Okay, you know how the Earth is divided by latitude and longitude? Well,
imagine a sort of latitude/longitude system projected on the sky. The
Greenwich Meridian (0 deg. 0 hrs.) of the Sky is in Aries. There are 360
degrees until we come back there again. The moon is about 1/2 deg. wide (and
by a wonderful coicidence so is the sun). If you If you extend your arm the
width of your little finger at arms length is 1 degree. Take your first 3
fingers and hold them at arms length this is about 5 deg.

So now that we know angular distances across the sky are measured in
degrees, what about the 40 arcminutes? Each degree is divided into 60
arcminutes, and each arcminute is divided into 60 arseconds. So Mars back in
August was 25 arcseconds across (it's now about 12).

As for the eyepiece, saying what the diameter is when looking through any
kind of scope is not relevant without knowing the power being employed.
There are very few 6-8" scopes which can see the entire Andromeda galaxy at
once. My widest eyepiece (which in my 10" dob is ) provides a true field of
just over 2 degrees, so not enough to take in the entirety of M31.

You can determine the field of your eyepieces/telescope with these formulae
(you can get the numbers from the manufacturers):

Power (magnification) = focal length of scope / focal length of eyepiece
Apparent Field of View = field of view through the eyepiece alone
True Field of View = apparent field / magnification

For example, a 26mm Plossl with a 50 degree field of view, placed in a
telescope with a focal length of 1200 mm, will have a magnification of about
46x (which is 1200 / 26), and a true field of view of just over 1 degree
(1.09 degrees, which is 50 / 46).
The same 26mm Plossl eyepiece, placed in a telescope with a focal length of
2032 mm, would have a magnification of 78x, and a true field of view of 0.64
degrees.

Hope this helps

"n3drk" wrote in message
om...
While reading the peterson field guide to the stars and planets, it gives
the diameter of M31 as 2 degrees and 40 arc minutes. It further states the
diameters are a guide to the size of the object when seen through a 6-8 in
reflector.
What is the normal diameter when looking through the eyepiece? Can someone
explain to me what degrees and arc minutes are?
thanks