CHAPTER VII

“As free oxygen and carbonic dioxide may exist in the atmosphere ofMars, vegetable and animal life is quite possible. If the temperature which prevails uponMarsis nearer to -36° C. than to -73° C., the existence of living beings like ourselves is possible. In fact, the ice of some Greenland and Alpine glaciers is covered by red algæ (Sphærella nivalis); we find there also different species of rotaloria, variegated spiders, and other animals on the snow fields illuminated by the sun; at the edges of glacier snows in the Tyrol we see violet bells ofSoldanella pusilla, the stalks of which make their way through the snow by producing heat which melts it round about them. Finally the Siberian town Verkhociansk, near Yakutsk, exists, though the temperature there falls to -69°·8 C. and the mean temperature of January to -51°·2, and the mean pressure of the vapour of water isless than 0·05mm. It is possible, therefore, that living beings have become adapted to the conditions now prevailing uponMarsafter the lapse of many ages, and live at an even lower temperature than upon the earth, developing the necessary heat themselves.”

M. Rogovsky adds, “Water in organisms is mainly a liquid or solvent, and many other liquids may be the same. We have no reason to believe that life is possible only under the same conditions and with the same chemical composition of organisms as upon the earth, although indeed we cannot affirm that they actually exist on Mars.”[105]With the above views the present writer fully concurs.

Prof. Lowell thinks that the polar regions of Mars, both north and south, are actually warmer than the corresponding regions of the earth, although the mean temperature of the planet is probably twelve degrees lower than the earth’s mean temperature.[106]

A writer inAstronomy and Astrophysics(1892, p. 748) says—

“Whether the planet Mars is inhabited or not seems to be the all-absorbing question with the ordinary reader. With the astronomer this query is almost the last thing about the planet that he would think of when he has an opportunity to study its surface markings ... noastronomer claims to know whether the planet is inhabited or not.”

Several suggestions have been made with reference to the possibility of signalling to Mars. But, as Mr. Larkin of Mount Lowe (U.S.A.) points out, all writers on this subject seem to forget the fact that the night side of two planets are never turned towards each other. “When the sun is between them it is day on the side of Mars which is towards us, and also day on the side of the earth which is towards Mars. When they are on the same side of the sun, it is day on Mars when night on the earth, and for this reason they could never see our signals. This should make it apparent that the task of signalling to Mars is a more difficult one than the most hopeful theorist has probably considered. All this is under the supposition that the Martians (if there are such) are beings like ourselves. If they are not like us, we cannot guess what they are like.”[107]These views seem to me to be undoubtedly correct, and show the futility of visual signals. Electricity might, however, be conceivably used for the purpose; but even this seems highly improbable.

Prof. Newcomb, in his workAstronomy for Everybody, says with reference to this question, “The reader will excuse me from saying nothing in this chapter about the possible inhabitants ofMars. He knows just as much about the subject as I do, and that is nothing at all.”

It is, however, quite possible that lifein some formmay exist on Mars. As Lowell well says, “Life but waits in the wings of existence for its cue to enter the scene the moment the stage is set.”[108]With reference to the “canals” he says—

“It is certainly no exaggeration to say that they are the most astonishing objects to be viewed in the heavens. There are celestial sights more dazzling, spectacles that inspire more awe, but to the thoughtful observer who is privileged to see them well, there is nothing in the sky so profoundly impressive as these canals of Mars.”[109]

The eminent Swedish physicist Arrhenius thinks that the mean annual temperature on Mars may possibly be as high as 50° F. He says, “Sometimes the snow-caps on the poles of Mars disappear entirely during the Mars summer; this never happens on our terrestrial poles. The mean temperature of Mars must therefore be above zero, probably about +10° [Centigrade = 50° Fahrenheit]. Organic life may very probably thrive, therefore, on Mars.”[110]He thinks that this excess of mean temperature above the calculated temperature may be due to an increased amount of carbonic acid in the planet’s atmosphere, and says “any doubling of the percentage of carbondioxide in the air would raise the temperature of the earth’s surface by 4°; and if the carbon dioxide were increased fourfold, the temperature would rise by 8°.”[111]

Denning says,—[112]

“A few years ago, when christening celestial formations was more in fashion than it is now, a man simply had to use a telescope for an evening or two on Mars or the moon, and spice the relation of his seeings with something in the way of novelty, when his name would be pretty certainly attached to an object and hung in the heavens for all time! A writer in theAstronomical Registerfor January, 1879, humorously suggested that ‘the matter should be put into the hands of an advertising agent,’ and ‘made the means of raising a revenue for astronomical purposes.’ Some men would not object to pay handsomely for the distinction of having their names applied to the seas and continents of Mars or the craters of the moon.”

An occultation of Mars by the moon is recorded by Aristotle as having occurred on April 4, 357B.C.[113]

Seen from Mars the maximum apparent distance between the earth and moon would vary from 3½′ to nearly 17′.[114]

The Minor Planets

Upto 1908 the number of minor planets (or asteroids) certainly known amounted to over 650.

From an examination of the distribution of the first 512 of these small bodies, Dr. P. Stroobant finds that a decided maximum in number occurs between the limits of distance of 2·55 and 2·85 (earth’s mean distance from sun = 1), “199 of the asteroids considered revolving in this annulus.” He finds that nearly all the asteroidal matter is concentrated near to the middle of the ring in the neighbourhood of the mean distance of 2·7, and the smallest asteroids are relatively less numerous in the richest zones.[115]

There are some “striking similarities” in the orbits of some of the asteroids. Thus, in the small planets Sophia (No. 251 in order of discovery) and Magdalena (No. 318) we have the mean distance of Sophia 3·10, and that of Magdalena 3·19 (earth’s mean distance = 1).The eccentricities of the orbits are 0·09 and 0·07; and the inclinations of the orbits to the plane of the ecliptic 10° 29′ and 10° 33′ respectively.[116]This similarity may be—and probably is—merely accidental, but it is none the less curious and interesting.

Some very interesting discoveries have recently been made among the minor planets. The orbit of Eros intersects the orbit of Mars; and the following have nearly the same mean distance from the sun as Jupiter:—

Achilles (1906 TG), No. 588,Patrocles (1906 XY), No. 617,Hector (1907 XM), No. 624,

and another (No. 659) has been recently found. Each of these small planets “moves approximately in a vertex of an equilateral triangle that it forms with Jupiter and the sun.”[117]The minor planet known provisionally as HN is remarkable for the large eccentricity of its orbit (0·38), and its small perihelion distance (1·6). When discovered it had a very high South Declination (61½°), showing that the inclination of the plane of its orbit to the plane of the ecliptic is considerable.[118]

Dr. Bauschinger has made a study of the minor planets discovered up to the end of 1900.He finds that the ascending nodes of the orbits show a marked tendency to cluster near the ascending node of Jupiter’s orbit, a fact which agrees well with Prof. Newcomb’s theoretical results. There seems to be a slight tendency for large inclinations and great eccentricities to go together; but there appears to be no connection between the eccentricity and the mean distance from the sun. The longitudes of the perihelia of these small planets “show a well-marked maximum near the longitude ofJupiter’sperihelion, and equally well-marked minimum near the longitude of his aphelion,” which is again in good agreement with Newcomb’s calculations.[119]Dr. Bauschinger’s diameter for Eros is 20 miles. He finds that the whole group, including those remaining to be discovered, would probably form a sphere of about 830 miles in diameter.

The total mass of the minor planets has been frequently estimated, but generally much too high. Mr. B. M. Roszel of the John Hopkins University (U.S.A.) has made a calculation of the probable mass from the known diameter of Vesta (319 miles, Pickering), and finds the volume of the first 216 asteroids discovered. From this calculation it appears that it would take 310 asteroids of the 6th magnitude, or 1200 of the 7th to equal the moon in volume. Mr. Roszel concludes that the probable mass of the wholeasteroidal belt is between1⁄50th and1⁄100th of that of the moon.[120]Subsequently Mr. Roszel extended his study to the mass of 311 asteroids,[121]and found a combined mass of about1⁄40th of the moon’s mass.

Dr. Palisa finds that the recently discovered minor planet (1905 QY) varies in light to a considerable extent.[122]This planet was discovered by Dr. Max Wolf on August 23, 1905; but it was subsequently found that it is identical with one previously known, (167) Urda.[123]The light variation is said to be from the 11th to the 13th magnitude.[124]Variation in some of the other minor planets has also been suspected. Prof. Wendell found a variation of about half a magnitude in the planet Eunomia (No. 15). He also found that Iris (No. 7) varies about a quarter of a magnitude in a period of about 6h12m.[125]But these variations are small, and perhaps doubtful. The variability of Eros is well known.

The planet Eros is a very interesting one. The perihelion portion of its orbit lies between the orbits of Mars and the earth, and the aphelion part is outside the orbit of Mars. Owing to the great variation in its distance from the earth the brightness of Eros varies from the 6th to the 12th magnitude. That is, when brightest, it is 250times brighter than when it is faintest.[126]This variation of light, is of course, merely due to the variation of distance; but some actual variation in the brightness of the planet has been observed.

It has been shown by Oeltzen and Valz that Cacciatore’s supposed distant comet, mentioned by Admiral Smyth in hisBedford Catalogue, must have been a minor planet.[127]

Dr. Max Wolf discovered 36 new minor planets by photography in the years 1892-95. Up to the latter year he had never seen one of these through a telescope! His words are, “Ich selsbt habe noch nie einen meinen kleinen Planeten am Himmel gesehen.”[128]

These small bodies have now become so numerous that it is a matter of much difficulty to follow them. At the meeting of the Royal Astronomical Society on January 8, 1909, Mr. G. F. Chambers made the following facetious remarks—

“I would like to make a suggestion that has been in my mind for several years past—that it should be made an offence punishable by fine or imprisonment to discover any more minor planets. They seem to be an intolerable nuisance, and are a great burden upon the literary gentlemen who have to keep pace with them and record them. I have never seen, during the last fewyears at any rate, any good come from them, or likely to come, and I should like to see the supply stopped, and the energies of the German gentlemen who find so many turned into more promising channels.”

Among the minor planets numbered 1 to 500, about 40 “have not been seen since the year of their discovery, and must be regarded as lost.”[129]

Jupiter

Thisbrilliant planet—only inferior to Venus in brightness—was often seen by Bond (Jun.) with the naked eye in “high and clear sunshine”; also by Denning, who has very keen eyesight. Its brightness on such occasions is so great, that—like Venus—it casts a distinct shadow in a dark room.[130]

The great “red spot” on Jupiter seems to have been originally discovered by Robert Hooke on May 9, 1664, with a telescope of 2 inches aperture and 12 feet focus. It seems to have existed ever since; at least the evidence is, according to Denning, in favour of the identity of Hooke’s spot with the red spot visible in recent years. The spot was also observed by Cassini in the years 1665-72, and is sometimes called “Cassini’s spot.” But the real discoverer was Hooke.[131]

The orbit of Jupiter is so far outside the earth’sorbit that there can be little visible in the way of “phase”—as in the case of Mars, where the “gibbous” phase is sometimes very perceptible. Some books on astronomy state that Jupiter shows no phase. But this is incorrect. A distinct, although very slight, gibbous appearance is visible when the planet is near quadrature. Webb thought it more conspicuous in twilight than in a dark sky. With large telescopes, Jupiter’s satellites II. and III. have been seen—in consequence of Jupiter’s phase—to emerge from occultation “at a sensible distance from the limb.”[132]

According to M. E. Rogovsky, the high “albedo of Jupiter, the appearance of the clear (red) and dark spots on its surface and their continual variation, the different velocity of rotation of the equatorial and other zones of its surface, and particularly its small density (1·33, water as unity), all these facts afford irrefragable proofs of the high temperature of this planet. The dense and opaque atmosphere hides its glowing surface from our view, and we see therefore only the external surface of its clouds. The objective existence of this atmosphere is proved by the bands and lines of absorption in its spectrum. The interesting photograph obtained by Draper, September 27, 1879, in which the blue and green parts are more brilliant for the equatorial zone than for theadjacent parts of the surface, appears to show thatJupiteremits its proper light. It is possible that the constant red spot noticed on its surface by several observers, as Gledhill, Lord Rosse, and Copeland (1873), Russel and Bredikhin (1876), is the summit of a high glowing mountain. G. W. Hough considers Jupiter to be gaseous, and A. Ritter inferred from his formulæ that in this case the temperature at the centre would be 600,000° C.”[133]

The four brighter satellites of Jupiter are usually known by numbers I., II., III., and IV.; I. being the nearest to the planet, and IV. the farthest. III. is usually the brightest, and IV. the faintest, but exceptions to this rule have been noticed.

With reference to the recently discovered sixth and seventh satellites of Jupiter, Prof. Perrine has suggested that the large inclination of their orbits to the plane of the planet’s equator seems to indicate that neither of these bodies was originally a member of Jupiter’s family, but has been “captured by the planet.” This seems possible as the orbits of some of the minor planets lie near the orbit of Jupiter (see “Minor Planets”). A similar suggestion has been made by Prof. del Marmol.[134]

Many curious observations have been recordedwith reference to Jupiter’s satellites; some very difficult of explanation. In 1711 Bianchini saw satellite IV. so faint for more than an hour that it was hardly visible! A similar observation was made by Lassell with a more powerful telescope on June 13, 1849. Key, T. T. Smyth, and Denning have also recorded unusual faintness.[135]A very remarkable phenomenon was seen by Admiral Smyth, Maclear, and Pearson on June 26, 1828. Satellite II., “having fairly entered on Jupiter, was found 12 or 13 minutes afterwardsoutside the limb, where it remained visible for at least 4 minutes, and then suddenly vanished.” As Webb says, “Explanation is here set at defiance; demonstrably neither in the atmosphere of the earth, nor Jupiter, where and what could have been the cause? At present we can get no answer.”[136]When Jupiter is in opposition to the sun—that is, on the meridian at midnight—satellite I. has been seen projected on its own shadow, the shadow appearing as a dark ring round the satellite.

On January 28, 1848, at Cambridge (U.S.A.) satellite III. was seen in transit lying between the shadows of I. and II. and so black that it could not be distinguished from the shadows, “except by the place it occupied.” This seems to suggest inherent light in the planet’s surface, as the satellite was at the time illuminated by fullsunshine; its apparent blackness being due to the effect of contrast. Cassini on one occasion failed to find the shadow of satellite I. when it should have been on the planet’s disc,[137]an observation which again points to the glowing light of Jupiter’s surface. Sadler and Trouvelot saw the shadow of satellite I. double! an observation difficult to explain—but the same phenomenon was again seen on the evening of September 19, 1891, by Mr. H. S. Halbert of Detroit, Michigan (U.S.A.). He says that the satellite “was in transit nearing egress, and it appeared as a white disc against the dark southern equatorial belt; following it was the usual shadow, and at an equal distance from this was a second shadow, smaller and not so dark as the true one, and surrounded by a faint penumbra.”[138]

A dark transit of satellite III. was again seen on the evening of December 19, 1891, by two observers in America. One observer noted that the satellite, when on the disc of the planet, was intensely black. To the other observer (Willis L. Barnes) it appeared as an ill-defineddarkimage.[139]A similar observation was made on October 9 of the same year by Messrs. Gale and Innes.[140]

A “black transit” of satellite IV. was seen by several observers in 1873, and by Prof. Barnard on May 4, 1886. The same phenomenon was observed on October 30, 1903, in America, by Miss Anne S. Young and Willis S. Barnes. Miss Young says—

“The ingress of the satellite took place at 8h50m(E. standard time) when it became invisible upon the background of the planet. An hour later it was plainly visible as a dark round spot upon the planet. It was decidedly darker than the equatorial belt.”[141]

The rather rare phenomenon of an occultation of one of Jupiter’s satellites by another was observed by Mr. Apple, director of the Daniel Scholl Observatory, Franklin and Marshall College, Lancaster, Pa. (U.S.A.), on the evening of March 16, 1908. The satellites in question were I. and II., and they were so close that they could not be separated with the 11·5-inch telescope of the Observatory.[142]One of the present writer’s first observations with a telescope is dated May 17, 1873, and is as follows: “Observed one of Jupiter’s satellites occulted (or very nearly so) by another. Appeared as one with power 133” (on 3-inch refractor in the Punjab). These satellites were probably I. and II.

Jupiter has been seen on several occasions apparently without his satellites; some beingbehind the disc, some eclipsed in his shadow, and some in transit across the disc. This phenomenon was seen by Galileo, March 15, 1611; by Molyneux, on November 12, 1681; by Sir William Herschel, May 23, 1802; by Wallis, April 15, 1826; by Greisbach, September 27, 1843; and by several observers on four occasions in the years 1867-1895.[143]The phenomenon again occurred on October 3, 1907, No. 1 being eclipsed and occulted, No. 2 in transit, No. 3 eclipsed, and No. 4 occulted.[144]It was not, however, visible in Europe, but could have been seen in Asia and Oceania.[144]The phenomenon will occur again on October 22, 1913.[145]

On the night of September 19, 1903, a star of magnitude 6½ was occulted by the disc of Jupiter. This curious and rare phenomenon was photographed by M. Lucien Rudaux at the Observatory of Donville, France.[146]The star was Lalande 45698 (= BAC 8129).[147]

Prof. Barnard, using telescopes with apertures from 5 inches up to 36 inches (Lick), has failed to see a satellite through the planet’s limb (an observation which has been claimed by other astronomers). He says, “To my mind this hasbeen due to either poor seeing, a poor telescope, or an excited observer.”[148]He adds—

“I think it is high time that the astronomers reject the idea that the satellites of Jupiter can be seen through his limb at occultation. When the seeing is bad there is a spurious limb to Jupiter that well might give the appearance of transparency at the occultation of a satellite. But under first-class conditions the limb of Jupiter is perfectly opaque. It is quibbling and begging the question altogether to say the phenomenon of transparency may be a rare one and so have escaped my observations. Has any one said that the moon was transparent when a star has been seen projected on it when it ought to have been behind it?”

Prof. Barnard and Mr. Douglass have seen white polar caps on the third and fourth satellites of Jupiter. The former says they are “exactly like those on Mars.” “Both caps of the fourth satellite have been clearly distinguished, that at the north being sometimes exceptionally large, covering a surface equal to one-quarter or one-third of the diameter of the satellite.”[149]This was confirmed on November 23, 1906, when Signor J. Comas Sola observed a brilliant white spot surrounded by a dark marking in the north polar region of the third satellite. There were other dark markings visible, and the satellite presented the appearance of a miniature of Mars.[150]

An eighth satellite of Jupiter has recently been discovered by Mr. Melotte at the Greenwich Observatory by means of photography. It moves in a retrograde direction round Jupiter in an orbit inclined about 30° to that of the planet. The period of revolution is about two years. The orbit is very eccentric, the eccentricity being about one-third, or greater than that of any other satellite of the solar system. When nearest to Jupiter it is about 9 millions of miles from the planet, and when farthest about 20 millions.[151]It has been suggested by Mr. George Forbes that this satellite may possibly be identical with the lost comet of Lexell which at its return in the year 1779 became entangled in Jupiter’s system, and has not been seen since. If this be the case, we should have the curious phenomenon of a comet revolving round a planet!

According to Humboldt the four bright satellites of Jupiter were seen almost simultaneously and quite independently by Simon Marius at Ausbach on December 29, 1609, and by Galileo at Padua on January 7, 1610.[152]The actual priority, therefore, seems to rest with Simon Marius, but the publication of the discovery was first made by Galileo in hisNuncius Siderius(1610).[153]Grant, however, in hisHistory of Physical Astronomy,calls Simon Marius an “impudent pretender”! (p. 79).

M. Dupret at Algiers saw Jupiter with the naked eye on September 26, 1890, twenty minutes before sunset.[154]

Humboldt states that he saw Jupiter with the naked eye when the sun was from 18° to 20° above the horizon.[155]This was in the plains of South America near the sea-level.

Saturn

Toshow the advantages of large telescopes over small ones, Mr. C. Roberts says that “with the 25-inch refractor of the Cambridge Observatory the view of the planet Saturn is indescribably glorious; everything I had ever seen before was visible at a glance, and an enormous amount of detail that I had never even glimpsed before, after a few minutes’ observation.”[156]

Chacornac found that the illumination of Saturn’s disc is the reverse of that of Jupiter, the edges of Saturn being brighter than the centre of the disc, while in the case of Jupiter—as in that of the sun—the edges are fainter than the centre.[157]According to Mr. Denning, Saturn bears satisfactorily “greater magnifying power than either Mars or Jupiter.”[158]

At an occultation of Saturn by the moon, which occurred on June 13, 1900, M. M. Honoratnoticed the great contrast between the slightly yellowish colour of the moon and the greenish tint of the planet.[159]

In the year 1892, when the rings of Saturn had nearly disappeared, Prof. L. W. Underwood, of the Underwood Observatory, Appleton, Wisconsin (U.S.A.), saw one of Saturn’s satellites (Titan) apparently moving along the needlelike appendage to the planet presented by the rings. “The apparent diameter of the satellite so far exceeded the apparent thickness of the ring that it gave the appearance of a beautiful golden bead moving very slowly along a fine golden thread.”[160]

In 1907, when the rings of Saturn became invisible in ordinary telescopes, Professor Campbell, observing with the great Lick telescope, noticed “prominent bright knots, visible ... in Saturn’s rings. The knots were symmetrically placed, two being to the east and two to the west.” This was confirmed by Mr. Lowell, who says, “Condensations in Saturn’s rings confirmed here and measured repeatedly. Symmetric and permanent.” This phenomenon was previously seen by Bond in the years 1847-56. Measures of these light spots made by Prof. Barnard with the 40-inch Yerkes telescope show that the outer one corresponded in position with the outer edgeof the middle ring close to the Cassini division, and the inner condensation, curious to say, seemed to coincide in position with the “crape ring.” Prof. Barnard thinks that the thickness of the rings “must be greatly under 100 miles, and probably less than 50 miles,” and he says—

“The important fact clearly brought out at this apparition ofSaturnis that the bright rings are not opaque to the light of the sun—and this is really what we should expect from the nature of their constitution as shown by the theory of Clerk Maxwell, and the spectroscopic results of Keeler.”[161]

Under certain conditions it would be theoretically possible, according to Mr. Whitmell, to see the globe of Saturn through the Cassini division in the ring. But the observation would be one of great difficulty and delicacy. The effect would be that, of the arc of the division which crosses the planet’s disc, “a small portion will appear bright instead of dark, and may almost disappear.”[162]

A remarkable white spot was seen on Saturn on June 23, 1903, by Prof. Barnard, and afterwards by Mr. Denning.[163]Another white spot was seen by Denning on July 9 of the same year.[164]From numerous observations of these spots, Denning found a rotation period for the planet of about10h39m21s.[165]From observations of the same spots Signor Comas Sola found a period 10h38m·4, a close agreement with Denning’s result. For Saturn’s equator, Prof. Hill found a rotation period of 10h14m23s·8, so that—as in the case of Jupiter—the rotation is faster at the equator than in the northern latitudes of the planet. A similar phenomenon is observed in the sun. Mr. Denning’s results were fully confirmed by Herr Leo Brenner, and other German astronomers.[166]

Photographs taken by Prof. V. M. Slipher in America show that the spectrum of Saturn is similar to that of Jupiter. None of the bands observed in the planet’s spectrum are visible in the spectrum of the rings. This shows that if the rings possess an atmosphere at all, it must be much rarer than that surrounding the ball of the planet. Prof. Slipher says that “none of the absorption bands in the spectrum ofSaturncan be identified with those bands due to absorption in the earth’s atmosphere,” and there is no trace of aqueous vapour.[167]

In September, 1907, M. G. Fournier suspected the existence of a “faint transparent and luminous ring” outside the principal rings of Saturn. He thinks that it may possibly be subject to periodical fluctuations of brightness, sometimes being visibleand sometimes not.[168]This dusky ring was again suspected at the Geneva Observatory in October, 1908.[169]M. Schaer found it a difficult object with a 16-inch Cassegrain reflector. Prof. Stromgen at Copenhagen, and Prof. Hartwig at Bamberg, however, failed to see any trace of the supposed ring.[170]It was seen at Greenwich in October, 1908.

A “dark transit” of Saturn’s satellite Titan across the disc of the planet has been observed on several occasions. It was seen by Mr. Isaac W. Ward, of Belfast, on March 27, 1892, with a 4·3-inch Wray refractor. The satellite appeared smaller than its shadow. The phenomenon was also seen on March 12 of the same year by the Rev. A. Freeman, Mr. Mee, and M. F. Terby; and again on November 6, 1907, by Mr. Paul Chauleur and Mr. A. B. Cobham.[171]

The recently discovered tenth satellite of Saturn, Themis, was discovered by photography, and has never been seen by the eye even with the largest telescopes! But its existence is beyond all doubt, and its orbit round the planet has been calculated.

Prof. Hussey of the Lick Observatory finds that Saturn’s satellite Mimas is probably larger than Hyperion. He also finds from careful measurements that the diameter of Titan is certainlyoverestimated, and that its probable diameter is about 2500 miles.[172]

The French astronomer, M. Lucien Rudaux, finds the following variation in the light of the satellites of Saturn:—

The variation of light is, he thinks, due to the fact that the period of rotation of each satellite is equal to that of their revolution round the planet; as in the case of our moon.[173]

The names of the satellites of Saturn are derived from the ancient heathen mythology. They are given in order of distance from the planet, the nearest being Mimas and the farthest Themis.

1. Mimas was a Trojan born at the same time as Paris.

2. Enceladus was son of Tartarus and Ge.

3. Tethys was wife of Oceanus, god of ocean currents. She became mother of all the chief rivers in the universe, as also the Oceanides or sea nymphs.

4. Dione was one of the wives of Zeus.

5. Rhea was a daughter of Uranus. She married Saturn, and became the mother of Vesta, Ceres, Juno, and Pluto.

6. Titan was the eldest son of Uranus.

7. Hyperion was the god of day, and the father of sun and moon.

8. Japetus was the fifth son of Uranus, and father of Atlas and Prometheus.[174]

9. Phœbe was daughter of Uranus and Ge.

10. Themis was daughter of Uranus and Ge, and, therefore, sister of Phœbe.

In a review of Prof. Comstock’sText Book of AstronomyinThe Observatory, November, 1901, the remark occurs, “We are astonished to see that Mr. Comstock alludes with apparent seriousness to theninesatellites of Saturn. As regards the ninth satellite, we thought that all astronomers held with Mrs. Betsy Prig on the subject of this astronomical Mrs. Harris.” This reads curiously now (1909) when the existence of the ninth satellite (Phœbe) has been fully confirmed, and a tenth satellite discovered.

Uranus and Neptune

Fromobservations of Uranus made in 1896, M. Leo Brenner concluded that the planet rotates on its axis in about 8½ hours (probably 8h27m). This is a short period, but considering the short periods of Jupiter and Saturn there seems to be nothing improbable about it.

Prof. Barnard finds that the two inner satellites of Uranus are difficult objects even with the great 36-inch telescope of the Lick Observatory! They have, however, been photographed at Cambridge (U.S.A.) with a 13-inch lens, although they are “among the most difficult objects known.”[175]

Sir William Huggins in 1871 found strong absorption lines (six strong lines) in the spectrum of Uranus. One of these lines indicated the presence of hydrogen, a gas which does not exist in our atmosphere. Three of the other lines seen were situated near lines in the spectrum of atmospheric air. Neither carbonic acid nor sodiumshowed any indications of their presence in the planet’s spectrum. A photograph by Prof. Slipher of Neptune’s spectrum “shows the spectrum of this planet to contain many strong absorption bands. These bands are so pronounced in the part of the spectrum between the Fraunhofer lines F and D, as to leave the solar spectrum unrecognizable.... Neptune’s spectrum is strikingly different from that ofUranus, the bands in the latter planet all being reinforced inNeptune. In this planet there are also new bands which have not been observed in any of the other planets. The F line of hydrogen is remarkably dark ... this band is of more than solar strength in the spectrum of Uranus also. Thus free hydrogen seems to be present in the atmosphere of both these planets. This and the other dark bands in these planets bear evidence of an enveloping atmosphere of gases which is quite unlike that which surrounds the earth.”[176]

With the 18-inch equatorial telescope of the Strasburgh Observatory, M. Wirtz measured the diameter of Neptune, and found from forty-nine measures made between December 9, 1902, and March 28, 1903, a value of 2″·303 at a distance of 30·1093 (earth’s distance from sun = 1). This gives a diameter of 50,251 kilometres, or about 31,225 miles,[177]and a mean density of 1·54 (water = 1;earth’s mean density = 5·53). Prof. Barnard’s measures gave a diameter of 32,900 miles, a fairly close agreement, considering the difficulty of measuring so small a disc as that shown by Neptune.

The satellite of Neptune was photographed at the Pulkown Observatory in the year 1899. The name Triton has been suggested for it. In the old Greek mythology Triton was a son of Neptune, so the name would be an appropriate one.

The existence of a second satellite of Neptune is suspected by Prof. Schaeberle, who thinks he once saw it with the 36-inch telescope of the Lick Observatory “on an exceptionally fine night” in 1895.[178]But this supposed discovery has not yet been confirmed. Lassell also thought he had discovered a second satellite, but this supposed discovery was never confirmed.[178]

The ancient Burmese mention eight planets, the sun, the moon, Mercury, Venus, Mars, Jupiter, Saturn, and another named Râhu, which is invisible. It has been surmised that “Râhu” is Uranus, which is just visible to the naked eye, and may possibly have been discovered by keen eyesight in ancient times. The present writer has seen it several times without optical aid in the West of Ireland, and with a binocular field-glass of 2 inches aperture he found it quite a conspicuous object.

When Neptune wasvisuallydiscovered by Galle, at Berlin, he was assisted in his observation by Prof. d’Arrest. The incident is thus described by Dr. Dreyer, “On the night of June 14, 1874, while observing Coggia’s comet together, I reminded Prof. d’Arrest how he had once said in the course of a lecture, that he had been present at the finding of Neptune, and that ‘he might say it would not have been found without him.’ He then told me (and I wrote it down the next day), how he had suggested the use of Bremiker’s map (as first mentioned by Dr. Galle in 1877) and continued, ‘We then went back to the dome, where there was a kind of desk, at which I placed myself with the map, while Galle, looking through the refractor, described the configurations of the stars he saw. I followed them on the map one by one, until he said: “And then there is a star of the 8th magnitude, in such and such a position,” whereupon I immediately exclaimed: “That star is not on the map.”’”[179]This was the planet. But it seems to the present writer that if Galle or d’Arrest had access to Harding’s Atlas (as they probably had) they might easily have found the planet with a good binocular field-glass. As a matter of fact Neptune is shown in Harding’s Atlas (1822) as a star of the 8th magnitude, having been mistaken for a star by Lalande on May 8 and 10, 1795; and the present writer hasfound Harding’s 8th magnitude stars quite easy objects with a binocular field-glass having object-glasses of two inches diameter, and a power of about six diameters.

Supposed Planet beyond Neptune.—The possible existence of a planet beyond Neptune has been frequently suggested. From considerations on the aphelia of certain comets, Prof. Forbes in 1880 computed the probable position of such a body. He thought this hypothetical planet would be considerably larger than Jupiter, and probably revolve round the sun at a distance of about 100 times the earth’s mean distance from the sun. The place indicated was between R.A. 11h24mand 12h12m, and declination 0° 0′ to 6° 0′ north. With a view to its discovery, the late Dr. Roberts took a series of eighteen photographs covering the region indicated. The result of an examination of these photographs showed, Dr. Roberts says, that “no planet of greater brightness than a star of the 15th magnitude exists on the sky area herein indicated.” Prof. W. H. Pickering has recently revived the question, and has arrived at the following results: Mean distance of the planet from the sun, 51·9 (earth’s mean distance = 1); period of revolution, 373½ years; mass about twice the earth’s mass; probable position for 1909 about R.A. 7h47m, north declination 21°, or about 5° south-east of the star κ Geminorum. The supposed planet would be faint, its brightnessbeing from 11½ to 13½, according to the “albedo” (or reflecting power) it may have.[180]

Prof. Forbes has again attacked the question of a possible ultra-Neptunian planet, and from a consideration of the comets of 1556, 1843 I, 1880 I, and 1882 II, finds a mean distance of 105·4, with an inclination of the orbit of 52° to the plane of the ecliptic. This high inclination implies that “during the greatest part of its revolution it is beyond the zodiac,” and this, Mr. W. T. Lynn thinks, “may partly account for its not having hitherto been found by observation.”[181]

From a consideration of the approximately circular shape of the orbits of all the large planets of the solar system, Dr. See suggests the existence of three planets outside Neptune, with approximate distances from the sun of 42, 56, and 72 respectively (earth’s distance = 1), and recommends a photographic search for them. He says, “To suppose the planetary system to terminate with an orbit so round as that of Neptune is as absurd as to suppose that Jupiter’s system terminates with the orbit of the fourth satellite.”[182]

According to Grant, even twenty years before the discovery of Neptune the error of Prof. Adams’ first approximation amounted to little more than 10°.[183]

Comets

Welearn from Pliny that comets were classified in ancient times, according to their peculiar forms, into twelve classes, of which the principal were:Pogonias, bearded;Lampadias, torch-like;Xiphias, sword-like;Pitheus, tun-like;Acontias, javelin-like;Ceratias, horn-like;Disceus, quoit-like; andHippias, horse-mane-like.[184]

Of the numerous comets mentioned in astronomical records, comparatively few have been visible to the naked eye. Before the invention of the telescope (1610) only those which were so visiblecould, of course, be recorded. These number about 400. Of the 400 observed since then, some 70 or 80 only have been visible by unaided vision; and most of these now recorded could never have been seen without a telescope. During the last century, out of 300 comets discovered, only 13 were very visible to the naked eye. Hence, when we read in the newspapers that a comet has been discovered thechances are greatly against it becoming visible to the naked eye.[185]

Although comparatively few comets can be seen without a telescope, they are sometimes bright enough to be visible in daylight! Such were those ofB.C.43,A.D.1106, 1402, 1532, 1577, 1744, 1843, and the “great September comet” of 1882.

If we except the great comet of 1861, through the tail of which the earth is supposed to have passed, the comet which came nearest to the earth was that of 1770, known as Lexell’s, which approached us within two millions of miles, moving nearly in the plane of the ecliptic. It produced, however, no effect on the tides, nor on the moon’s motion, which shows that its mass must have been very small. It was computed by Laplace that if its mass had equalled that of the earth, the length of our year would have been shortened by 2 hours 47 minutes, and as there was no perceptible change Laplace concluded that the comet’s mass did not exceed1⁄5000th of the earth’s mass. This is the comet which passed so near to Jupiter that its period was reduced to 5½ years. Owing to another near approach in 1779 it became invisible from the earth, and is now lost.[186]Its identity with the recently discovered eighth satellite of Jupiter has been suggested by Mr. George Forbes (see under “Jupiter”). At the near approach of Lexell’s comet to the earth in 1770, Messier, “the comet ferret,”found that its head had an apparent diameter of 2½°, or nearly five times that of the moon!

Another case of near approach to the earth was that of Biela’s comet at its appearance in 1805. On the evening of December 9 of that year, the comet approached the earth within 3,380,000 miles.[187]

The comet ofA.D.1106 is stated to have been seen in daylight close to the sun. This was on February 4 of that year. On February 10 it had a tail of 60° in length, according to Gaubil.[188]

The comet of 1577 seems to have been one of the brightest on record. According to Tycho Brahé, it was visible in broad daylight. He describes the head as “round, bright, and of a yellowish light,” with a curved tail of a reddish colour.[189]

The comet of 1652 was observed for about three weeks only, and Hevelius and Comiers state that it was equal to the moon in apparent size! This would indicate a near approach to the earth. An orbit computed by Halley shows that the least distance was about 12 millions of miles, and the diameter of the comet’s head rather less than 110,000 miles, or about 14 times the earth’s diameter.

According to Mr. Denning, “most of the periodical comets at perihelion are outside the earth’s orbit, and hence it follows that they escapeobservation unless the earth is on the same side of the sun as the comet.”[190]

It was computed by M. Faye that thevolumeof the famous Donati’s comet (1858) was about 500 times that of the sun! On the other hand, he calculated that itsmass(or quantity of matter it contained) was only a fraction of the earth’s mass. This shows how almost inconceivably tenuous the material forming the comet must have been—much more rarefied, indeed, than the most perfect vacuum which can be produced in an air-pump. This tenuity is shown by the fact that stars were seen through the tail “as if the tail did not exist.” A mist of a few hundred yards in thickness is sufficient to hide the stars from our view, while a thickness of thousands of miles of cometary matter does not suffice even to dim their brilliancy!

At the time of the appearance of the great comet of 1843, it was doubtful whether the comet had transited the sun’s disc. But it is now known, from careful calculations by Prof. Hubbard, that a transit really took place between 11h28mand 12h29mon February 27, 1843, and might have been observed in the southern hemisphere. The distance of this remarkable comet from the sun at its perihelion passage was less than that of any known comet. A little before 10 p.m. on February 27, the comet passed within 81,500 miles ofthe sun’s surface with the enormous velocity of 348 miles a second! It remained less than 2¼ hours north of the ecliptic, passing from the ascending to the descending node of its orbit in 2h13m·4.[191]The great comet of 1882 transited the sun’s disc on Sunday, September 17, of that year, the ingress taking place at 4h50m58s, Cape mean time. When on the sun the comet was absolutely invisible, showing that there was nothing solid about it. It was visible near the sun with the naked eye a little before the transit took place.[192]This great comet was found by several computors to have been travelling in an elliptic orbit with a period of about eight centuries. Morrison found 712 years; Frisby, 794; Fabritius, 823; and Kreutz, 843 years.[193]

The great southern comet of 1887 may be described as a comet without a head! The popular idea of a comet is a star with a tail. But in this case there was no head visible—to the naked eye at least. Dr. Thome of the Cordoba Observatory—its discoverer—describes it as “a beautiful object—a narrow, straight, sharply defined, graceful tail, over 40° long, shining with a soft starry light against a dark sky, beginning apparently without a head, and gradually widening and fading as it extended upwards.”[194]

The great southern comet of 1901 had five tails on May 6 of that year. Two were fairly bright, and the remaining three rather faint. Mr. Gale saw a number of faint stars through the tails. The light of these seem to have been “undimmed.” Mr. Cobham noticed that the stars Rigel and β Eridani shone through one of the faint tails, and “showed no perceptible difference.”[195]

Prof. W. H. Pickering says that “the head of a comet, as far as our present knowledge is concerned, seems therefore to be merely a meteor swarm containing so much gaseous material that when electrified by its approach to the sun it will be rendered luminous” (Harvard Annual, vol. xxxii. part ii. p. 295) “... if the meteors and their atmospheres are sufficiently widely separated from one another, the comet may be brilliant and yet transparent at the same time.”

In the case of Swift’s comet of 1892 some periodical differences of appearance were due, according to Prof. W. H. Pickering, to a rotation of the comet round an axis passing longitudinally through the tail, and he estimated the period of rotation at about 94 to 97 hours. He computed that in this comet the repulsive force exerted by the sun on the comet’s tail was “about 39·5 times the gravitational force.”[196]

The comet known as 1902bapproached theplanet Mercury within two millions of miles on November 29 of that year. Prof. O. C. Wendell, of Harvard Observatory, made some observations on the transparency of this comet. He found with the aid of a photometer and the 15-inch telescope of the observatory that in the case of two faint stars over which the comet passed on October 14, 1902, the absorption of light by the comet was insensible, and possibly did not exceed one or two hundredths of a magnitude,[197]an amount quite imperceptible to the naked eye, and shows conclusively how almost inconceivably rarefied the substance of this comet must be.

The comet known as Morehouse (1908c) showed some curious and wonderful changes. Mr. Borelly found that five tails are visible on a photographic plate taken on October 3, 1908, and the trail of an occulted star indicates a slight absorption effect. According to M. L. Rabourdin, great changes took place from day to day, and even during the course of an hour! Similar changes were recorded by G. M. Gauthier; and Prof. Barnard, who photographed the comet on 30 nights from September 2 to October 13, states that the photographs of September 30 “are unique, whilst the transformation which took place between the taking of these and the taking of the next one on October 1 was very wonderful.”[198]The spectrumshowed the lines of cyanogen instead of the hydrocarbon spectrum shown by most comets.

Prof. Barnard has suggested that all the phenomena of comets’ tails cannot be explained by a repulsive force from the sun. Short tails issuing from the comet’s nucleus at considerable angles with the main tail point to eruptive action in the comet itself. The rapid changes and distortions frequently observed in the tails of some comets suggest motion through a resisting medium; and the sudden increase of light also occasionally observed points in the same direction.[199]

It was computed by Olbers that if a comet having a mass of1⁄2000th of the earth’s mass—which would form a globe of about 520 miles in diameter and of the density of granite—collided with the earth, with a velocity of 40 miles a second, our globe would be shattered into fragments.[200]But that any comet has a solid nucleus of this size seems very doubtful; and we may further say that the collision of the earth withanycomet is highly improbable.

It seems to be a common idea that harvests are affected by comets, and even “comet wines” are sometimes spoken of. But we know that the earth receives practically no heat from the brightest comet. Even in the case of the brilliant comet of 1811, one of the finest on record, it wasfound that “all the efforts to concentrate its rays did not produce the slightest effect on the blackened bulb of the most sensitive thermometer.” Arago found that the year 1808, in which several comets were visible, was a cold year, “and 1831, in which there was no comet, enjoyed a much higher temperature than 1819, when there were three comets, one of which was very brilliant.”[201]We may, therefore, safely conclude that even a large comet has no effect whatever on the weather.

From calculations on the orbit of Halley’s comet, the next return of which is due in 1910, Messrs. Cowell and Crommelin find that the identity of the comet shown on the Bayeux Tapestry with Halley’s comet is now “fully established.” They find that the date of perihelion passage was March 25, 1066, which differs by only 4 days from the date found by Hind. The imposing aspect of the comet in 1066 described in European chronicles of that time is confirmed by the Chinese Annals. In the latter records the brightness is compared to that of Venus, and even with that of the moon! The comparison with the moon was probably an exaggeration, but the comet doubtless made a very brilliant show. In the Bayeux Tapestry the inscription on the wall behind the spectators reads: “isti mirant stella.” Now, this is bad Latin, and Mr. W. T.Lynn has made the interesting suggestion that some of the letters are hidden by the buildings in front and that the real sentence is “isti mirantur stellam.”[202]The present writer has examined the copy of the Bayeux Tapestry which is in the Dublin Museum, and thinks that Mr. Lynn’s suggestion seems very plausible. But the last letter ofstellamis apparently hidden by the comet’s tail, which does not seem very probable!

The conditions under which the comet will appear in 1910 are not unlike those of 1066 and 1145. “In each year the comet was discovered as a morning star, then lost in the sun’s rays; on its emergence it was near the earth and moved with great rapidity, finally becoming stationary in the neighbourhood of Hydra, where it was lost to view.”[203]In 1910 it will probably be an evening star before March 17, and after May 11, making a near approach to the earth about May 12. About this time its apparent motion in the sky will be very rapid. As, however, periodical comets—such as Halley’s—seem to become fainter at each return, great expectations with reference to its appearance in 1910 should not be indulged in.

The appearance of Halley’s comet inA.D.1222 is thus described by Pingré—a great authority on comets—(quoting from an ancient writer)—

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