The Rings and Satellites of Saturn

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Nature Magazine, February 1949

On February 21 Saturn will be in opposition to the sun and above the horizon from sunset to sunrise. At this time its brightness will be very nearly equal to that of Procyon, in Canis Minor, now visible in the evening sky. By the end of the year, however, it will have decreased in brightness until it will be no brighter than Regulus in Leo which is now not far distant from Saturn.

In the year 1944, at the time of opposition of Saturn to the sun in December, the planet was so brilliant that it surpassed all stars in brightness with the exception of Sirius and Canopus. At that time the southern side of the rings of Saturn was tipped toward the earth at an angle of 27 degrees, which is close to the maximum amount of elevation of the earth above the plane of the rings of Saturn. The exceptional brilliancy of Saturn at that time arose from the fact that the rings were then at their widest extent as seen from the earth and were very considerably brighter than the surface of Saturn.

From 1944 on, due to the continual decrease in the elevation of the earth above the ring-plane, the width of the rings has steadily lessened. At the beginning of this year the elevation of the earth above the southern surface of the rings was only about 7 degrees and the elevation is still approximately that amount this month at opposition. The angle will fluctuate a few degrees during the year, due to the earth's changing position in its orbit, and will increase to about 10 degrees during the next few months so that they will be fairly well placed for observation from the earth for the first half of 1949; there will be, however, a rapid decrease in the angle at which the rings are tipped to the earth after Saturn's conjunction with the sun in September. By the end of December the elevation of the earth above the ring-plane of Saturn will be only 1 ½ degrees and the rings will be very narrow and poorly placed for observation from the earth.

Twice during its 29 ½-year period of revolution around the sun, Saturn is at a point where the plane of the rings, which lies in the plane of Saturn's equator, crosses the ecliptic. These two points of intersection of the ring-plane with the ecliptic are on opposite sides of the planet's orbit and are called the "nodes" of the ring-plane on the ecliptic. When Saturn is at or near these points in its orbit the earth passes through the ring-plane from one side of the rings to the other, and at that time they become invisible for a brief period even in the most powerful telescopes. Saturn is now approaching one of these points and will reach it on September 14, 1950. At that time the earth will cross from the south to the north side of the rings and they will temporarily be invisible. Unfortunately, at that time the planet will be within a day of conjunction with the sun and so the disappearance of the rings at that time cannot be observed. For more than a year before and after that date, however, certain interesting phenomena will be observable that cannot be seen at other times. At and near the time of disappearance of the rings the surface of Saturn is seen to the best advantage. For then our view of the disc of the planet is little obstructed by the rings, which appear almost on edge as seen from the earth. The remarkable flattening of Saturn at the poles, greater than that of any other planet, is very conspicuous at this time. It amounts to about one-tenth of the equatorial diameter and as a result of it the polar diameter of Saturn is about 7500 miles shorter than its equatorial diameter. The belts in the atmosphere of Saturn, which parallel the planet's equator and are similar to those of Jupiter, are also seen to the best advantage at this time, as are also other temporary spots and markings in the planet's atmosphere.

The rings of Saturn, compared to their great extent in the plane of Saturn's equator, are unbelievably thin. The extent of the ring-system from the outer edge of the outer ring to the corresponding point on the opposite side of Saturn is approximately 171,000 miles. The inner edge of the inner ring, called the dusky or crepe ring, is only 7,000 miles above the surface of Saturn and its width is about 11,500 miles. It is the least dense of the rings and is faintly luminous. The central, brightest, and largest ring is 16,000 miles wide and is separated by a gap of about 3000 miles, known as Cassini's Division, from the outer ring which is about 10,000 miles wide. The inner and outer rings are semi-transparent and stars have been seen through them, but the central ring is very brilliant and quite opaque. The thickness of the rings has been estimated to be not more than ten miles and the individual moonlets of which the rings are composed are merely small meteoric or dust-like particles, each of which is pursuing its own orbit around the planet. It is probable that collisions between particles in the rings are frequent, and as a result of them it is probable that no large meteoric masses still remain.

It has been suggested that the rings are the pulverized remains of a satellite that came so close to the surface of the planets that tidal action eventually tore it asunder.

In addition to its rings Saturn has a large family of nine satellites which have been divided into three groups according to their distance from the planet. Within the innermost group are the satellites Mimas, Enceladus, Tethys, Dione and Rhea, named in the order of their distance outward from the planet. All lie within a distance of 300,000 miles of the surface of the planet and all are either in or very close to the ring-plane and equator of die planet. Mimas skirts around the edge of the outermost ring at a distance of only 30,000 miles from it. Its orbit and that of all the satellites in this group are very nearly circular. Mimas and Enceladus are difficult objects to observe because of their nearness to the bright ball of the planet. Enceladus can at times be seen with a six-inch telescope, however. Tethys and Dione are visible in a four-inch telescope. Rhea, which is the brightest of the five, can be seen with a 2 ½-inch telescope. The diameters of Mimas and Enceladus are about 370 and 460 miles, respectively, of Tethys 750 miles, Dione 900 miles, and Rhea 1150 miles. A gap of 400,000 miles separates this group of inner satellites from Titan and Hyperion, which form an interesting group together. Hyperion is about 160,000 miles more distant from Saturn than Titan. The diameter of Hyperion is only 300 miles. Titan has a diameter of 3550 miles and is the largest satellite in the solar system, not even with the exception of our moon, and the only one that is known to possess an atmosphere. It can be seen with any instrument greater than an inch in diameter and can easily be recognized not only by its superior brightness but by its decidedly reddish color as well. Dr. G. P. Kuiper discovered in 1944 with the 82-inch reflector of the McDonald Observatory, that this satellite showed the presence of both methane and ammonia in its atmosphere. The orbits of both Titan and Hyperion lie in the plane of the rings but the orbit of Hyperion is greatly perturbed by Titan. Hyperion has been easily found in a six-inch telescope when in conjunction with Titan, which always occurs when the two bodies are farthest apart. A gap of more than a million miles separates Hyper ion from the third group of satellites, which consists of Japetus and Phoebe. The diameter of Japetus is about 1000 miles and that of Phoebe is about 150 miles. This remarkable little body is moving in backward motion around Saturn, that is, in an east to west instead of west to east direction, and its orbit, like that of Japetus, is highly inclined to the plane of the rings and very eccentric. It is the most distant from Saturn of all its satellites, more than 8,000,000 miles, and it takes 550 days to complete one revolution around Saturn. Japetus is remarkable for the fact that it varies very noticeably in brightness. It seems probable, from the observations, that one side is brighter than the other and that it always keeps the same side turned toward the planet. Under favorable circumstances Japetus can be seen in a three-inch telescope. At time of disappearance of the rings and, in fact, for a year or more before and after that time, the six inner satellites of Saturn present the same phenomena that are so easily observable and so interesting a feature of the four historic satellites of Jupiter. These are the eclipses in the shadow of Saturn, occultations of the satellites behind the disk of the planet, and transits of the satellites and their shadows across the disk of Saturn. Even before the end of 1948 some of these phenomena were observable in the case of Tethy and Dione. The phenomena were observable for Mimas and Enceladus even earlier, but were not generally predicted because of the difficulty of observing them. Phenomena for Rhea and Titan become visible this year.

On March 2, 1921, about the time the earth was passing through the plane of the rings from the south to the north side, a remarkably interesting photograph was taken showing the rings practically as a line and the six innermost satellites strung out along the plane of the rings like beads on a wire, small brilliant points of light. Enceladus was on one side of Saturn and the other five satellites on the opposite side.

In February Saturn will be the only bright planet visible in the evening sky. Mars, though still in the evening sky, is too close to the horizon at sunset to be observed. Mercury may be seen in the morning sky ten days or more before the date of its greatest western elongation on February 28, and Jupiter, which passed to the morning sky on January 1, may now be seen in the morning sky in Sagittarius. Venus is still in the morning sky, as always a brilliant object but slowly drawing nearer to the horizon at sunrise.