Chapter XV: The Great Nebula in Orion (4)

Errors have certainly been made by astronomers in some of their observations of the so-called polar snow-spots, other objects occupying their place having been mistaken for them. A regular series of observations on this planet, which I have now continued for seven years, has revealed the fact that during the winter seasons of the southern hemisphere of Mars, the polar spots are most of the time invisible, being covered over by white, opaque, cloud-like forms, strongly reflecting light. In 1877, during more than a month, I, myself, mistook for the polar spots such a canopy of clouds, which covered at least one-fifth of the surface of the whole disk. I only became aware of my error when the opaque cloud, beginning to dissolve at the approach of the Martial summer, allowed the real polar spot to be seen through its vapors, as through a mist at first, and afterwards with great distinctness. In this particular case, the snow-spot was considerably smaller than the cloudy cap which covered it, and it is to be remarked that it was not situated at the centre of this cloudy cap, but was east of that centre; a fact which may account for the so-called polar spots not being always observed on exactly opposite sides of the disk. From my observations of 1877, 1878 and 1880, it appears that at the approach of the autumnal equinox of the southern hemisphere of Mars, large, opaque masses, like cumulus clouds in form, began to gather in the polar regions of that hemisphere, and continued through autumn and winter, dissolving only at the approach of spring. These clouds, which varied in form and extent, were very unsteady at first, but as the winter drew nearer they enlarged and became more permanent, covering large surfaces for months at a time.

That the large white spots under consideration are real clouds in the atmosphere of Mars, and are not due to a fall of snow, is proved by the fact that these spots covered both seas and continents with equal facility, even in the equatorial regions of the planet. Snow, of course, could not cover the seas of Mars, unless these were all frozen over, even in the equatorial zones; therefore, if the dark spots of Mars are assumed to be due to water, these large white spots cannot well be ascribed to snow.

The real polar spots of Mars seem to be in relief on the surface of the planet, since the southern spot often appeared slightly shaded on the side opposite to the Sun during my observations in 1877. In certain cases, when they are on or very near the limb, they have been observed, both by others and by myself, to project from the disk slightly.

The polar spots of Mars are doubtless composed of a material which, like our snow or ice, melts under the rays of the Sun; although it seems difficult to admit that the Martial snow is identical with our terrestrial snow, and that it melts at a like temperature. The south polar spot of Mars entirely disappeared from sight in its summer season in 1877, although the planet receives less than one-half as much heat as we receive from the Sun; yet on our globe the arctic or antarctic ices and snow are perpetual--never melting entirely. An important fact disclosed by the melting away of the southern polar spot is, that in melting it is always surrounded with a very dark surface, which takes the place of the melted portion of the spot, as observed by myself in 1877-78. When the polar spot had entirely disappeared, its place was occupied by a very dark spot. Now, if the polar spot is really ice, and the dark spots are actual seas, this polar spot must be situated in mid-ocean, since, on melting away, it is replaced by a dark spot. If the polar spots are composed of a white substance melting under the rays of the Sun, as seems altogether probable, its melting point must be above that of terrestrial snow.

Many of the dark spots of Mars, and especially those whose northern border forms an irregular belt upon the equatorial regions of this planet, are bordered on that side by a white luminous belt, following all their sinuosities. These white borders are variable. Sometimes they are very prominent and intensely bright, especially at some points, which occasionally almost equal the polar spots in brilliancy; while at other times they are so faint, that they can hardly be distinguished, or are even invisible; although the atmosphere is clear and the dark spots appear perfectly well defined. While these white borders were invisible, I have sometimes watched for several hours at a time to see if I could detect any traces of them in places where they usually appear the most prominent, but generally without success. On a few occasions, however, I had the good fortune to see some of these spots forming gradually in the course of one or two hours, at places where nothing of them could be seen before.

These whitish fringes forming and vanishing along the coasts of the Martial seas have been very little studied by astronomers. From my observations made during the last seven years, it appears very probable that this belt and its white spots are mainly due to the condensation of vapors around, and over high peaks, and extensive mountain chains, forming the Martial sea-coasts, as the Andes and Rocky Mountains form the sea-coasts of the Pacific Ocean. These high mountains on Mars, condensing the vapors into fogs or clouds above them, or at their sides, as often happens in our mountainous districts, would certainly suffice to produce the phenomena observed. Some of the highest peaks among these mountain chains may even have their summits covered with perpetual snow, or some substance partaking of the nature of snow. The temporary visibility and invisibility of the white spots seen on Mars, as well as the rapid transformations they sometimes undergo, may be explained as caused by clouds having a high reflective power and a liability to form and disappear quickly.

The assumption that these irregular whitish bands and spots are formed by the condensation of vapors on mountain chains, and elevated table lands, is supported by my observations made in 1877 and 1879. When such white spots were traversing the terminator at sunrise, they very often projected far into the night side, thus indicating that they were at a higher level than that of the general surface. Indentations in the terminator, corresponding to large dark spots crossing its line, also clearly indicated the depression of the dark spots below the general surface. The highest mountainous districts thus observed on Mars, are situated between 60° and 70° of south latitude, towards the western extremity of Gill Land. The mountain chain, which almost completely forms the surface of this land, is so elevated at some points, that they not only change the form of the terminator when they are seen upon it, but also the limb of the planet, as seen by myself. They then appear so brilliant, that the principal summit among them has been mistaken by several observers for the polar spot itself, as proved by the wrong position assigned to it on their drawings. It seems probable that this high peak, which appears always white, is constantly covered with snow, or the similar material replacing it on Mars. This high region is situated between longitudes 180° and 190°.

The highest mountainous parts belonging to the hemisphere represented on Plate VIII., which are nearly always more or less visible as whitish spots and bands, form a coast line along the northern (lower) border of De La Rue Ocean. This great spot, which is not so simple as it has been represented by observers, is in fact divided by two narrow isthmuses, one in the north, the other in the east, both joining, in the interior of the great ocean, a peninsula heretofore known as Hall Island. Upon the south-eastern extremity of this peninsula, a white spot, called Dawes Ice Island, was observed in 1865, but it soon disappeared, and was after that seen only now and then. It is very probable that this so-called Ice Island was due to clouds forming around the summit of some high peak of this peninsula.

On the opposite hemisphere to that represented on Plate VIII., the white fringes bordering the dark spots are much more conspicuous than they are on this side. On the eastern side of a remarkable dark spot called Kaiser Sea, they are very bright, and almost always present, although they vary considerably, both in brightness and in extent. To the south of Kaiser Sea, they are very conspicuous on the eastern border of Lockyer Land, forming an elevated and deeply indented coast-line along Lambert Sea. There the white spots never disappear entirely, being always visible on the north side, where they turn westward along Dawes' Ocean--the mountain chain attaining there its greatest altitude. Very frequently Lockyer Land, which seems to be a vast plateau, appears throughout white and brilliant, this occurring usually towards the sunrise or sunset of that region, probably from the condensation of vapors and the formation of fogs, but generally this whiteness gradually disappears with the progress of the sun above this plateau. Inside of the great continents of Mars these temporary white spots are not so frequent, but when visible they occupy always the same positions--a fact which probably indicates that they occupy the culminating points of these continents. One of these temporary white spots inside of the continents is represented on Plate VIII., on the left-hand side, below De La Rue Ocean, on Maedler Continent.

Although large, opaque, cumulus-shaped, cloud-like forms are seen in the polar regions of Mars, such forms are very seldom seen in the tropical zones, or, at least, it appears so, from the fact that my observations, continued during the last seven years, have disclosed no real opaque cloudy forms there. Although the Martial sky is frequently overcast by dense vapors or thick fogs in these regions, yet no real opaque clouds were ever seen; the most prominent among the dusky spots being faintly visible through the vapory veil, when they approached the centre of the disk.

Besides these phenomena, which prove that Mars is surrounded by an atmosphere having a great deal of similarity to our own, a further proof is afforded by the fact that the dark spots, which appear sharply defined and black when they are seen near the centre, become less and less visible as they advance towards the limb, and are totally invisible before they reach it. Moreover, the spectroscope also indicates the existence of an atmosphere, and even the presence of watery vapor in it. A very curious state of the Martial atmosphere is revealed by my observations of 1877-78. During eight consecutive weeks, from December 12th to February 6th, a whole hemisphere of the planet--precisely that represented on Plate VIII.--was completely covered by dense vapors, or a thick fog which barely allowed the dark spots to be seen through it, even when they were in the centre of the disk. The opposite hemisphere of Mars appeared just as clear and calm as possible; there all the spots and their minutest details could be seen, and when the planet was observed at the proper time, the line separating the foggy from the clear side was plainly visible.

The reddish tint observed on the continents of Mars has been supposed by some astronomers to be the real color of the atmosphere of this planet. But, for many reasons, this explanation is not acceptable. Besides the fact that the border of the planet appears white, while it should be more red than the other part, owing to the greater depth of atmosphere there presented to us, the polar spots, the white bands along the sea-coasts, and the cloud-like forms appear perfectly white, not the slightest tint of red being visible on them, as would be the case if these objects were seen through an atmosphere tinted red. Other astronomers have supposed that the vegetation of this planet has a reddish color; but this is not supported by observation. It has been again supposed, with much more probability, that the surface of Mars is composed of an ochreous material which gives the planet its predominant ruddy color.

Until lately Mars was supposed to be without a satellite, but in August, 1877, Professor Hall, of the Washington Observatory, made one of the most remarkable discoveries of the time, and found two satellites revolving around this planet. These satellites are among the smallest known heavenly bodies, their diameter having been estimated at from 6 to 10 miles for the outer satellite, and from 10 to 40 miles for the inner one.

The most extraordinary feature of these bodies is the proximity of the inner satellite to the planet, and the consequent rapidity of its motion. The distance of the inner satellite from the centre of Mars is about 6,000 miles, and from surface to surface it is less than 4,000 miles, or a little more than the distance from New York to San Francisco. The shortest period of revolution of any satellite previously known, is that of the inner satellite of Saturn, which is a little more than 22½ hours; but the inner satellite of Mars accomplishes its revolution in 7h. 38m., or in 17 hours less than the period of rotation of the planet upon its axis. The period of revolution of the outer satellite is greater, of course, and equals 30h. 7m.

From this rapidity of motion of the inner satellite of Mars, a very curious result follows, which at first sight may appear in contradiction with the fact that this body has a direct motion, like that of all the planets of the solar system, and moves around Mars from west to east. While the outer satellite of this planet, in company with all the stars and planets, rises in the east and sets in the west, the inner satellite, on the contrary, rises in the west and sets in the east. Since the period of rotation of Mars is greater than is the period of revolution of this satellite, it necessarily follows that this last body must constantly be gaining on the rotation, and, consequently, that the satellite sets in the east and rises in the west, compassing the whole heavens around Mars three times a day, passing through all its phases in 11 hours, each quarter of this singular Moon lasting less than 3 hours.

It has been shown above that Mars has many points of resemblance to the Earth. It has an atmosphere constituted very nearly like ours; it has fogs, clouds, rains, snows, and winds. It has water, or at least some liquids resembling it; it has rivers, lakes, seas and oceans. It has also islands, peninsulas, continents, mountains and valleys. It has two Moons, which must create great and rapid tides in the waters of its seas and oceans. It has its days and nights, its warm and cold seasons, and very likely its vegetation, its prairies and forests, like the Earth. On the other hand, its year and seasons are double those of the Earth, and its distance from the Sun is greater.

Is this planet, which is certainly constituted very nearly like our globe, and seems so nearly fitted for the wants of the human race, inhabited by animals and intelligent beings?

To answer this question, either in the negative or in the affirmative, would be to step out of the pure province of science, and enter the boundless domain of speculation, since no observer has ever seen anything indicating that animal life exists on Mars, or on any other planet or satellite. So far as observation goes, Mars seems to be a planet well suited to sustain animal life, and we may reason from analogy that if animal life can exist at all outside of the Earth, Mars must have its flora and fauna; it must have its fishes and birds, its mammalia and men; although all these living beings must inevitably be very different in appearance from their representatives on the Earth, as can easily be imagined from the differences existing between the two planets. Although all this is possible, and even very probable, yet it must be remembered that we have not the slightest evidence that it is so; and until we have acquired this evidence, we may only provisionally accept this idea as a pleasing hypothesis, which, after all, may be wrong and totally unfounded.

THE PLANET JUPITER

PLATE IX

Jupiter, the giant of the planetary world, is the fifth in order of distance from the Sun, and is next to Mars, our ruddy neighbor. To the naked eye, Jupiter appears as a very brilliant star, whose magnitude, changing with the distance of this planet from the Earth, sometimes approaches that of Venus, our bright morning and evening star.

The mean distance of Jupiter from the Sun is 475,000,000 miles, but owing to the eccentricity of its orbit, its distance varies from 452 to 498 millions of miles. The distance of this planet from the Earth varies still more. When nearest to our globe, or in opposition, its distance is reduced to 384,000,000 miles, and its apparent diameter increased to 50"; while when it is farthest, or in conjunction, its distance is increased to 567,000,000 miles, and its apparent diameter reduced to 30"; Jupiter being thus 183,000,000 miles nearer our globe while in opposition than when it is in conjunction.

This planet revolves around the Sun in 11 years, 10 months and 17 days, or in only 50 days less than 12 terrestrial years. Such is the year of this planet. The plane of its orbit is inclined 1° 19' to the ecliptic. No planet, except Uranus, has an orbit exhibiting a smaller inclination. The planet advances in its orbit at the mean rate of 8 miles a second; which is a little less than half the orbital velocity of the Earth.

Jupiter is of enormous proportions. Its equatorial diameter measures 88,000 miles, and its circumference no less than 276,460 miles, these dimensions being 11 times greater than those of the Earth. This planet, notwithstanding its huge size, rotates on its axis in not far from 9h. 55m. 36s., which period constitutes its day. Owing, however, to the changeable appearance of its surface, this period cannot be ascertained with very great exactitude. In consequence of its rapid rotation, the planet is far from spherical, its polar diameter being shorter than the equatorial by about ¹⁄₁₆, or 5,500 miles. Its surface is 124 times the surface of the earth; while its volume is 1,387 times as great. If Jupiter occupied the place of our satellite in the sky, it would appear 40 times as large as the Moon appears to us, and would cover a surface of the heavens 1,600 times that covered by the full Moon, and would subtend an angle of 21°. Jupiter's mass does not correspond with its great bulk, and is only ¹⁄₁₀₄₇ of the mass of the Sun, and 310 times the mass of the earth; its density being only ¼ of that of our globe. The force of gravitation at the surface of this planet is over 2½ times what it is on the Earth, so that a terrestrial object carried to the surface of Jupiter would weigh over two and a half times as much as on our globe.

Observed with a telescope, even of moderate aperture, Jupiter, with its four attending satellites and its dazzling brilliancy, appears as one of the most magnificent objects in the sky. The general appearance of the disk is white; but unlike that of Mars, it is brightest towards its central parts, and a little darker around the limb, especially on the side opposite to the Sun. Although an exterior planet, and so far from us, Jupiter shows faint traces of phases when observed near its quadratures, but this gibbosity of its disk is very slight, and is indicated only by a kind of penumbral shadow on the limb.

When observed with adequate power, the disk of Jupiter is found to be highly diversified. The principal features consist of a series of alternate light and dark streaks or bands, disposed most of the time parallel with the Jovian equator. These bands differ from each other in intensity as well as in breadth; those near the equator being usually much more prominent than those situated in higher latitudes north and south.

The equatorial zone of Jupiter is occupied most of the time by a broad, prominent belt 20° or 30° wide, limited on each side by a very dark narrow streak. Between these two dark borders, but seldom occupying the whole space between them, appears an irregular white belt, apparently composed of dense masses of clouds strongly reflecting the Sun's light, some of these cloudy masses being very brilliant. The spaces left between the cloudy belt and the dark borders, usually exhibit a delicate pink or rosy color, which produces a very harmonious effect with the varying grayish and bluish shades of some of the belts and streaks seen on the disk. Quite often the cloudy belt is broken up, and consists of independent cloudy masses, separated by larger or smaller intervals, these intervals disclosing the rosy background of this zone.

On each side of the equatorial belt there is usually a broad whitish belt, succeeded by a narrow gray band; the space left on each hemisphere between these last bands and the limb being usually occupied by two or three alternate white and gray bands. A uniform gray segment usually forms a sort of polar cap to Jupiter.

When observed under very favorable conditions, all the lighter belts appear as if composed of masses of small cloudlets, resembling the white opaque clouds seen in our atmosphere. This, as already stated, is particularly noticeable on the equatorial belt. It is not unusual, when Jupiter is in quadrature, to see some of the most conspicuous white spots casting a shadow opposite to the Sun; a fact which sufficiently indicates that these spots are at different levels. They probably form the summits of vast banks of clouds floating high up in the atmosphere of Jupiter.

What we see of Jupiter is chiefly a vaporous, cloudy envelope. If our sight penetrates anywhere deeper into the interior, it can only be through the narrow fissures of this envelope, which appear as gray or dark streaks or spots. That most of the visible surface of Jupiter is simply a cloudy covering, is abundantly proved by the proper motion of its spots, which sometimes becomes very great.

In periods of calm, very few changes are noticeable in the markings of the planet, except, perhaps, some slight modifications of form in the cloudy, equatorial belt which, in general, is much more liable to changes than the other belts. But the Jovian surface is not always so tranquil, great changes being observed during the terrific storms which sometimes occur on this mighty planet, when all becomes disorder and confusion on its usually calm surface; and nothing on the Earth can give us a conception of the velocity with which some of its clouds and spots are animated. New belts quickly form, while old ones disappear. The usual parallelism of the belts no more exists. Huge, white, cumulus-like masses advance and spread out, the rosy equatorial belt enlarges sometimes to two or three times its usual size, and occupies two-thirds, or more, of the disk, the rosy tint spreading out in a very short time. At times very dark bands extending across the disk are transformed into knots or dark spots, which encircle the planet with a belt, as it were, of jet black beads. Sometimes, also, a secondary but narrower rosy belt forms either in the northern or the southern hemisphere, and remains visible for a few days or for years at a time.

On May 25, 1876, I witnessed one of the grandest commotions which can be conceived as taking place in an atmosphere. All the southern hemisphere of Jupiter, from equator to pole, was in rapid motion, the belts and spots being transported entirely across the disk, from the eastern to the western limb, in one hour's time, during which the equatorial belt swelled to twice its original breadth, towards the south.

Now, when one stops for a moment to think what is signified by that motion of the dark spots across the little telescopic disk of Jupiter in an hour's time, he may arrive at some conception of the magnitude of the Jovian storms, compared with those of our globe. The circumference of Jupiter's equator, as stated above, is 276,460 miles; half this number, or 138,230 miles, represents the length of the equatorial line seen from the Earth. Now, after taking into account the rotation of the planet, which somewhat diminishes the apparent motion, we arrive at the astonishing result that the spots and markings were carried along by this Jovian storm, at the enormous rate of 110,584 miles an hour, or over 30.7 miles a second. On our globe, a hurricane or tornado, which blows at the rate of 100 miles an hour, sweeps everything before it. What, then, must be expected from a velocity over 1,105 times as great? Enormous as this motion may appear, its occurrence cannot be doubted, since it is disclosed by direct observation.

The surface of Jupiter, it would seem, has its periods of calm and activity like that of the Sun, although it is not yet known, as it is for the latter, that they recur with approximate regularity.

My observations of this planet, which embrace a period of ten years, seem to point in that direction, for they show, at least, that Jupiter has its years of calm and its years of disturbances. The year 1876 was a year of extraordinary disturbance on Jupiter. Changes in the markings were going on all the time, and no one form could be recognized the next day, or even sometimes the next hour, as shown above. The cloudy envelope of the planet was in constant motion, the equatorial belt, especially, showing the signs of greatest disturbance, being, for the most part, two or three times as wide as in other years. After 1876 the calm was very great on the planet, only a slight change now and then being noticeable, the same forms being recognized day after day, month after month, and even year after year. In one case the same marking has been observed for seventeen consecutive months, and in another for twenty-eight months. This state of quietude lasted until October, 1880, when considerable commotion occurred on the northern hemisphere, where large round black spots, somewhat resembling the Sun-spots, formed in the cloudy atmosphere, and finally changed, towards the end of December, into a narrow pink belt, which still exists.

[PLATE IX.--THE PLANET JUPITER.

Observed November 1, 1880, at 9h. 30m. P.M.]

The most curious marking ever seen on Jupiter is undoubtedly the great Red Spot, observed on the southern hemisphere of this planet for the last three years. This interesting object, seen first in July, 1878, disappeared for a time, reappeared on September 25 of the same year, and has remained visible until now. When seen by me in September, it was much elongated, and sharply pointed on one side, like a spear-head, but it subsequently acquired an irregular form, with short appendages protruding from its northern border. At first, the changes were great and frequent, but at length it acquired the regular oval form, which, with but slight modifications, it has retained until now. During the month of November, 1880, I noticed two small black specks upon this Red Spot, and they were seen again in January of the succeeding year, by Mr. Alvan Clark, Jr. When the spot had attained its oval shape, it appeared part of the time surrounded with a white luminous ring of cloudy forms which, however, was changing more or less all the time, being sometimes invisible. The color of this curious spot is a brilliant rosy red, tinged with vermilion, and altogether different in shade from the pinkish color of the equatorial belt. The size of the spot varies, but of late its changes have been slight. Its longer diameter may be estimated at 8,000 miles, and the shorter at 2,200 miles. The Red Spot is represented on Plate IX. with its natural color, and as it appears at the moments most favorable for observation. In ordinary cases its color does not appear so brilliant, but paler.

It is difficult to account for the color of the equatorial belt and that of the Red Spot; but it is known, at least, that the material to which they are due cannot be situated at the level of the general surface visible to us, and especially that of the cloudy forms of the equatorial zone. Undoubtedly the red layer lies deeper than the superficial envelope of the planet, although it does not seem to be very deeply depressed.

Jupiter is attended by four satellites, which revolve around the planet at various distances, and shine like stars of the 6th and 7th magnitude. It is said that under very favorable circumstances, and in a very clear sky, the satellites can be seen with the naked eye, but this requires exceptionally keen eyes, since the glare of the planet is so strong as to overpower the comparatively faint light of the satellites. However, I myself have sometimes seen, without the aid of the telescope, two or three of the satellites as a single object, when they were closely grouped on the same side of Jupiter.

The four moons of Jupiter are all larger than our Moon, except the first, which has about the same diameter. They range in size from 2,300 to 3,400 miles in diameter, the third being the largest; the determination of their diameter is by no means accurate, however, as it is difficult to measure such small objects with precision. Their mean distance from the centre of Jupiter varies from 267,000 to 1,192,000 miles, the first satellite, the nearest to the planet, being a little farther from Jupiter then our satellite is from us. The four satellites revolve around the planet in orbits whose planes have a slight inclination to the equator of Jupiter, and consequently to the ecliptic. The diameter of the largest satellite is nearly half that of the Earth, or 3,436 miles; while its volume is five times that of our Moon. The period of revolution of these satellites varies from 1d. 18h. for the first, to 16d. 16h. for the last.

Owing to the slight inclination of the plane of their orbits to that of the planet, the three first satellites, and generally the fourth, pass in front of the disk and also through the shadow of the planet at every revolution, and are accordingly eclipsed. Their passages behind Jupiter's disk are called occultations; those in front of it, transits. The eclipses, the occultations and the transits of the moons of Jupiter are interesting and important phenomena; the eclipses being sometimes observed for the rough determination of longitudes at sea.

The satellites in transit present some curious phenomena. When they enter the disk, they appear intensely luminous upon its grayish border; but as they advance, they seem by degrees to lose their brightness, until they finally become undistinguishable from the luminous surface of Jupiter. It sometimes happens, however, that the first, the third and the fourth satellites, after ceasing to appear as bright spots, continue to be visible as dark spots upon the bright central portions of the planet's disk; but in these cases their disks appear smaller than the shadows they cast. Undoubtedly these satellites have extensive atmospheres, since they sometimes pass unperceived across the central parts of Jupiter, this being probably when their atmospheres are condensed into clouds, strongly reflecting light; while when these clouds are absent, we can see their actual surface, with traces of the dark spots upon them similar to those on Mars.

From the variation in the brightness of these satellites, which is said to be always observed in the same part of their orbit, William Herschel was led to suppose that these bodies, like our Moon, rotate upon their axes in the same period in which they move round the planet, so that they always present the same face to Jupiter; but these conclusions have been denied. From my observations it is apparent, however, that the light reflected by them varies in intensity as well as in color. But this is rather to be attributed to the presence of an atmosphere surrounding these bodies, which when cloudy reflects more light than when clear, with corresponding changes in the color of the light.

The satellites in transit are sometimes preceded or followed, according to the position of the Sun, by a round black spot having about the same size as the satellite itself. This black spot is the shadow of the satellite cast on the vapory envelope of Jupiter, similar to the shadow cast by the Moon on the Earth, during eclipses of the Sun; in fact, all the Jovian regions traversed by these shadows have the Sun totally eclipsed. Sometimes it happens that the shadow appears elliptical. This occurs either when it is observed very near the limb, or when entering upon a round, cloud-like spot. This effect is attributable to the perspective under which the shadow is seen on the spherical globe or spot.

The proper motion of the satellites in the Jovian sky is much more rapid than that of the Moon in our sky. During one Jovian day of ten hours, the first satellite advances 84°; the second, 42°; the third, 20° and the fourth, 9°. The first satellite passes from New Moon to its first quarter in a little more than a Jovian day, while the fourth occupies ten such days in attaining the same phase.

In density, as well as in physical constitution, Jupiter differs widely from the interior planets, and especially from the Earth; and, as has been shown, it is surrounded by a dense, opaque, cloudy layer, which is almost always impenetrable to the sight, and hides from view the nucleus, which we may conceive to exist under this vaporous envelope. In 1876, the year of the great Jovian disturbances, I observed frequently in the northern hemisphere of the planet a very curious phenomenon, which seems to prove that its cloudy envelope is at times partially absent in some places, its vapors being apparently either condensed, or transported to other parts of its surface, and that, therefore, a considerable part of the real globe of the planet was visible at these places. The phenomenon consisted in the deformation of the northern limb, which had a much shorter radius on all of this hemisphere situated northward of the white belt which adjoins the equatorial zone. The deformation of the limb on both sides, where it passed from a longer to a shorter radius, was abrupt, and at right angles to the limb, forming there a steplike indentation which was very prominent. The polar segment having a smaller radius, appeared unusually dark, and was not striped, as usual, but uniform in tint throughout. On September 27th, the third satellite passed over this dark segment, and emerged from the western border, a little below the place where the limb was abruptly deformed, as above described. When the satellite had fully emerged from this limb, it was apparent that if the portion of the limb having a longer radius had been prolonged a little below, and as far as the satellite, it would have enclosed it within its border, and thus retarded the time of emersion. The depth of deformation of the limb was accordingly greater than the diameter of the third satellite, and certainly more than 4,000 miles. That the phenomenon was real, is proved by the fact that the egress of this satellite occurred at least four minutes sooner than the time predicted for it in the American Ephemeris. Other observations seem to point in the same direction, since some of the satellites which were occulted have been seen through the limb of Jupiter by different astronomers, as if this limb was sometimes semi-transparent. Another observation of mine seems to confirm these conclusions. On April 24th, 1877, at 15h. 25m. the shadow of the first satellite was projected on the dark band forming the northern border of the equatorial belt, the shadow being then not far from the east limb. Close to this shadow, and on its western side, it was preceded by a secondary shadow, which was fainter, but had the same apparent size. This round dark spot was not the satellite itself, as I had supposed at first, since this object was yet outside of the planet, on the east, and entered upon it only at 16h. 4m. I watched closely this strange phenomenon, and at 16h. 45m., when the shadow had already crossed about ¾ of the disk, it was still preceded by the secondary, or mock shadow, as it may be called; the same relative distance having been kept all the while between the two objects, which had therefore traveled at the same rate. It is obvious that this dark spot could not be one of the planet's markings, since the shadow of the first satellite moves more quickly on the surface of Jupiter than a spot on the same surface travels by the effect of rotation, so that in this case the shadow would soon have passed over this marking, and left it behind, during the time occupied by the observation. From these observations it seems very probable that Jupiter has a nucleus, either solid or liquid, which lies several thousand mites below the surface of its cloudy envelope. It is also probable that the uniformly shaded dark segment seen in 1876, was a portion of the surface of this nucleus itself. When the cloudy envelope is semi-transparent at the place situated on a line with an occulted satellite and the eye of an observer, this satellite may accordingly remain visible for a time through the limb, as shown by observation. The phenomenon of the mock shadow may also be attributed to a similar cause, where semi-transparent vapors receive the shadow of a satellite at their surface, while at the same time part of this shadow, passing through the semi-transparent vapors, may be seen at the surface of the nucleus, or of a layer of opaque clouds situated at some distance below the surface.

Some astronomers are inclined to think that Jupiter is at a high temperature, and self-luminous to a certain extent. If this planet is self-luminous to any degree, we might expect that some light would be thrown upon the satellites when they are crossing the shadow cast into space by the planet; but when they cross this shadow they are totally invisible in the best telescopes, a proof that they do not receive much light from the non-illuminated side of Jupiter. It would, indeed, seem probable that some of the intensely white spots occasionally seen on the equatorial belt of the planet are self-luminous in a degree, yet not enough to render the satellites visible while they are immersed in Jupiter's shadow. It does not seem impossible that the planet should have the high temperature attributed to it, when we remember the terrific storms observed in its atmosphere, which, owing to the great distance of Jupiter from the Sun, do not seem to be attributable to this body, but rather to some local cause within the envelope of the planet.

Astronomy, which is a science of observation, is naturally silent with regard to the inhabitants of Jupiter. If there are any such inhabitants, they are confined to the domain of conjecture, under the dense cloudy envelope of the planet. The conditions of habitability on Jupiter must differ very widely from those of our globe. Comparatively little direct light from the Sun reaches the surface of the globe of Jupiter, except that which passes through the narrow openings forming the dark clouds. All the rest of the planet's surface, being covered perpetually by opaque clouds, receives only diffused light. On Jupiter there are practically no seasons, since its axis is nearly perpendicular to its orbit. The force of gravity on the surface of Jupiter being more than double what it is on the Earth, living bodies would there have more than double the weight of similar bodies on the Earth. Furthermore, Jupiter only receives 0.011 of the light and heat which we receive from the Sun; and its year is nearly equal to 12 of our years. If there are living beings on Jupiter, they must, then, be entirely different from any known to us, and they may have forms never dreamed of in our most fantastic conceptions.

The two round black spots represented towards the central parts of Plate IX. are the shadows of the first and second satellite; while the two round white spots seen on the left of the disk, are the satellites themselves, as they appeared at the moment of the observation. The first satellite and its shadow are the nearest to the equator; while the second satellite and its shadow are higher, the last being projected on the Great Red Spot.[2] The row of dark circular spots represented on the northern, or lower hemisphere, when they first appeared, had some resemblance to Sun-spots without a penumbra, with bright markings around them, resembling faculæ. These round spots subsequently enlarged considerably, until they united along the entire line, encircling the planet, and finally forming a narrow pink belt, which is still visible.

[Footnote 2: By an accidental error in enlarging the original drawing, the satellites and shadows appear in Plate IX. of double their actual size. The error is one easy of mental correction.]

THE PLANET SATURN

PLATE X

Saturn, which is next to Jupiter in order of distance from the Sun, while not the largest, is certainly the most beautiful and interesting of all the planets, with his grand and unique system of rings, and his eight satellites, which, like faithful servants, attend the planet's interminable journey through space.

Seen with the naked eye, Saturn shines in the night like a star of the first magnitude, whose dull, soft whiteness is, however, far from attaining the brilliancy of Venus or Jupiter, although it sometimes approaches Mars in brightness. Saturn hardly ever exhibits the phenomenon of scintillation, or twinkling, a peculiarity which makes it easily distinguishable among the stars and planets of the heavens.

The synodical period of Saturn occupies 1 year and 13 days, so that every 378 days, on an average, this planet holds the same position in the sky relatively to the Sun and the Earth.

The mean distance of Saturn from the Sun is a little over 9½ times that of our globe, or 872,000,000 miles. Owing to the orbital eccentricity, this distance may increase to 921,000,000 miles, when the planet is in aphelion; or decrease to 823,000,000 miles, when it is in perihelion; Saturn being therefore 98,000,000 miles nearer to the Sun when in perihelion than in aphelion. If gravitation were free to exert its influence alone, Saturn would fall into the Sun in 5 years and 2 months.

The distance of Saturn from the Earth varies, according to the position of the two planets in their respective orbits. At the time of opposition, when the Earth lies between the Sun and Saturn, this distance is smallest; while, on the contrary, at the time of conjunction, when the Sun lies between the Earth and Saturn, it is greatest. Owing, however, to the eccentricity of the orbits of Saturn and our globe, and the inclination of their planes to each other, and owing also to the variable heliocentric longitude of the perihelion, the distance of the two planets from each other at their successive conjunctions and oppositions is rendered extremely variable. At present it is when the oppositions of Saturn occur in December that this planet comes nearest to us; while when the conjunctions take place in June, the distance of Saturn from the Earth is the greatest possible. In the former case the distance of the planet from our globe is only 730,000,000 miles; while in the last it is 1,014,000,000 miles, the difference between the nearest and farthest points of Saturn's approach to us being no less than 284,000,000 miles, or over three times the mean distance of the Sun from the Earth.

From the great variations in the distance of Saturn from the Earth, necessarily result corresponding changes in the brightness and apparent diameter of this body. When it is farthest from us, its angular diameter measures but 14"; while, when it is nearest, it measures 20".

The orbit of Saturn is inclined 2°30' to the ecliptic, and its eccentricity, which equals 0,056, is over three times that of the Earth's orbit.

This planet revolves around the Sun in a period of 29 years and 5½ months, or 10,759 terrestrial days, which constitutes its sidereal year. The extension of the immense curve forming the orbit of this planet, is no less than 5,505,000,000 miles, which is traversed by the planet with a mean velocity of a little less than 6 miles per second, or three times less than the motion of our globe in space.

The real dimensions of the globe of Saturn are not yet known with accuracy, and the equatorial diameter has been variously estimated by observers, at from 71,000 to 79,000 miles. If we adopt the mean of these numbers, 75,000 miles, the circumference of the Saturnian equator would measure 235,620 miles, or 9½ times the circumference of our globe; the surface of Saturn would be 86 times, and its volume over 810 times that of the Earth.

However great the volume of Saturn, its mass is proportionally small, being only 90 times greater than that of our globe; the mean density of the materials composing this planet being less than that of cork, and only 0.68 the density of water. The force of gravitation at the surface of Saturn is greater, by a little over ⅑, than it is at the surface of the Earth; a body falling in a vacuum at its surface, would travel 17.59 feet during the first second.

From observations of markings seen on the surface of Saturn, and from the study of their apparent displacements on the disk, William Herschel found that the planet rotated upon its axis in 10h. 16m. 0.24s. Since Herschel's determination, new researches have been made, and lately, Professor Hall, noticing a bright spot, followed it for nearly a month, observing its transits across the central meridian of the disk. From these observations he has obtained for the rotation period 10h. 14m. 23.8s., a result which agrees very closely with that obtained 82 years earlier by Herschel, considering the fact that the markings from which the period of rotation is ascertained are not fixed on the planet, but are always more or less endowed with proper motion. The velocity of rotation at the equator is 21,538 miles per hour, or nearly 6 miles per second.

The axis of rotation of Saturn is inclined 64° 18' to the plane of the orbit, so that its equator makes an angle of 25° 42' with the same plane. The seasons of this planet therefore present greater extremes of temperature than those of the Earth, but not quite so great variations as the seasons of Mars.

The globe of Saturn is not a perfect sphere, but its figure is that of an oblong spheroid, flattened at the poles. The polar compression of Saturn is greater than that of any other planet, surpassing even that of Jupiter. Though not yet determined with a great degree of accuracy, the compression is known to be between ²⁄₁₈ and ⅒ of the equatorial diameter; that is, a flattening of about 3,894 miles, at each pole, the polar diameter being 7,788 miles shorter than the equatorial.

The internal condition of the planet Saturn, whether solid, liquid or gaseous, cannot be discovered from the examination of its surface, as its globe is enwrapped in a dense opaque layer of vapors and cloud-like forms, through which the sight fails to penetrate. The appearance of this vapory envelope is like that of _cumulus_ clouds, and one of its characteristics is to arrange itself into alternate bright and dark parallel belts, broader than those seen on Jupiter, and also more regular and dark. These belts, which are parallel to the equator of the planet, vary in curvature with the inclination of its axis of rotation to the line of sight.

The belts of Saturn, like those of Jupiter, are not permanent, but keep changing more or less rapidly. Sometimes they have been observed to be quite numerous; while at other times they are few. Occasionally conspicuous white or dark spots are seen on the surface, although the phenomenon is quite rare. It is from the observation of such spots that Saturn's period of rotation has been determined, as stated above. The equatorial zone of Saturn always appears more white and brilliant than the other parts, as it also appears more mottled and cloud-like. In late years the globe has been characterized, and much adorned, by a pale pinkish tint on its equatorial belt, resembling that of Jupiter, but somewhat fainter. On either side of the equatorial belt there is a narrower band, upon which the mottled appearance is visible. Below these, one or two dark belts, separated by narrow white bands, are usually seen; but, of late, the bands have been less numerous, being replaced in high latitudes by a dark segment, which forms a polar cap to Saturn. The globe of Saturn does not anywhere appear perfectly white, and when compared with its ring, it looks of a smoky yellowish tint, which becomes an ashy gray on its shaded parts. It usually appears darker near the limb than in its central portions; although on some occasions I have seen portions of the limb appear brighter, as if some white spots were traversing it.

Some observers have seen the limb deformed and flattened at different places, and W. Herschel even thought such a deformation to be a permanent feature of this globe, which he termed diamond-shaped, or "square shouldered." But this was evidently an illusion, since the planet's limb usually appears perfectly elliptical, although it occasionally appears as if flattened at some points, especially where it comes in apparent contact with the shadow cast by the globe on the ring, as observed by myself many times. But with some attention, it is generally found that this deformation is apparent rather than real, and is caused by the passage of some large dark spots over the limb, which is thus rendered indistinguishable from the dark background upon which it is projected.

What distinguishes Saturn from all known planets, or heavenly bodies, and makes it unique in our universe, is the marvelous broad flat ring which encircles its equator at a considerable distance from it. With a low magnifying power this flat ring appears single, but when carefully examined with higher powers, it is found to consist of several distinct concentric rings and zones, all lying nearly in the same plane with the planet's equator.

[PLATE X.--THE PLANET SATURN.

Observed on November 30, 1874, at 5h. 30m. P.M.]

At first sight only two concentric rings are recognized, the _outer_ and the _middle_, or _intermediary_, which are separated by a wide and continuous black line, called the _principal division_. This line, and indeed all the features of the surface of the rings are better seen, and appear more prominent on that part of the ring on either side called the _ansa_, or handle. Besides these two conspicuous rings, a third, of very dark bluish or purplish color, lies between this middle ring, to which it is contiguous, and the planet. This inner ring, which is quite wide, is called the _gauze_ or _dusky ring_. Closer examination shows that the outer ring is itself divided by a narrow, faint, grayish line called the pencil line, which, from its extreme faintness, is only visible on the ansæ. Moreover, the middle ring is composed of three concentric zones, or belts, which, although not apparently divided by any interval of space, are distinguished by the different shadings of the materials composing them. The outer zone of this compound middle ring is, by far, the brightest of all the system of rings and belts, especially close to its external border, where, on favorable occasions, I have seen it appear on the ansæ as if mottled over, and covered throughout with strongly luminous cloud-like masses. On the ansæ of the double outer ring, similar cloudy forms have also been seen at different times. The second zone of the middle ring is darker than the first, the innermost being darker still. All the characteristic points which have thus been described, are shown in Plate X.