Quote:
Originally Posted by -George-
Hi, If I may ask...
The moon is 1/2 degree in diameter looking at it with your eyes. That means mars through the telescope at 1 degree should look like 2 moon sizes by eye - no scope.
A moon generally looks big at night... if mars is 2x the size of that (1 degree vs 1/2) then Mars should look absolute huge. I am confused on this degree business.
And when is the moon 1/2 degree because some nights it looks as big as the sun. So what is all this degree based on and why isn't mars double the size of a big moon since its 1 degree not half?
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The full sphere of the sky is 360 degrees around, or 180 degrees from horizon to horizon. The discs of the Moon and the Sun are 1/2 degree across - or to put it another way, you would need a "train" of 360 Moons (or Suns) to reach from one horizon to the other.
The Moon and the Sun are
BOTH 1/2 degree across (plus or minus a small amount) - it's one of the great coincidences of the universe. The Sun is much bigger than the Moon, but it is also much further away, so they are the same size as viewed from Earth. That's why we can get total solar eclipses (where the Moon JUST covers the disc of the Sun) and annular eclipses (when the Moon is a little further away, so it doesn't quite cover the disc of the Sun).
The fact that the Moon looks bigger on the horizon than it it does when it is higher in the sky is just an optical illusion - it is basically 1/2 degree across wherever it is in the sky (barring minuscule effects of refraction etc). Somehow, having nearby reference points to compare it against (the horizon, etc) tricks us into thinking it is bigger. You can prove this yourself if you check out how much of the eyepiece it fills when low in the sky (and looks huge to the naked eye), and when it is higher - it is the same size.
If you can get a clear 240x view of Mars, it will look about twice as big through the eyepiece as the full Moon does with the naked eye (but probably not as bright) - and you should be able to see some surface shading etc.
Remember though that Mars is significantly bigger and further away than than the Moon, so even with 240x magnification, features of a size of a few hundred km that you can see on the Moon with the naked eye (the dark Mare, etc) will still be smaller in your 240x telescope view of Mars, so you will not be able to see mountains and craters on Mars. If you point your telescope at the Moon, however, and get a 240x view, you will be able to see features that are just a couple of km across.
Hope this helps!