L and III. are in conjunction II. is in opposition to both, is that the three satellites are never in conjunction together. Thus, starting from a conjunction of I. and III., with II. in opposition, we have, after almost exactly one revolution of I., II. and I. in conjunction, and III. in quadrature to both ; after another, II. and III. are in conjunction, and I. in opposition to both; after another, II. and I. are in con junction, and III. in quadrature to both ; and after a fourth, the same arrangement is resumed as at first, I. and III. being in conjunction, and II. in opposition to both, but the line on which all four satellites now lie holding a slightly different position from that occupied when these circuits began. Thus the satellites can never be all occulted
together, or all in transit together.
Chapter XV.—The Planet Saturn.
Saturn is the largest planet but one of the solar system, being surpassed in mass by Jupiter alone. His mass exceeds the combined mass of all the other planets (Jupiter excepted) nearly threefold. He is the sixth planet in order of distance from the sun, and was the remotest planet known to the ancients. He travels at a mean dis tance of 872,137,000 miles from the sun, his greatest distance being 920,973,000 miles, his least distance 823,301,000 miles. The eccentricity of his orbit is 0-055996, and the inclination 2 29 28" to the plane of the ecliptic. Saturn s mean sidereal period of revolution in his orbit is 29 years 167 days (see the Elements, p. 782); his mean synodical revolution, or the mean interval between his successive returns to opposition, is 378 - 090 days. The planet s mean diameter is about 70,000 miles, his polar diameter about 3500 miles less, his equatorial diameter about 3500 miles greater. In volume he exceeds the earth nearly 700 times, but in mass only about 90 times, his density being less than the earth s in the pro portion of about 13 to 100. He rotates on his axis in about 10 hours, the plane of Ms equator being inclined nearly 27 to the plane of his orbit.
This planet, in consequence of a luminous ring with which he is surrounded, is one of the most interesting objects in the heavens. This singular appendage was first noticed by Galileo, to whom the planet presented a triple appear ance, as if a large orb were situated between two small bodies or ansce. He observed that sometimes the ans were so enlarged as to present the appearance of a continuous ring ; at other times they entirely disappeared, and Saturn ap peared round like the rest of the planets. After a certain time they again became visible, and gradually increased in magnitude. Thess curious appearances were shown by Huyghens to be occasioned by an opaque, thin, circular ring surrounding the equator of Saturn, and at a considerable distance from the planet. Saturn moving in the plane of his orbit, carries the ring along with him, which, presenting itself to the earth under different inclinations, occasions all the phenomena which have been described. The ring being only luminous in consequence of its reflecting the solar light, it is evident that it can be visible only when the sun and the earth are both on the same side of it ; if they are on opposite sides, it will be invisible. It will likewise be invisible in two other cases, namely, st, when its plane produced passes through the centre of the earth, for then none of the light reflected from it can reach us ; and 2d, when its plane passes through the sun, because only its edge is then enlightened, and being very thin, the whole quantity of reflected light will scarcely be sufficient to render it visible. It is, however, evident that in these two cases the effect will be modified in some degree by the power of the telescope. In ordinary telescopes the ring disappears sometimes before its plane comes into either of the situations mentioned ; but Herschel never lost sight of it, either when its plane passed through the earth or the sun. In the last case the edge of the ring appeared as a luminous line on the round disk of the planet, measuring scarcely a second in breadth , but at the distance of Saturn a second corresponds to 4000 miles, which is equal to the semidiameter of the terrestrial globe. The reason of the ring s disappearance will be easily understood by referring to tig. 49, where the circle abed represents the orbit of the earth, A B C D that of Saturn 9 times more distant from the sun. When Saturn is at A, the earth and sun are both in the plane of the ring ; its edge is consequently turned towards us, and it will be invisible unless telescopes of very high power are used. See Plate XXIX. fig. 1. As Saturn advances from A to B, the ring gradually opens, see Plate XXIX. fig. 2, and it attains its greatest breadth at C, where a straight line perpendicular to its plane makes a more acute angle with the visual ray than in any other situation, see Plate XXIX. fig. 3. As the planet advances towards D, the plane of the ring becomes more oblique to the visual ray ; the breadth of the ring consequently contracts, and it again disappears at E. From E to F, G, and A, the same phenomena will be repeated, only in this case it is the southern side of the ring which is visible to the earth, whereas, while Saturn was in the other half of his orbit, it was the northern side. These changes take place in a period whose full length is Saturn s year, or about 29 of our years. Thus the interval between successive disappearances is about 15 years.
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Fig. 49.—Diagram illustrating Different Appearances of Saturn's Rings.
The brothers W. and J. Ball discovered a division sepa- Divisions rating the ring into two concentric rings, the inner being the riD 8 s - the wider. Cassini confirmed this discovery. Sir W. Herschel concluded that the matter of the ring is no less solid than that of Saturn, as it casts a strong shadow upon the planet. The light of the ring is generally brighter than that of the planet; for it appears sufficiently luminous when the telescope affords scarcely light enough for Saturn. The outer ring is much less brilliant than the inner.
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Fig. 50.—Saturn, as seen at Malta by Mr Lassell with a 20 feet Reflector on 9th October 1852.
In observing the ring with very powerful telescopes, some astronomers have remarked, not one only, but several dark concentric lines on its surface, which divide it into as many distinct circumferences. In common telescopes these are not perceptible ; for the irradiation, by enlarging the space occupied by each ring, causes the intervals between them to disappear, and the whole seems blended together in one belt of uniform appearance. (See ng. 50 .)
