Using the telescope if you will try to see any heavenly body such as moon, planet, etc then it will appear like a disc. Now we have to find the angle alpha made by the diameter of the planet at a point present on the earth’s surface. Let r be the distance of the heavenly body from the earth and D is the diameter of the body. So the formula will be:
= D / r
Or it can be written as
D=r
If the far off star seems to be a small point in the telescope then this method will not be applicable.
