Astronomy for Everybody/Part 4/Chapter 7

Among the planets, Saturn is next to Jupiter in size and mass. It performs its revolution round the sun in twenty-nine and a half years. When the planet is visible the casual observer will generally be able to recognise it without difficulty by the slightly reddish tint of its light, and by its position in the heavens. During the next few years it will be in opposition first in summer and then in autumn, about twelve or thirteen days later each year. Starting from August, 1903, opposition will occur in August of 1904-'05, September of 1906-'08, October of 1909-'10, and so on. At these times Saturn will be seen each evening after dark in the eastern or southeastern sky, moving toward the south as the evening advances. It looks a good deal like Arcturus, which, for a few years to come, will be visible at the same seasons, only high up in the south or southwest, or lower down in the west.

Although Saturn is far from being as bright as Jupiter, its rings make it the most magnificent object in the solar system. There is nothing else like them in the heavens, and it is not surprising that they were an enigma to the early observers with the telescope. To Galileo they first appeared as two handles to the planet. After a year or two they disappeared from his view. We ​now know that this occurred because, owing to the motion of the planet in its orbit, they were seen edge-on, and are then so thin as to be invisible in a telescope as imperfect as Galileo's. But the disappearance was a source of great embarrassment to the Tuscan philosopher, who is said to have feared that he had been the victim of some illusion on the subject, and ceased to observe Saturn. He was then growing old, and left to others the task of continuing his observations. Of course the handles soon reappeared, but there was no way of learning what they were. After more than forty years the riddle was solved by Huyghens, the great Dutch astronomer and physicist, who announced that the planet was surrounded by a thin plane ring, nowhere touching it, and inclined to the ecliptic.

Besides his rings, Saturn is surrounded by a retinue of eight satellites—a greater number than any other planet. The existence of a ninth has been suspected, but awaits confirmation. They are very unequal in size and distance from the planet. One, Titan, may be seen with a small telescope; the faintest, only in very powerful ones.

Titan was discovered by Huyghens just as he had made out the true nature of the rings. And hereby hangs a little tale which has only recently come out through the publication of Huyghens's correspondence. Following a practice of the time, the astronomer sought to secure priority for his discovery without making it known, by concealing it in an anagram, a collection of letters which, when properly arranged, would inform the ​reader that the companion of Saturn made its revolution in fifteen days. A copy of this was sent to Wallis, the celebrated English mathematician. In his reply the latter thanked Huyghens for his attention, and said he also had something to say, and gave a collection of letters longer than that of Huyghens. When the latter interpreted his anagram to Wallis, he was surprised to receive in reply a solution of the Wallis anagram announcing the very same discovery, but, of course, in different language and at greater length. It turned out that Wallis, who was expert in ciphers, wanted to demonstrate the futility of the system, and had managed to arrange his own letters so as to express the discovery, after he knew what it was. Huyghens did not appreciate the joke.

The Paris Observatory was founded in 1666 as one of the great scientific institutions of France which adorned the reign of Louis XIV. Here Cassini discovered the division in the ring, showing that the latter was really composed of two, one outside the other, but in the same plane. The outer of these rings has somewhat the appearance of being again divided, by a line called the Encke division, after the astronomer who first noticed it, but the exact nature of this division is still in doubt. It certainly is not sharp and well defined like the Cassini division, but only a slight shade.

To understand the varying appearance of the rings we give a figure showing how they and the planet would look if we could see them perpendicularly (which we ​never can). We notice first the dark Cassini division, separating the rings into two, an inner and an outer one, the latter being the narrower. Then, on the outer ring, we see the faint and grey Encke division, which is much
Fig. 39.—Perpendicular View of the Rings of Saturn. less marked and much harder to see than the other. Passing to the inner ring, the latter shades off gradually on the inner edge, where there is a grey border called the "crape ring." This was first described by Bond, of the Harvard Observatory, and was long supposed to be a separate and distinct ring. But careful observation shows that such is not the case. The crape ring joins on to the ring outside of it, and the latter merely fades away into the other.

The rings of Saturn are inclined about twenty-seven degrees to the plane of its orbit, and they keep the same direction in space as the planet revolves round the sun. The effect of this will be seen by the figure, which shows the orbit of the planet round the sun in perspective. When the planet is at A the sun shines on the north (upper) side of the ring. Seven years later, when the ​planet is at B, the ring is presented to the sun edgewise. After passing B the sun shines on the south (lower) side at an inclination which continually increases till the planet makes C, when the inclination is at its greatest,

twenty-seven degrees. Then it diminishes as the planet passes to D, at which point the edge of the ring is again presented to the sun. From this point to A and B the sun again shines on the north side.

The earth is so near the sun in comparison with Saturn that the rings appear to us nearly as they would to an observer on the sun. There is a period of fifteen years, during which we see the north side of the ring, and at the middle of which we see them at the widest angle. As the years advance, the angle grows narrower and the rings are seen more and more edgewise till they close up into a mere line crossing the planet, or perhaps disappear entirely. Then they open out again, to close up in another fifteen years. A disappearance occurred in 1892 and another will take place in 1907.

​With this view of what the shape of the rings really is, we may understand their appearance to us. The rings are always seen very obliquely, never at a greater angle than twenty-seven degrees. The general outline presented

by the planet and rings is that seen in Figure 40. The best views are obtained when the rings are seen at a considerable angle. The divisions and the crape ring are then seen. The shadow of the globe of the planet on the ring will be seen as a dark notch. A dark line cross​ing the planet like a border to the inner ring is the shadow of the ring on the planet.

Very interesting are rather rare occasions when the plane of the ring passes between the earth and the sun. Then the sun shines on one side of the ring while the other side is presented to us, though, of course, at a very small angle. The chances for observing Saturn at such times are rather few, especially in recent times. At both the last occasions, 1877 and 1892, this only happened for a few days, when the planet was not well situated for these observations. Nevertheless, in October, 1892, Barnard got a look at it from the Lick Observatory, and found that the rings were totally invisible, though their shadow could be seen on the planet. This shows that the rings are so thin that their edges are invisible in a powerful telescope.

When it became accepted that the laws of mechanics, as we learn them on the earth, govern the motions of the heavenly bodies, another riddle was presented by the rings of Saturn. What keeps the rings in place? What keeps the planet from running against the inner ring and producing, to modify Addison's verse, a "wreck of matter and crush of worlds" that would lay the whole beautiful structure in ruins? It was for a time supposed that a liquid ring might be proof against such a catastrophe, and then it was shown that such was not the case. Finally it was made clear that the rings could not be cohering bodies of any kind, but were merely clouds of ​minute bodies, perhaps little satellites, perhaps only particles like pebbles or dust, or perhaps like a cloud of smoke. This view had to be accepted, but was long without direct proof. The latter was finally brought out by Keeler with his spectroscope. He found that when the light of the rings was spread out into a spectrum, the dark spectral lines did not go straight across it, but were bent and broken in such a way as to show that the matter of the rings was revolving round the planet at unequal speeds. At the outer edge it revolved most slowly; the speed continually increased toward the inner edge, and was everywhere the same that a satellite would have if it revolved round the planet at that distance. This most beautiful discovery was made at the Allegheny Observatory near Pittsburg, Pa.

In making known his discovery of the satellite Titan, Huyghens congratulated himself that the solar system was now complete. There were now seven great bodies and seven small ones, the magic number of each. But within the next thirty years Cassini exploded all this mysticism by discovering four more satellites. Then, after the lapse of a century, the great Herschel found yet two more. Finally, the eighth was found by Bond at the Harvard Observatory in 1848.

In 1898 photographs of the sky taken at the South American branch of the Harvard Observatory showed a star near Saturn, but farther than the outermost known satellite, which seemed to be in a different position each ​night. It has not yet been decided whether this was a satellite, because Saturn has been among the countless faint stars of the Milky Way, among which the satellite might be lost.

The following is a list of the eight satellites, with their distances from the planet in radii of the latter, their times of revolution, and the discoverer of each:

The most noteworthy features of this list are the wide range of distances among the satellites, and the relation between the times of revolution of the four inner ones. The five inner ones seem to form a group by themselves. Then there is a gap exceeding in breadth the distance of the innermost of the five, when we have another group of two, Titan and Hyperion. Then there is a gap wider than the distance of Hyperion, outside of which comes Japetus, the outermost yet known.

A curious relation among the periods is that the period of the third satellite is almost exactly twice that of the first; and that of the fourth almost twice that of the ​second. Also, four periods of Titan are almost exactly equal to three of Hyperion.

The result of the latter relation is a certain very curious action of these two satellites on each other, through their mutual gravitation. To show this we give a diagram of the orbits. That of Hyperion, the outer of the

two, is very eccentric, as will be seen by the figure. Suppose the satellites to be in conjunction at a certain moment; Titan, the inner and larger of the two at a point A, Hyperion at the point a just outside. At the end of sixty-five days Titan will have made three revolutions and Hyperion four, which will bring them again into ​conjunction at very nearly, but not exactly, the same point. Titan will have reached the point B, and Hyperion b. At a third conjunction the two will be a little above the line Bb, and so on. Really the conjunctions occur closer together than we have been able to draw them in the figure. In the course of nineteen years the point of conjunction will have slowly moved all round the circle, and the satellites will again be in conjunction at A.

Now the effect of this slow motion of the conjunction-point round the circle is that the orbit of Hyperion, or, more exactly, its longer axis, is carried round with the conjunction-point, so that the conjunctions always occur where the distance of the two orbits is greatest. The dotted line shows how the orbit of Hyperion is thus carried halfway round in nine years.

An interesting feature of this action is that it is, so far as we know, unique, there being no case like it elsewhere in the solar system. But there may be something quite similar in the mutual action of the first and third, and of the second and fourth satellites of Saturn on each other.

A yet more striking effect of the mutual attraction of the matter composing the rings and satellites is that, excepting the outer satellite of all, these bodies all keep exactly in the same plane. The effect of the sun's attraction, if there were nothing to counteract it, would be that in a few thousand years the orbits of these bodies would be drawn around into different planes, all having, however, the same inclination to the plane of the orbit ​of Saturn. But, by their mutual attraction, the planes of the orbits are all kept together as if they were solidly attached to the planet.

There is a remarkable resemblance between the physical make-up of this planet and that of its neighbour Jupiter. They are alike remarkable for their small density, that of Saturn being even less than that of water. Another point of likeness is the rapid rotation, Saturn turning on its axis in 10 hours 14 minutes, a little more than the period of Jupiter. The surface of the planet also seems to be variegated with cloud-like forms, similar to those of Jupiter, but far fainter, so that they cannot be seen with any distinctness.

What has been said of the probable cause of the small density of Jupiter applies equally to Saturn. The probability is that the planet has a comparatively small but massive nucleus, surrounded by an immense atmosphere, and that what we see is only the outer surface of the atmosphere.

A curious fact which bears on this view is that the satellite Titan is far denser than the planet. Its cubical contents are about one ten-thousandth those of the planet. But its mass, as found from the motion of Hyperion, is one forty-three-hundredth that of the planet.