"Kilolani" wrote in message
ink.net...
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
Just a couple of added 'comments', that may help to give an idea of what the
sizes 'mean'. If you hold your fist out at arms length, this covers about 10
degrees. The width of a finger at the same distance, is about 1 degree. The
Moon is about 1/2 degree (so about the size of a small 'pea' held at arms
length). Jupiter is about the size of a small coin, 100 yards from you.
When magnifications are applied, the effect (in terms of the 'angle' the
objects take), is to divide the distance away by the magnification. So in
the case of Jupiter, applying a 100* magnification, makes it cover the angle
in the eyepiece, of the small coin, now just one yard away. With this
magnification, it is actually larger than the 'naked eye' moon, but will
often not appear so!. This is a classic 'optical illusion', which is quite
interesting to demonstrate for yourself. Look at the Moon 'naked eye', and
then look at again through a tube (a cardboard core from a kitchen towel
roll for instance). Though the Moon itself does not change size, you 'see'
it as smaller through the roll. The change is not much, but is enough, so
that if you look at a planet like Jupiter through an eyepiece (which has the
same effect on the human visual system as the cardboard tube), with the
magnification carefully selected to make it appear the same size, it still
tends to look smaller...
As a general 'rule' on apparent sizes, 1" at 100yards, is one 'arc minute'.

Best Wishes