A Glimpse of Saturn's Rings: Galileo's Observations and Discoveries

On a memorable evening in the year 1610 Galileo sat in the tower of his observatory in Florence, and gazed through his newly-invented “perspective glass” at Saturn, which was then regarded as the most distant of the planets. The astronomer was astonished to see the planet flanked by two smaller globes, one on each side, and with characteristic prudence, and by the fashion of the age, he made a record of his discovery in the form of an anagram, which admitted an almost infinite number of interpretations, and sorely tried the patience of the indomitable Kepler, until the answer to the riddle appeared in Galileo's letter to Giuliano de Medici. Two years later, however, Galileo found that these attendants of Saturn had vanished. The inexplicable character of these discoveries is said to have vexed GaJileo so greatly, that he never deigned to cast another glance on the mysterious planet. Later observers were deceived in the same way. Saturn surrounded by its rings appeared to Scheiner and Hevel, in 1614, as a disk with two projecting ears. The Jesuit priest Eustachius de Divinis, in 1647, and Riccioli, in 1648, made a close approximation to the real form of the planet, but the mathematically-trained Huygens, in 1655, first described in FIG. l.-SATURN AND ITS RINGS, AS DRAWN IN 1894 BY LEO BRENNER, DIRECTOR OF THE MANORA OBSERVATORY AT LUSSINPICCOLO.
1. Encke's line.
2. Casf. line's division.
3. The gauze or crape ring.
correctly as being “surrounded by a thin flat ring, nowhere in contact with it, and inclined to the ecliptic.” Saturn, with its rings and satellites, forms so true a picture of an early stage in the development of the solar system according to Laplace's theory of cosmogony, that one is almost tempted to regard it as a living proof of the correctness of that hypothesis. From the newer and more probable meteoric theory, the late English astronomer Proctor derived the conclusion that Saturn is the second planet of the solar system in order of creation. This theory assumes that in the beginning the entire field of the solar system was filled with solid particles of cosmical matter which, coalescing in consequence of mutual collisions, formed first a principal nucleus of attraction (the sun) and then subordinate nuclei (the planets), each of which, as it revolved around the sun, captured all the remaining particles that came within its sphere of attraction. Saturn is but faintly illuminated by the sun, its mean distance from which is nearly 900 million miles. The Hindus called it “Sanaistshara,” (slowly moving— a name given also to Vishnu in the Vedas), because the planet so leisurely follows the eastward course of the sun among the stars, tarrying two years and six 6 5 4 3 2 1 FIG. 2.--SATURN AND ITS RINGS, AS DRAWN IN 1898 BY LEO BRENNER. 1. Encke's pencil line. 2. Caseini's division. 3. Secchi's line. 4. Bond's line. 5. Manora line. Ii. Struve's line. months in each constellation of the zodiac, through which the sun passes in a single month. This apparent motion of the planet, however, is not uniform but is affected by the periodic shifting of our point of view, the earth, so Saturn is not always easily found and distinguished among the stars. When the planet is viewed through a large telescope, it is seen to be greatly flattened at the poles. Saturn has been weighed in the astronomer's balance and found to contain twice as much matter as Mercury, Venus, the earth, Mars, Uranus, and Neptune combined. Its mass is 93 times that of the Earth, and Jupiter alone exceeds it in magnitude. The equatorial diameter of Saturn is about 73,000 miles, but the polar diameter is only 66,000 miles. The great planet consumes 29% of our years in making one revolution around the sun, from which its mean distance is about 895 million miles. At this distance, the intensity of solar radiation is only 1-90 of that which falls on the earth. It might be inferred from this fact that Saturn must have an Arctic climate and be covered with glaciers, but ice caps, like those of Mars, have never been observed at the poles of Saturn.
SATURN'S RINGS AS THEY WOULD APPEAR TO AN OBSERVER ON THE PLANET, FIFTY DEGREES FROM THE EQUATOR. Hence we are justified in assuming that Saturn is still hot and, to some extent, self-luminous. Very probably it is yet in the stage of crust formation and preparation for the development of organic life, or in the stage of cooling through which the earth passed millions of years ago when it was covered with forests of huge ferns and grasses and inhabited by gigantic saurians. Each of the four seasons of Saturn's year continues through seven of our years. The inclination of its equator to the plane of its orbit, 27 degrees, is such as to make the distribution of climate and the phenomena of change of seasons similar to those of Mars and the Earth. Saturn not only possesses an atmosphere, in which Joussen has found spectroscopic indications of the presence of hydrogen but is enveloped in a mantle of dense clouds, as the earth was millions of years ago and Jupiter is today. The spectrum of Saturn is very similar to that of Jupiter. Both show dark absorption bands in the red and orange, which point to a close resemblance in the physical constitution of the two great planets. In the vapors that envelop both, also, peculiar cloud masses have also been observed, including dark bands parallel to the equator. In the case of Saturn these phe- SATURN'S RINGS AS THEY WOULD APPEAR TO AN OBSERVER ON THE PLANET, SEVENTY DEGREES FROM THE EQUATOR. noumena are very variable and are indistinct because of the feeble illumination that the planet receives from the sun. From them, however, Sir William Herschel computed the period of rotation at 10 hours and 16 minutes. In 1876 a very bright spot appeared near the equator of Saturn and remained visible for several months. Like the red spot once seen on Jupiter, it was probably the result of some great convulsion on the surface of the planet. From the motion of this spot, Prof. Asaph Hall, director of the Naval Observatory at Washington, deduced a period of rotation of 10 hours, 14 minutes, and 24 seconds. These figures, however, were only provisional. The atmospheric strata of Saturn do not rotate at the same rate as the body of the planet, and these spots and clouds have relative motion concerning the atmosphere. In 1903 very conspicuous but variable dark and bright spots appeared on the northern hemisphere of Saturn, indicating another series of great atmospheric disturbances. From all the spectroscopic and other observations that bear upon the rotation of Saturn, a period of 10 hours, 14 minutes, and 6 seconds—nearly identical to Hall's result—has been deduced as the most probable value. This rapid rotation greatly reduces the weight of objects at the equator, where gravity would be entirely neutralized by centrifugal force if Fig. 5.-RELATIVE DISTANCES OF THE SATELLITES FROM SATURN. The actual mean distance between Saturn and Phoebe is 8,000,000 miles. the planet rotated 2% times faster than it does. For the time of rotation of Saturn's ring, Laplace gave the value 10 hours, 29 minutes, and 17 seconds. More recent observations give 7 hours and 45 minutes for the inner edge of the first bright ring, and 5 hours and 39 minutes for the inner edge of the innermost, or “gauze” ring. The mean time of rotation of the rings, however, is greater than this and is estimated as equal to 10 hours and 29 minutes, a value almost identical to that given by Laplace. The series of nearly concentric rings that encircle Saturn in the equatorial plane appears like a survival of primordial conditions. It is the most astonishing object that is visible in the solar system or that could well be imagined. There are three principal rings and several minor subdivisions. The innermost, dimly- shining ring, called the “gauze” or “crape” ring, begins at a distance of 6,400 miles from the surface of the planet, and is 8,400 miles wide. This ring was discovered in 1836 by Galle, the discoverer of Neptune, and was carefully studied by Bond and Dawes. Astronomers have detected or fancied numerous subdivisions in this faint ring, which is now generally regarded as a cloud of cosmical dust, similar to the cloud that causes the phenomenon of zodiacal light. The disk of Saturn is seen through this ring in un-diminished brightness, and in May 1905, Saturn's satellite Iapetus passed bodily through the gauze ring. The circumstances and consequences of this passage proved that the gauze ring is composed of separate particles, which are either smaller or less closely aggregated than those that form the outer rings. The gauze ring merges by imperceptible gradations into a moderately bright ring about 18,000 miles wide, which is separated by an interval of 1,450 miles known as “Cassini's division,” from the outer very bright ring, the breadth of which is 10,000 miles. The outer edge of the exterior ring is about 45,000 miles distant from the surface of the planet. This remarkable series of rings, which surrounds Saturn as the brim of a hat surrounds the wearer's head, possesses no atmosphere, for the characteristic atmospheric line in the red is absent from its spectrum. Twice in each revolution of the planet about the sun the rings vanish or dwindle to a narrow line, and twice they open into the broad ovals shown in Fig. 2. These are the changes which so greatly puzzled and annoyed Galileo. When the plane of the rings passes through the sun, an event which occurs whenever, as at present, Saturn in opposition is found in the constellation Pisces or the constellation Leo, the rings disappear. When the planet is in opposition in Sagittarius, or between Taurus and Gemini, the rings appear broadest, and Saturn shines more brightly than a star of the first magnitude. Finally, when the rings are invisible and the planet is near conjunction, it appears like a star of the magnitude 1%. An interval of seven years and four months elapses between the maximum brightness and the disappearance of the rings as described above. But the rings are likewise invisible whenever their plane passes between the earth and the sun because their illuminated side is turned away from us. In this case, we see only the shadow cast by the rings on the planet. A very fine drawing of the shadow as observed in 1856 was made by Secchi, and the shadow has often been drawn from observation by other astronomers. From the breadth of the shadow, it is inferred that the rings are about 60 miles thick. To an inhabitant of Saturn, the rings would present a wonderful picture at night, spanning the landscape like an immense rainbow, colorless butdazzling. By day, however, they would eclipse the sun in the regions covered by their shadow. Viewed through the small telescope of early observers, the rings appeared as one. In 1665 Ball discovered indications of the principal dark line of division, which was afterward seen more distinctly by Maraldi and Cassini and named “Cassini's division.” Encke discovered a much finer line, which is called “Encke's pencil line,” and later observers have increased the number of distinct rings to seven or more (Fig. 2). The intervals between the rings may be compared with the void spaces in the asteroid belt, where asteroid orbits are made impossible by the disturbing influence of the great planets. The rings of Saturn, if they are not continuous masses of cosmical matter, must be swarms of billions of small satellites, each of which travels in its orbit subject to the attractions of all the rest and all other bodies in the universe, by the inexorable law of gravitation. Probably the complex motions of this vast swarm of satellites have become so adjusted that no collisions occur. Saturn's ring was long believed to be a continuous solid mass. Bond and Peirce regarded it as liquid, but in 1884 Pratt discovered a distinct granulation that suggested its true nature. Now we know with certainty from the photometric investigations of See- liger, of Munich, that the rings are made up of countless separate particles, which change their relative positions, and that the apparent continuity is merely an effect of distance. The talented and prematurely deceased American astronomer Keeler proved by spectroscopic observations interpreted by DOppler's principle that the inner parts of the ring move more swiftly in a radial direction (in the line of vision) than the outer parts. Hence the ring can not be a rigid mass. Saturn also. possesses at least nine larger satellites or moons, and probably a tenth (Thermis). The following table gives the principal data concerning these satellites: Q 0:1 10 7 Name of Satellite Mimas Enceladus Thetis... Dione Rhea Titan Hyperion. Tapetus... Phoobe.... Themis Discover. Sir Wm. Herschel. SirWm. Herschel. D. Cassini ]>. Cassini D. Cassini Huygens Bond and Lasse!.. D. Cassini H. W. Pickering. H. W. Pickering. u cy o a t/] lime of Revolution o Q Days HrP. O 1789 o 1789 1 9 1684 I 21 1684 2 18 1672 4 12 1655 15 23 1818 21 7 1671 79 8 1898 546 12 1905 3 4 5 6 8^ 20 24 57 207 Phrebe was first actually seen by Barnard in 1904, after its motions had been photographically traced for years. It is only about 60 miles in diameter. Its motion was long thought to be retrograde, but this question, with that of the existence of the tenth satellite, must be left to the future for the final answer. The satellite Lapetus is peculiarly interesting because of its periodic changes in brightness, which are probably due to differences in the character of its surface at different points. Our present knowledge of Saturn makes it certain that neither the planet nor its rings can be inhabited by creatures at all resembling human beings. It is not improbable, however, that the surface of the planet may have cooled sufficiently to develop a flora and fauna similar to those which occupied the earth in the carboniferous period.—Translated for the SCIENTIFIC American Supplement from Illustrirte Zeitung.