Question

how to calculate distance between two planets or two stars

Answer 1

Astronomers can measure a star's position once, and then again 6 months later and calculate the apparent change in position. The star's apparent motion is called stellar parallax. The distance d is measured in parsecs and the parallax angle p is measuredin arcseconds.

Astronomers can use parallax to find distances to objects much farther even than planets. To calculate the distance to a star, astronomers observe it from different places along Earth's orbit around the Sun.