1911 Encyclopædia Britannica/Saturn (planet)
SATURN, in astronomy, the sixth major planet in the order of distance from the sun, and the most distant one known before the discovery of Uranus in 1781. Its symbol is ♄. Its periodic time is somewhat less than 30 years, and the interval between oppositions is from 12 to 13 days greater than a year. It appears as a star of at least the first magnitude, but varies much in brightness with its orbital position, owing to the varying phases of its rings. It seems to resemble Jupiter in its physical constitution, but the belts and cloud-like features so conspicuous on that planet are so faint on Saturn that they can be seen only in a general way as a slight mottling. In colour the planet has a warmish tint, not dissimilar to that of Arcturus. Its density is the smallest known among the planets, being only 0·13 that of the earth, and therefore less than that of water.
Owing to the difficulty of distinguishing any individual feature, the rotation of the planet has been observed only on a few rare occasions when a temporary bright spot appeared and continued during several days. The first observation of such a spot was made by the elder Herschel, who derived a rotation period of 10 h. 16 m. In December 1876 a bright spot appeared near the equator of the planet, which was observed by Asaph Hall at Washington for more than a month. It gradually spread out in longitude, and finally faded away. The time of rotation found by Hall was 10 h. 14 m. 24 s. A third spot appeared in 1903 on the northern hemisphere, and had a rotation period of about 10 h. 38 m. The deviation of this period from the others indicates that, as in the case of Jupiter and the sun, the time of rotation is least at the equator, and increases toward the poles. Both from this difference and from the appearance presented by the planet it is clear that the visible surface is not a solid, as in the case of Mars, but consists of a layer of cloudy or vaporous matter, which conceals from view the solid body of the planet, if any such exists. Owing to the rapid rotation the figure of the disk is markedly elliptical, but when, owing to the rings being seen edgewise, the entire disk is visible, the latter sometimes seems to have the form of a square with its edges rounded off. This may be an illusion.
The most remarkable feature associated with Saturn is its magnificent system of ring and satellites. The former is unique in the solar system. The ring, the seeming ends of which were first seen by Galileo as handles to the planet, was for some time a mystery. After Galileo had seen it at one or two oppositions, it faded from sight, a result which we now know was due to the advance of the planet in its orbit, bringing our line of sight edgeways to the ring. When it reappeared, Galileo seems to have abandoned telescopic observation, but the “ansae” of Saturn remained a subject of study through a generation of his successors without any solution of their mystery being reached. The truth was at length worked out in 1656 by Huygens, who first circulated his solution in the form of an anagram. When arranged in order the letters read:
“Annulo cingitur tenui, plano, nusquam cohaerente, ad eclipticam inclinato.”
This designation of a plain thin ring surrounding the planet, but disconnected from it, and inclined to the ecliptic, is accurate and as complete as the means of observation permitted.
The varying phases presented by the ring arise from its having an inclination of 27° to the orbit of the planet, while its plane remains invariable in direction as the planet performs its orbital revolution There are therefore two opposite points of the orbit, at each of which the plane of the ring passes through the sun, and is seen nearly edgeways from the earth. At the two intermediate points the ring is seen as opened out at an angle of 27°. The apparent illuminated surface which it then presents to us exceeds that presented by the planet, so that the brightness of the entire system to the naked eye is more than double.
In 1665 William Ball or Balle, joint-founder and first treasurer of the Royal Society, discovered that the ring was apparently formed of two concentric rings, separated by a fine dark line. This was afterwards independently discovered by G. D. Cassini at the Paris Observatory. As the telescope was improved, yet other shaded lines concentric with the ring itself were found. These were sometimes regarded as divisions, but if they are such they are by no means complete and sharp. The universal rule is that, if we consider any portion of the ring contained between two circles concentric with the ring itself, the general aspect and brightness of this circular portion are alike through its whole circumference. That is to say, if the brightness of different parts of the ring be compared, it is found to be constant when the parts compared are equally distant from the centre, but subject to variation as we pass from the circumference towards the centre. The inner and broader of the two rings is brightest near the outer part and shades off toward the planet, gradually at first, and more rapidly afterwards. Its inner portion is so dark that it was at one time regarded as separate, and called the “crape” or “dusky” ring. This supposed discovery of an inner ring was made independently by W. R. Dawes of England and G. P. Bond of the Harvard Observatory, though J. G. Galle at Berlin noticed the actual appearance at an earlier date. The more powerful telescopes of the present time show this dusky ring to be continuous with the inner portions of the main ring, and transparent, at least near its inner edge.
The physical constitution of the rings is unlike that of any other object in the solar system. They are not formed of a continuous mass of solid or liquid matter, but of discrete particles of unknown minuteness, probably widely separated in proportion to their individual volumes, yet so close as to appear continuous when viewed from the earth. This constitution was first divined by Cassini early in the 18th century. But, although the impossibility that a continuous ring could surround a planet without falling upon it was shown by Laplace, and must have been evident to all investigators in celestial mechanics, Cassini’s explanation was forgotten until 1848. In that year James Clerk Maxwell, in an essay which was the first to gain the newly-founded Adams prize of the university of Cambridge, made an exhaustive mathematical investigation of the satellite constitution, showing that it alone, could fulfil the conditions of stability. Although this demonstration laced the subject beyond doubt, it was of great interest when J. E. Keeler at the Allegheny Observatory roved this constitution by spectroscopic observation in 1895. He found by measuring the velocity of different parts of the ring to or from the earth that, as we pass from the outer to the inner regions of the ring, the velocity of revolution around the planet increases, each concentric portion of the ring having the speed belonging to a satellite revolving in a. circular orbit at the same distance from the planet. A remarkable feature of the rings is that they are so thin as to elude measurement and nearly disappear from view when seen edgeways even in powerful telescopes. As this can happen only at the rare moments when the plane of the ring passes accurately through the earth, precise observations of the phenomenon with powerful telescopes are few. But before or after the epochs at which the plane passes through the sun, there is sometimes a period of several weeks, during which the sun shines on one face of the ring while the other is presented to the earth. In October 1907 the appearance presented by the rings was studied by W. W. Campbell at the Lick Observatory, and E. E. Barnard at the Yerkes Observatory. The position of the ring as seen against the planet is marked by a dark line stretching across the equator, which is the thin shadow of the ring, on which the sun shines at a very acute angle.
An interesting question still open is the nature of the so-called divisions of the rings. Are these divisions real or are they simply apparent, arising from a darker colour in the matter which composes them? In the case of the sharpest and best-known division, to which the name of Cassini has been given from its first observer, there would seem to be little doubt that the division is real. But there is some doubt in the case of the other divisions. While many excellent observers have sometimes thought they saw a complete separation between the bright and the crape rings, no such phenomenon has been seen in the great telescopes of our times, and it'is almost certain that the dark colour of the crape ring arises merely from its tenuity and transparency. From Barnard's observation of the passage of Iapetus through the shadow of Saturn and its rings it appears that the transparency gradually diminishes from the centre of this ring to its line of junction with the bright ring. If there should ever be a transit of Saturn centrally past a bright star, many questions as to the constitution of the rings may be settled by noting the times at which the star was seen through the divisions of the ring.
Elements of the Satellites of Saturn.
| Mean Longitude. |
Epoch Greenwich Mean Noon. |
Mean Daily Motion. |
Mean Distance. |
Eccentricity. |
Long. of Pericentre. |
Mass Saturn. |
|
| Mimas | 127° 19·0′ | 1889, April | 381·9945° | 26·814″ | Small | Doubtful | 16,340,000 |
| Enceladus | 199° 19·8 | 1889, April | 272·73199° | 34·401 | Small | Doubtful | 4,000,000 |
| Tethys | 284° 31·0 | 1889, April | 190·69795° | 42·586 | Small | Doubtful | 921,500 |
| Dione | 253° 51·4 | 1889, April | 131·534975° | 54·543 | Small | Doubtful | 536,000 |
| Rhea | 358° 23·8 | 1889, April | 79·690087° | 76·170 | Small | Doubtful | 250,000 |
| Titan | 260° 25·1 | 1890, Jan. | 22·5770093° | 176·578 | ·02886 | 276° 15′ + 31·7′t | 4,700 |
| Hyperion | 304° 31·8 | 1890, Jan. | 16·919983° | 213·92 | ·1043 | 255° 47′ – 18·663°t | unknown |
| Japetus | 75° 26·4 | 1885, Sept. 1 | 4·537997° | 514·59 | ·02836 | 354° 30′ + 7·9°t | 100,000 |
| Phoebe | 343° 9·0 | 1900, Jan. | −0·65398° | 1871·6 | ·1659 | 291° 2′ – 0·27°t | unknown |
Saturn is surrounded by a system of nine or (perhaps) ten satellites. The brightest of these was discovered by Huygens in 1665 while pursuing his studies of the ring. The following table shows the names, distances, times of revolution, discoverer and date of discovery of the nine whose orbits are well established:
| Name. | Dis- stance. |
Periodic Time. |
Discoverer. | Date of Discovery. |
| d. h. | ||||
| 1 Mimas | 3·1 | 0 23 | W. Herschel | 1789, Sept. 17 |
| 2 Enceladus | 4·0 | 1 9 | W. Herschel | 1789, Aug. 28 |
| 3 Tethys | 5·0 | 1 21 | G. D. Cassini | 1684, March |
| 4 Dione | 6·3 | 2 18 | G. D. Cassini | 1684, March |
| 5 Rhea | 8·9 | 4 12 | G. D. Cassini | 1672, Dec. 23 |
| 6 Titan | 20·5 | 15 23 | Huygens | 1655, Mar. 25 |
| 7 Hyperion | 25·1 | 21 7 | W. C. Bond | 1848, Sept. 16 |
| 8 Japetus | 59·6 | 79 8 | J. D. Cassini | 1671, October |
| 9 Phoebe | 209·3 | 546 12 | W. H. Pickering | 1898, August |
The five inner satellites seem to form a class by themselves, of which the distinguishing feature is that the orbits are so nearly circular that no eccentricity has been certainly detected in them, and that the planes of their orbits coincide with that of the ring and, it may be inferred, with the plane of the planet's equator. Thus, so far as the position of the planes of rotation and revolution are concerned, the system keeps together as if it were rigid. This results from the mutual attraction of the various bodies. A remarkable feature of this inner system is the near approach to commensurability in the periods of revolution. The period of Tethys is nearly double that of Mimas, and the period of Enceladus nearly double that of Dione. The result of this near approach to commensurability is a wide libration in the longitudes of the satellites, having periods very long compared with the times of revolution.
Each of the four outer satellites has some special feature of interest. Titan is much the brightest of all and has therefore been most accurately observed. Hyperion is so small as to be visible only in a powerful telescope, and has a quite eccentric orbit. Its time of revolution is almost commensurable with that of Titan, the ratio of the period being 3 to 4. The result is that the major axis of the orbit of Hyperion has a retrograde motion of 18° 40' annually, of such a character that the conjunction of the two satellites always occurs near the apocentre of the orbit, when the distance of the orbit from that of Titan is the greatest. This is among the most interesting phenomena of celestial mechanics. japetus has, the peculiarity of always appearing brighter when seen to the west of the planet than when seen to the east. This is explained by the supposition that, like our moon, this satellite always presents the same face to the central body, and is darker in colour on one side than on the other.
In studying a series of photographs of the sky in the neighbourhood of Saturn, taken at the branch Harvard observatory at Arequipa, Peru, W. H. Pickering found on each of three plates a very faint star which was missing on the other two. He concluded that these were the images of a satellite moving around the planet. The latter was then entering the Milky Way, where minute stars were so numerous that it was not easy to confirm the discovery. When the planet began to emerge from the Milky Way no difficulty was found in relocating the object, and proving that it was a ninth satellite. Its motion was found to be retrograde, a conclusion confirmed by Frank E. Ross. This phenomenon may be regarded as unique in the solar system, for, although the motion of the satellite of Neptune is retrograde, it is the only known satellite of that planet.
Another extremely faint satellite has probably been established by Pickering, but its orbit is still in some doubt. The conclusions from the spectrum of Saturn, and numerical particulars relating to the planet, are found in the article Planet. The planes of the orbits of the inner six satellites are coincident the plane of the ring system, of which the elements are as follow:
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(S. N.)