In 1889 Barnard was fortunate enough to observe an eclipse of one of Saturn’s satellites by the ring, but the eclipse, that is the concealment of the satellite, was only effected when it passed behind the bright rings; the dusky ring did not obliterate it, and hence there was obtained a conclusive proof of the transparency of the dusky ring. Barnard further concluded from his observations that there was no separating space or division between the inner bright ring and the dusky ring, as has frequently been represented in drawings. This transparency of the Dusky Ring, as a matter of fact, is therefore undoubted; yet what are we to consider to be the meaning of an observation by Wray in 1861, that whilst looking at the dusky ring edgeways the impression was conveyed to his eye that that ring was very much thicker than the bright rings?
A very interesting question which has been much discussed has reference to the stability of the rings. It is generally agreed that the constituent particles of the rings must be in motion round the primary or their equilibrium could not be maintained: almost equally certain is it, and for the like reason, that the rings cannot be solid. Of actual change in the rings as regards their dimensions, we have no satisfactory proof, though authorities differ on the point, some thinking that the rings are expanding inwards, so that ultimately they will come into contact with the ball, whilst others consider no proof whatever of such change can be obtained from any of the observations yet made in the way of measurements.
We must now proceed to consider the satellites of Saturn. These are 8 in number, 7 of which move in orbits whose planes coincide nearly with the planet’s equator, whilst the remaining one is inclined about 12° thereto. One consequence of this coincidence in the planes of these satellites, which, it should be stated, are the 7 innermost, is that they are always visible to the inhabitants of both hemispheres when they are not actually undergoing eclipse in the shadow of Saturn. The satellites are of various sizes, and succeed one another in the following order, reckoning from the nearest, outwards:—Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion and Iapetus. Any good 2-inch telescope will show Titan; a 3-inch will sometimes show Iapetus; a 4-inch will show Iapetus well, together with Rhea and Dione, but hardly Tethys; all the others require large telescopes. If Saturn has any inhabitants at all constituted like ourselves, which is highly improbable, they will have a chance of seeing celestial phenomena of the greatest interest. What with the rings surrounding the planet and 8 moons in constant motion, there will be an endless succession of astronomical sights for them to study. The amount of light received from the Sun cannot be much—barely 1/100th what the earth receives. The ring and satellites will therefore be useful as supplementary sources of light; yet the satellites will not furnish much, for it has been calculated that the surface of the sky occupied by all the satellites put together would to a dweller on Saturn only amount to 6 times the area of the sky covered by our Moon; whilst the intrinsic brightness of all put together would be no more than 1/16th part of the light which we receive from our Moon.
The only physical fact worth noting here in connection with the satellites concerns Iapetus. Cassini two centuries ago with his indifferent telescopes thought he had ascertained that this satellite was subject to considerable variations of brilliancy. Sir W. Herschel confirmed Cassini as to this. He found that it was much less brilliant when traversing the eastern half of its orbit than at other times. Two conclusions have been drawn from this fact. One is that the satellite rotates once on its axis in the same time that it performs one revolution round its primary; and that there are portions of its surface which are almost entirely incapable of reflecting the rays of the Sun. This last named supposition may perhaps be well founded, but the former needs more proof than is as yet forthcoming. Iapetus on the whole may be said to shine as a star of the 9th magnitude. To this it may be added that Titan is of the 8th magnitude, but all the others much smaller.
Fig. 18.—Saturn with the shadow of Titan on it, March 11, 1892 (Terby).
Saturn revolves round the Sun in a little under 29½ years at a mean distance of 886 millions of miles. Its apparent diameter varies between 15″ and 20″; its true diameter may be put at 75,000 miles. The flattening of the poles, or “polar compression” as it is called, is greater than that of any other planet, but is usually less noticeable than in the case of Jupiter, because the ring is apt to distract the eye, except when near the edgeways phase. The compression may be taken at 1/9.
CHAPTER XI.
URANUS.
To the Ancients Saturn was the outermost planet of the System, nothing beyond it being known. Nor indeed was it to be assumed that any more could possibly exist, because Mercury, Venus, the Earth, Mars, Jupiter, and Saturn, with the Sun, made 7 celestial bodies of prime importance; and 7 was the number of perfection; and there was thus provided one celestial body to give a name to each of the days of the week.
But Science is not sentimental; and when men of Science come upon what looks like a discovery they do their best to bring their discovery to a successful issue, however much people’s prejudices may seem to stand in the way at the moment.