Part 17

The density of Saturn is less than that of any other planet, and it is ten times less than that of the earth. No other planet is less dense than water; but Saturn would float in water, and is not more dense than cork. On account of its mass its gravity is greater than that of the earth by about one-tenth. This is not enough to make a very interesting difference in the weight of objects on Saturn and on the earth. The average person weighing one hundred and fifty pounds here would weigh only one hundred and sixty-five pounds on Saturn. The numerous penny-in-the-slot weighing-machines vary almost that much. Saturn has eighty-three times more surface than the earth, and more than seven hundred and fifty times the earth’s volume.

SURFACE ASPECTS AND CONSTITUTION

It is not at all certain that Saturn, more than Jupiter, has any solid surface. Indeed, it is almost certain that it has not. It is surrounded by an atmosphere of great density, and we do not at any time see the surface of the planet. It is believed probable that it is at least largely in a liquid state, if not to a great extent even gaseous.

The planet is certainly not in any way dependent on the sun for the extraordinary heat that everything indicates it to have, and its surface is brighter than it is believed it could be if shining only by the reflected light of the sun. This does not mean that Saturn is self-luminous; but it is nearly certain that it is extremely hot and glowing, and its brightness may be in part due to its own internal fires and the extremely luminous and dense atmosphere that surrounds it. It receives one hundred times less heat and light from the sun than we do. If it depended entirely upon the sun for its heat, the temperature would be nearly three hundred degrees below zero, Fahrenheit. It is probably not only very hot itself, but its heavy atmospheric envelope perhaps allows comparatively little heat to escape.

Its surface is belted and spotted somewhat after the manner of Jupiter’s, but, being so much farther from us than Jupiter, it does not disclose its surface features with the same distinctness. Apparently it is much less turbulent than Jupiter; but even this we are not quite certain of, and it may seem more placid because we do not so well see its agitations.

Like all the outer planets, it differs in its constitution from the earth and the other inner planets. Its atmosphere contains compounds with which we are not familiar, and the body of the planet itself is rarer and lighter, and less condensed, and in a much earlier stage of evolution than the earth and the small planets so comparatively near us.

DAY AND NIGHT

The length of Saturn’s day, or its period of rotation on its axis, is about ten hours and a quarter. Like Jupiter, it has slightly different rates of rotation in different latitudes, thus showing its lack of solidity. The rate of rotation has been determined, as in the case of Jupiter, by observation of the spots on its surface, which, while they are not exactly permanent, yet remain apparently in the same positions for months and even years at a time, and are thus sufficiently stable to measure a rotation of so short a time as ten hours.

Whirling over at this rate would cause the sun to appear to skim across the sky very swiftly as viewed from Saturn. In size, it would not seem more than three times as large as Venus at her brightest seems to us, and every minute it would cover a distance about equal to the diameter of the full moon as we see it. In an hour it would seem to move more than six times as far as the distance between the “pointers.” At the time of Saturn’s equinox the little five-hour day, followed by the equally short night, must present a lively aspect with the sun racing thus swiftly across the sky in daylight and the stars sweeping as swiftly over at night. If things remain as they now are, it will be a splendid panorama for the people there when, in the far-distant future, Saturn may have cooled and solidified sufficiently to maintain life somewhat as we know it. The earth, though, and Venus and Mars would be from Saturn only telescopic objects to eyes like ours, and Jupiter no brighter than he is to us. Thus does our brother Saturn pay the price of his remoteness from the rest of the solar family.

THE RINGS AND MOONS OF SATURN

But the circling stars and the swift-moving sun are the least part of the splendid spectacle that might be seen from Saturn. He is surrounded with no less than ten moons of more or less imposing size, and in addition has three rings circling around with him, composed of myriads of small satellites, together forming a band the outer diameter of which is something more than twenty-one times broader than the diameter of the earth. These are the famous rings of Saturn, the only objects of their kind in the solar system, intensely interesting to scientific observers, wonderful to the curious, and splendidly beautiful to everybody. It is this profusion of rings and moons that entitles Saturn to be called, as he often has been, the most spectacular of all the planets.

The outer ring is nearly ten thousand miles broad, and is separated from the next one by a space of about seventeen hundred miles. The second ring is nearly eighteen thousand miles across. It is very bright on the outer edge, but gradually grows less so, until, with a not very perceptible division, it fades into the inner ring, which is but slightly luminous, and is called the crape ring. This is about nine thousand miles broad and nearly ten thousand miles from Saturn. This gradual fading of the rings to a dusky hue toward the center, and then the blackness of the space between them and the planet, gives them from certain points of view a nest-like appearance; and my first impression of Saturn, when I saw him through the telescope, was that he was nestling in a concave body of light--an appearance that is intensified by his extreme flatness at the poles.

Notwithstanding the imposing breadth of these rings, they are less than a hundred miles in thickness. They are, in fact, nothing more than an untold number of tiny satellites revolving about Saturn in the same plane and close enough together to appear, at the distance they are from us, as if they were one body. Just how close they are together, and how they appear when near by, we do not yet know. It was first shown by mechanical laws that they must be composed of separate bodies; the spectroscope shows that they are; and it has recently been thought that they have even been seen to be so through a telescope.

Being all in the same plane, they form a flat, broad, thin ring, so thin that when the edge of the ring is turned toward us we cannot see them at all. We never see them at their full breadth. If we did, Saturn would be much brighter at times than he ever is. The plane in which they revolve is the plane of Saturn’s equator; and the axis of Saturn, with the rings, has a tilt of twenty-seven degrees in his orbit. The result of this is that at the time of Saturn’s equinoxes the edge of the rings is turned toward us, and they practically disappear. Half-way between the equinoxes they are open again as far as they ever are to our view. This is why Saturn alternates in brightness. The times of his equinoxes occur every fourteen and eight-tenths years, and he is then alternately in Leo and Aquarius and is least bright. The times at which the rings are most open occur at intervals of the same length, and he is then alternately in Scorpio and Taurus and at his brightest.

Photographed at Mt. Wilson by E. E. Barnard, the six exposures being made on one plate.]

It is believed that Saturn’s rings were never a part of the planet, but are mere particles of cosmic materials which happened to be left over, and which he has gathered up by his force of gravity and compelled to revolve about him.

Saturn, more fortunate than Jupiter, has escaped the unimaginative naming of his moons by number, though one would think that, having such a numerous offspring, a shortage in names would be more likely to occur in his than in any other planet family. They all have names more or less connected with the great god whose name the planet bears, and are, in order of their distance from Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Japetus, Phœbe, and Themis. The largest and brightest of them all is Titan. It is larger than our moon, which is one of the large moons in the solar system, or than Mercury, and is not much smaller than Mars. It is more than three-quarters as large as all the other moons of Saturn put together. Naturally, it was the first to be discovered, and was under observation as long ago as 1655. Rhea and Japetus are next in size, and were discovered in 1671–72; Dione and Tethys were both discovered in 1684, and Enceladus and Mimas in 1789.

Until 1848 seven moons were all that were known to belong to Saturn. In that year little Hyperion, whose diameter, it is thought, can hardly exceed two hundred miles, came into our view. A little more than fifty years later (in 1898) Phœbe made her bright mark on a photographic plate at Harvard, and was caught. By tracing her from one plate to another her orbit was computed, her probable size determined, and practically all that is known about her was found out before she was seen, which was not until 1904. She is not much larger than a good-sized mountain, but is a unique and interesting little satellite that, far outside of the paths of any of the other moons, circles in an eccentric orbit around Saturn in an opposite direction from the rest of the satellites, and thus gives rise to many interesting astronomical speculations. Themis, also a tiny body, was discovered in the same way in 1906, and is thought to be the smallest body in the solar system. Titan is the only one of this group of satellites whose true disc we can see even with a telescope. Only one other (Rhea) can be seen in transit across the planet. The others are not much more than bright points of light, while Phœbe and Themis are almost at the limit of visibility.

On account of their great distance from the sun Saturn’s moons are, of course, not very bright, and all of them put together do not give one-tenth as much light to Saturn as we receive from our moon. But, such as they are, they may some day be very useful to Saturn as a means of illumination. Receiving as he does a hundred times less light from the sun than we do, he may be some day much in need of the light reflected from all his rings and moons.

SEASONS

The seasons on Saturn are somewhat like ours in the succession of spring, summer, autumn, and winter; but the inclination of its axis to its orbit being twenty-seven degrees instead of twenty-three and a half, as ours is, each season is much more accentuated than ours. The sun climbs higher during the northern summer, and sinks correspondingly lower during the winter. But in length Saturn’s seasons are very different from ours. Like his year, they are about twenty-nine and one-half times as long as ours. Each one is more than seven years long. Even the agreeable seasons might grow monotonous to one in that time; but to be spinning through the rapidly alternating days and nights of Saturn during seven long years of winter is a situation that one does not care to contemplate. It is with world personalities as with human personalities: however much we may admire their superior grandeur, when we consider details we would not change places with them.

The symbol of Saturn is an ancient scythe (♄), which gets its appropriateness from the fact that the deity of that name was the special protector of agriculture.

URANUS

Venus, Mars, Jupiter, and Saturn, brilliant beauties that they are, have always been distinguished features of the heavenly view. The records of Mercury do not go back so far as those of these more easily seen planets, yet there is no reason to think that he has not been always known, though less widely, perhaps, than the four planets more frequently in view. To Uranus belongs the distinction of being the first planet that was discovered--a distinction that one cannot help but feel was too long delayed, for it did not come until 1781. For ages and ages his lovely pale beams had been shining down upon us from his little disc, no fainter in brilliancy than many a sixth-magnitude star (a degree of brightness which we think is within the limit of good vision, even in these days), and no human being had been conscious that this bright body was only another member of the solar family, circling with the rest of us around our parent, the sun, and having nothing in common with the far-off stars among which we had numbered him. Nineteen times he had been charted as a fixed star before his identity was suspected, and after he became known to us as a planet he was, by means of these charts, traced back for one hundred and thirty years, and much information was thus gained concerning his orbit and movements.

Uranus was not, however, discovered through observation of his movement among the stars. A view of his actual disc was caught by the musician and astronomer, Herschel, as he gleaned with his telescope in that part of the sky where the planet lay, one hundred and seventy-one years after the invention of that aid to vision. It was at first thought that a comet had been discovered, but later investigation showed a much more important member of the solar system, and the discovery of a new planet was announced.

George III. was then King of England, and the loyal Herschel called the planet Georgium Sidus in honor of that monarch. Fortunately, the world-wide interest in this newly discovered body saved it from so local an appellation, and it finally came to be called after Uranus, the father of Saturn, a name somewhat more in keeping with its place among the planets. In England, however, a very commendable loyalty to Herschel has resulted in the planet’s sometimes being called Herschel, after its discoverer, and we see this name often in English books on astronomy, especially the older books; but Uranus is now the generally accepted name.

The symbol of the planet as it appears in all almanacs--at least in all English almanacs--is a capital H with a planet swinging from the cross-bar in the letter, thus ♅. And to this extent the discovery of the planet by Herschel is commemorated. In American almanacs the symbol is contracted into this figure ♅.

It is a matter for regret that Uranus does not come more easily within our view; for he is a very beautiful planet, pale green in color, and unlike any of the others in his aspect. There are, however, very few persons nowadays who can see him without the aid of at least a small glass, and to most of us he must ever remain a body with which we can have no personal acquaintance. None the less he must have an interest to us such as attaches always to anything so closely related to us, and sharing with us a common origin and a common destiny. To those who have unusually keen vision--or a small telescope--there will be much pleasure in viewing the planet. But even to those who have not these facilities for seeing, it ought to be interesting to know in what region of the skies this far-off member of our family dwells, what his wanderings are, and something of his personality and habits.