Question

How astronomers calculate the distance between two or more celestial body ?

Answer 1

First, one would need to measure the distance of the body from earth. This is done using parallax. So what is parallax? Hold your finger a foot away from your eye. Close one eye and observe the location. Now close the open eye and use your other eye to see the finger. With reference to the background, your finger seems to have moved. This is exactly what you do on a larger scale.