Laurent -
I thought this might be a video card issue - not rendering the curve
properly - but when I tried it I had the same issue along with the
disappearing rings.

I guess then I can only confirm the issue...

James

--
Then I noticed that the line for the limb of the Moon is not a curve,
but
a polygon (very close to the real limb, I guess). When the Moon was
partly
over Saturn, I could see that in fact Guide draws that line and also a
vast disk filled with black, just a little smaller than the polygon
line
and inside it, but with its edge not maching it. Eventually, just at
the
moment the saturnian globe was completely covered by the Moon, the
part of
the rings that remained visible disappeared also. So I was unable to
simulate the end of the disappearance of the rings.