What about Mars magnification?

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??

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/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

On Tue, 22 Jul 2003 18:39:21 GMT, "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 were missing a full moon near the same line of sight ;-)

During one of my recent Mars observations (using a refractor without a
star diagonal) I couldn't help but notice how *small* (and bright!)
the moon was compared to my telescopic view of Mars. The scope's
magnification was between 200 and 300x.

Without the moon near the same line of sight it's *very* difficult to
accurately judge which would appear to be larger -- assuming you don't
"cheat" and use a little math!

An absent full moon isn't as large as most of our memories of it ;-)

Bill Greer