a standard candle placed at a distance of one metre (39·37 inches) from the eye.[84]

Mr. H. H. Kimball finds that no less than 52 per cent. of the observed changes in intensity of the “earth-shine” visible on the moon when at or near the crescent phase is due to the eccentricityof the lunar orbit, and “this is probably much greater than could be expected from any increase or diminution in the average cloudiness over the hemisphere of the earth reflecting light to the moon.”[85]

The “moon maiden” is a term applied to a fancied resemblance of a portion of the Sinus Iridum to a female head. It forms the “promontory” known as Cape Heraclides, and may be looked for when the moon’s “age” is about 11 days. Mr. C. J. Caswell, who observed it on September 29, 1895, describes it as resembling “a beautiful silver statuette of a graceful female figure with flowing hair.”

M. Landerer finds that the angle of polarization of the moon’s surface—about 33°—agrees well with the polarizing angle for many specimens of igneous rocks (30° 51′ to 33° 46′). The polarizing angle for ice is more than 37°, and this fact is opposed to the theories of lunar glaciation advanced by some observers.[86]

Kepler states in hisSomniumthat he saw the moon in the crescent phase on the morning and evening of thesameday (that is, before and after conjunction with the sun). Kepler could see 14 stars in the Pleiades with the naked eye, so his eyesight must have been exceptionally keen.

Investigations on ancient eclipses of the moon show that the eclipse mentioned by Josephus ashaving occurred before the death of Herod is probably that which took place on September 15,B.C.5. This occurred about 9.45 p.m.; and probably about six months before the death of Herod (St. Matthew ii. 15).

The total lunar eclipse which occurred on October 4, 1884, was remarkable for the almost total disappearance of the moon during totality. One observer says that “in the open air, if one had not known exactly where to look for it, one might have searched for some time without discovering it. I speak of course of the naked eye appearance.”[87]On the other hand the same observer, speaking of the total eclipse of the moon on August 23, 1877, which was a bright one, says—

“The moon even in the middle of the total phase was a conspicuous object in the sky, and the ruddy colour was well marked. In the very middle of the eclipse the degree of illumination was as nearly as possible equal all round the edge of the moon, the central parts being darker than those near the edge.”

In Roger de Hovedin’sChronicle(A.D.756) an account is given of the occultation of “a bright star,” by the moon during a total eclipse. This is confirmed by Simeon of Durham, who also dates the eclipseA.D.756. This is, however, a mistake, the eclipse having occurred on the evening of November 23,A.D.755. Calvisius supposed thatthe occulted “star” might have been Aldebaran. Pingré, however, showed that this was impossible, and Struyck, in 1740, showed that the planet Jupiter was the “star” referred to by the early observer. Further calculations by Hind (1885) show conclusively that Struyck was quite correct, and that the phenomenon described in the old chronicles was the occultation of Jupiter by a totally eclipsed moon—a rather unique phenomenon.[88]

An occultation of Mars by the moon is recorded by the Chinese, on February 14,B.C.69, and one of Venus, on March 30,A.D.361. These have also been verified by Hind, and his calculations show the accuracy of these old Chinese records.

It has been suggested that the moon may possibly have a satellite revolving round it, as the moon itself revolves round the earth. This would, of course, form an object of great interest. During the total lunar eclipses of March 10 and September 3, 1895, a careful photographic search was made by Prof. Barnard for a possible lunar satellite. The eclipse of March 10 was not very suitable for the purpose owing to a hazy sky, but that of September 3 was “entirely satisfactory,” as the sky was very clear, and the duration of totality was very long. On the latter occasion “six splendid” photographs were obtained of the total phase with a 6-inch Willard lens. The resultwas that none of these photographs “show anything which might be taken for a lunar satellite,” at least any satellite as bright as the 10th or 12th magnitude. It is, of course, just possible that the supposed satellite might have been behind the moon during the totality.

With reference to the attraction between the earth and moon, Sir Oliver Lodge says—

“The force with which the moon is held in its orbit would be great enough to tear asunder a steel rod 400 miles thick, with a tenacity of 30 tons to the square inch, so that if the moon and earth were connected by steel instead of gravity, a forest of pillars would be necessary to whirl the system once a month round their common centre of gravity. Such a force necessarily implies enormous tensure or pressure in the medium. Maxwell calculates that the gravitational stress near the earth, which we must suppose to exist in the invisible medium, is 3000 times greater than what the strongest steel can stand, and near the sun it should be 2500 times as great as that.”[89]

With reference to the names given to “craters” on the moon, Prof. W. H. Pickering says,[90]“The system of nomenclature is, I think, unfortunate. The names of the chief craters are generally those of men who have done little or nothing for selenography, or even for astronomy, while the men who should be really commemorated arerepresented in general by small and unimportant craters,” and again—

“A serious objection to the whole system of nomenclature lies in the fact that it has apparently been used by some selenographers, from the earliest times up to the present, as a means of satisfying their spite against some of their contemporaries. Under the guise of pretending to honour them by placing their names in perpetuity upon the moon, they have used their names merely to designate the smallest objects that their telescopes were capable of showing. An interesting illustration of this point is found in the craters of Galileo and Riccioli, which lie close together on the moon. It will be remembered that Galileo was the discoverer of the craters on the moon. Both names were given by Riccioli, and the relative size and importance of the craters [Riccioli large, and Galileo very small] probably indicates to us the relative importance that he assigned to the two men themselves. Other examples might be quoted of craters named in the same spirit after men still living.... With the exception of Maedler, one might almost say, the more prominent the selenographer the more insignificant the crater.”

The mathematical treatment of the lunar theory is a problem of great difficulty. The famous mathematician, Euler, described it asincredibile stadium atque indefessus labor.[91]

With reference to the “earth-shine” on the moon when in the crescent phase, Humboldt says, “Lambert made the remarkable observation(14th of February, 1774) of a change of the ash-coloured moonlight into an olive-green colour, bordering upon yellow. The moon, which then stood vertically over the Atlantic Ocean, received upon its night side the green terrestrial light, which is reflected towards her when the sky is clear by the forest districts of South America.”[92]Arago said, “Il n’est donc pas impossible, malgré tout ce qu’un pareil résultat exciterait de surprise au premier coup d’œil qu’un jour les météorologistes aillent puiser dans l’aspect de la Lune des notions précieuses surl’etat moyende diaphanité de l’atmosphère terrestre, dans les hemisphères qui successivement concurrent à la production de la lumière cendrée.”[93]

The “earth-shine” on the new moon was successfully photographed in February, 1895, by Prof. Barnard at the Lick Observatory, with a 6-inch Willard portrait lens. He says—

“The earth-lit globe stands out beautifully round, encircled by the slender crescent. All the ‘seas’ are conspicuously visible, as are also the other prominent features, especially the region aboutTycho.AristarchusandCopernicusappear as bright specks, and the light streams fromTychoare very distinct.”[94]

Kepler found that the moon completely disappeared during the total eclipse of December 9,1601, and Hevelius observed the same phenomenon during the eclipse of April 25, 1642, when “not a vestige of the moon could be seen.”[95]In the total lunar eclipse of June 10, 1816, the moon during totality was not visible in London, even with a telescope![95]

The lunar mountains arerelativelymuch higher than those on the earth. Beer and Mädler found the following heights: Dörfel, 23,174 feet; Newton, 22,141; Casatus, 21,102; Curtius, 20,632; Callippus, 18,946; and Tycho, 18,748 feet.[96]

Taking the earth’s diameter at 7912 miles, the moon’s diameter, 2163 miles, and the height of Mount Everest as 29,000 feet, I find that

From which it follows that the lunar mountains areproportionatelyabout three times higher than those on the earth.

According to an hypothesis recently advanced by Dr. See, all the satellites of the solar system, including our moon, were “captured” by their primaries. He thinks, therefore, that the “moon came to earth from heavenly space.”[97]

Mars

Marswas called by the ancients “the vanishing star,” owing to the long periods during which it is practically invisible from the earth.[98]It was also called πυρόεις and Hercules.

I have seen it stated in a book on the “Solar System” by a well-known astronomer that theaxisof Mars “is inclined to the plane of the orbit” at an angle of 24° 50′! But this is quite erroneous. The angle given is the angle betweenthe plane of the planet’s equatorand the plane of its orbit, which is quite a different thing. This angle, which may be called the obliquity of Mars’ ecliptic, does not differ much from that of the earth. Lowell finds it 23° 13′ from observations in 1907.[99]

The late Mr. Proctor thought that Mars is “far the reddest star in the heavens; Aldebaran and Antares are pale beside him.”[100]But this does notagree with my experience. Antares is to my eye quite as red as Mars. Its name is derived from two Greek words implying “redder than Mars.” The colour of Aldebaran is, I think, quite comparable with that of the “ruddy planet.” In the telescope the colour of Mars is, I believe, more yellow than red, but I have not seen the planet very often in a telescope. Sir John Herschel suggested that the reddish colour of Mars may possibly be due to red rocks, like those of the Old Red Sandstone, and the red soil often associated with such rocks, as I have myself noticed near Torquay and other places in Devonshire.

The ruddy colour of Mars was formerly thought to be due to the great density of its atmosphere. But modern observations seem to show that the planet’s atmosphere is, on the contrary, much rarer than that of the earth. The persistent visibility of the markings on its surface shows that its atmosphere cannot be cloud-laden like ours; and the spectroscope shows that the water vapour present is—although perceptible—less than that of our terrestrial envelope.

The existence of water vapour is clearly shown by photographs of the planet’s spectrum taken by Mr. Slipher at the Lowell Observatory in 1908. These show that the water vapour bandsaand near D are stronger in the spectrum of Mars than in that of the moon at the same altitude.[101]

The dark markings on Mars were formerly supposed to represent water and the light parts land. But this idea has now been abandoned. Light reflected from a water surface is polarized at certain angles. Prof. W. H. Pickering, in his observations on Mars, finds no trace of polarization in the light reflected from the dark parts of the planet. But under the same conditions he finds that the bluish-black ring surrounding the white polar cap shows a well-marked polarization of light, thus indicating that this dark ring is probably water.[102]

Projections on the limb of the planet have frequently been observed in America. These are knownnotto be mountains, as they do not reappear under similar conditions. They are supposed to be clouds, and one seen in December, 1900, has been explained as a cloud lying at a height of some 13 miles above the planet’s surface and drifting at the rate of about 27 miles an hour. If there are any mountains on Mars they have not yet been discovered.

The existence of the so-called “canals” of Mars is supposed to be confirmed by Lowell’s photographs of the planet. But what these “canals” really represent, that is the question. They have certainly an artificial look about them, and they form one of the most curious and interesting problems in the heavens. Prof. Lowell says—

“Most suggestive of all Martian phenomena are the canals. Were they more generally observable the world would have been spared much scepticism and more theory. They may of course not be artificial, but observations here [Flagstaff] indicate that they are; as will, I think, appear from the drawings. For it is one thing to see two or three canals and quite another to have the planet’s disc mapped with them on a most elaborate system of triangulation. In the first place they are this season (August, 1894) bluish-green, of the same colour as the seas into which the longer ones all eventually debouch. In the next place they are almost without exception geodetically straight, supernaturally so, and this in spite of their leading in every possible direction. Then they are of apparently nearly uniform width throughout their length. What they are is another matter. Their mere aspect, however, is enough to cause all theories about glaciation fissures or surface cracks to die an instant and natural death.”[103]

Some of the observed colour-changes on Mars are very curious. In April, 1905, Mr. Lowell observed that the marking known as Mare Erythræum, just above Syrtis, had “changed from a blue-green to a chocolate-brown colour.” The season on Mars corresponded with our February.

Signor V. Cerulli says that, having observed Mars regularly for ten years, he has come to the conclusion that the actual existence of the “canals” is as much a subject for physiologicalas for astronomical investigation. He states that “the phenomena observed are so near the limit of the range of the human eye that in observing them one really experiences an effect accompanying the ‘birth of vision.’ That is to say, the eye sees more and more as it becomes accustomed, or strained, to the delicate markings, and thus the joining up of spots to form ‘canals’ and the gemination of the latter follow as a physiological effect, and need not necessarily be subjective phenomena seen by the unaccustomed eye.”[104]

The possibility of life on Mars has been recently much discussed; some denying, others asserting. M. E. Rogovsky says—

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