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Relative distances on the sky (celestial sphere) are measured based on angles. If the diameter (D) of an object is small compared to its distance (d) from the observer O (this is the `small angle approximation'), its angular measurement (theta) is calculated as D/d in Radians.


What's a Radian, you ask? We know there are 360^o in a full circle. There are 2*Pi rad in a full circle. So: 2*3.14 rad = 360^o, 1^o = 0.0174 rad, 1 rad = 57.3^o.

You can use your hands and fingers to measure angles in the sky. Your fingers are about 1/2^o wide when you view it at arm's length. Your fist is about 10^o wide. Your hand with fingers spread wide, thumb-tip to pinkie-tip is about 20^o.