What is the small-angle formula?

Answer 1

The small angle formula is used for measuring the distance to a far away object when the actual size and angular size are known, or for finding out the actual size of a faraway object when the distance to the object and angular size are known.

In arc-seconds:

a = 206265 x D/d

where a = the angular size of the object in arc-seconds

D = the actual linear size of an object in km

d = the distance to the object in km

206265 = the number of arc-seconds in a complete circle divided by 2pi

In Radians:

a = D/d

where a = angular size of object in radians