204. As Saturn goes round the Sun, his obliquely posited ring, like our
Earth’s Axis, keeps parallel to itself, and is therefore turned edgewise
to the Sun twice in a Saturnian year, which is almost as long as 30 of
our years § 81. But the ring, though considerably broad, is too thin to
be seen when it is turned round edgewise to the Sun, at which time it is
also edgewise to the Earth; and therefore it disappears once in every
fifteen years to us. As the Sun shines half a year on the north pole of
our earth, then disappears to it, and shines as long on the south pole;
so, during one half of Saturn’s year the Sun shines on the north side of
his ring, then disappears to it, and shines as long on it’s south side.
When the Earth’s Axis inclines neither to nor from the Sun, but sidewise
to him, he instantly ceases to shine on one pole, and begins to
enlighten the other; and when Saturn’s Ring inclines neither to nor from
the Sun, but sidewise to him, he ceases to shine on the one side of it,
and begins to shine upon the other.
[Sidenote: Fig. III.]
Let _S_ be the Sun, _ABCDEFGH_ Saturn’s Orbit, and _IKLMNO_ the Earth’s
Orbit. Both Saturn and the Earth move according to the order of the
letters, and when Saturn is at _A_ his ring is turned edgewise to the
Sun _S_, and he is then seen from the Earth as if he had lost his ring,
let the Earth be in any part of it’s Orbit whatever, except between _N_
and _O_; for whilst it describes that space, Saturn is apparently so
near the Sun as to be hid in his beams. As Saturn goes from _A_ to _C_
his ring appears more and more open to the Earth: at _C_ the ring
appears most open of all; and seems to grow narrower and narrower as
Saturn goes from _C_ to _E_; and when he comes to _E_, the ring is again
turned edgewise both to the Sun and Earth: and as neither of it’s sides
are illuminated, it is invisible to us, because it’s edge is too thin to
be perceptible: and Saturn appears again as if he had lost his ring. But
as he goes from _E_ to _G_, his ring opens more and more to our view on
the under side; and seems just as open at _G_ as it was at _C_; and may
be seen in the night-time from the Earth in any part of it’s Orbit,
except about _M_, when the Sun hides the Planet from our view. As Saturn
goes from _G_ to _A_ his ring turns more and more edgewise to us, and
therefore it seems to grow narrower and narrower; and at _A_ it
disappears as before. Hence, while Saturn goes from _A_ to _E_ the Sun
shines on the upper side of his ring, and the under side is dark; but
whilst he goes from _E_ to _A_ the Sun shines on the under side of his
ring, and the upper side is dark.
[Sidenote: Fig. I and III.]
It may perhaps be imagined that this Article might have been placed more
properly after § 81 than here: but when the candid reader considers that
all the various Phenomena of Saturn’s Ring depend upon a cause similar
to that of our Earth’s seasons, he will readily allow that they are best
explained together; and that the two Figures serve to illustrate each
other.
[Sidenote: PLATE VI.
The Earth nearer the Sun in winter than in summer.