“[The planet] is surrounded by a slender flat ring inclined to the ecliptic, but which nowhere touches [the body of the planet.]”
Huygen’s discovery was not a mere piece of guesswork, for he spent several years carefully observing the alterations of form which Saturn underwent, before he came to the conclusion that it was only the existence of a ring surrounding the planet which would explain the various observed changes.
It was by way of guarding himself from being robbed of the fruits of his discovery whilst he was accumulating the necessary proof of its truth, that he buried his thoughts in the logogriph or anagram just quoted. Having arrived at the conclusion which he did, he thought himself sufficiently sure of his facts to predict that in July or August 1671, the planet would again appear round, the ring becoming invisible. This surmise proved practically correct, in so far, that in May 1671, or within 2 months of the time predicted by Huygens, Cassini saw the planet as a simple ball unaccompanied by any ring.
This is a convenient place at which to offer a brief explanation of the changes of appearance as regards the ball and rings which Saturn undergoes. These changes depend jointly on Saturn’s motion in its orbit round the Sun, and on the corresponding motion of the Earth in its orbit. Neither Saturn nor the Earth revolve round the Sun exactly in the ecliptic, and this want of coincidence results in the fact, that twice in the 29½ years occupied by Saturn in journeying round the Sun, the plane of its ring is seen edgeways by us on the Earth; whilst at two other periods intermediate but equi-distant the ring is seen opened out to the widest possible extent; that is, so far as we on the Earth can by any possibility have a chance of seeing it.
[Illustration: Fig. 17.—Phases of Saturn’s Rings at specified dates.]
The appearances presented by the rings when undergoing the transformations to which they are subject, will be readily understood by an inspection of the annexed engravings. Fig. 17, indicates the actual appearances in the years specified, and these years may be considered as carried forward and brought up to date by substituting 1877 for 1848, 1885 for 1855, 1891 for 1862, and 1898 for 1869.