Mick wrote:
Am I missing something? Is my arithmetic off? Presently Mars is some 20 arc
seconds wide. This implies that at 100 X's magnification, it should be 2000
arc seconds across...or just over 1/2 a degree, which is the diameter of a
full moon!! (1/2 degree = 1800"). Last night, the image at 100 x's is no
where near the size of a full moon. What am I missing here??

You only think it is smaller than the size of the Full Moon. Planets
in the eyepiece look smaller (at least, at first) than you think they
might, given the magnification. It's a well-known effect, without (I
don't think) a well-known cause.

It's been discussed on SAA a number of times.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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