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Fred Hebard wrote-

>I have no idea how Saturn is currently oriented, but sometimes it's
>pole axis is parallel to Earth's.  When that happens the rings are
>viewed edge on and are more-or-less two blips outside the equator.  If
>you can see the equatorial belts of Jupiter, you should be able to see
>Saturn's rings.

Response from George.

This is a real nitpicker's nit-pick; but the Earth's polar axis has nothing
to do with the matter, though Saturn's has.

From Saturn's point of view, the Earth is rather close to the Sun, so to
start with, just simplify matters by thinking of Saturn being illuminated
by the Sun, and leave the Earth out of it.

Saturn is going round the Sun in an orbit which takes nearly 30 Earth-years
to make a circuit, because it's so far out.

Saturn's polar axis is tilted to the plane of its orbit (similar to the way
our Earth's is), so it goes through a similar successions of "seasons" to
our own. Because the rings are very precisely in the plane of Saturn's
equator, then in Saturn's Northern summer they are lit by sunlight slanting
in on their Northern face, and vice versa in winter. Between those two, at
15-Earthyear intervals, is an equinox, spring or autumn, when Saturn
presents the edge only of the ring system to the Sun. The Sun's
"declination", to a Saturnian, is then zero, as it passes through the plane
of the equator and the rings. The ring system is very thin and when it's
lit that way, light can't penetrate radially through it for any distance,
so it becomes invisible, for a short time, from any direction in the solar
system.

Now we can bring the Earth into the picture. Seen from Saturn, the Earth,
though always close to the Sun, can be a few degrees out of Saturn's orbit
plane. There are times when the Earth, but not the Sun, passes exactly
through the plane of the rings. At those times also, the ring plane,
although illuminated by the Sun, is invisible from Earth, because it
appears to be so damn thin.

So the result is that whenever the Earth or the Sun pass through Saturn's
ring-plane, the rings disappear. The Earth-crossings are always grouped, in
time, around a Sun-crossing. At present, Saturn's rings remain quite wide
(they were at their widest in 2002) so the rings remains a fine spectacle.
But they are gradually closing, and will vanish around August-September
2009. Catch them soon.

George.

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