How do I measure the angle between stars?

On Tue, 4 May 1999 you wrote:

> Can you help me with an astronomy lab? I'm stuck!
>
> What are the number of degrees that separate these stars from each other?
>
> a. Betelgeuse - Pollux
> b. Arcturus - Regulus
> c. Deneb - Vega

Hello,

While I don't know the answers to these off the top of my head, I can
tell you a way to get an approximate answer (given that this is an
astronomy lab, I doubt that your instructor wants you to go through the
detailed spherical angle calculations). First, find a star map with the
above stars. Look in old issues of Sky and Telescope or Astronomy
magazines. Stars in (a) are visible in the winter, (b) in the spring, and
(c) in the summer. It will work best if the stars are near the center of
the map rather than on the edges.

Find both stars on the star maps. Then, using a ruler, measure the
diameter of the circle of the star map. Say that
diameter is "d". Now, measure the distance between the two stars, and
let's call that distance "x". Now, the circumference of the sky map is a
full circle, or 360 degrees. The actual circumference is equal to pi
(3.14159) times the diameter, or 3.14159*d. The distance between the two
stars is some fraction of this distance. So, we can set up a ratio:

angle between stars 360 degrees
-------------------- = --------------
x 3.14159*d

Solving for the angle between the stars, we get

360 * x
angle = -----------
3.14159 * d

This answer should be roughly correct, although the closer you are to the
edge, the more off your answer will be. This arises because a star map
is the projection of the sphere of the sky onto a flat page and, like a
map of the Earth, it is distorted.

I hope this helps a little!

Thanks for writing.

Sincerely,
Kurtis Williams

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