How Parallax Works, Parallax Measurements
Astronomers must use very indirect methods to measure the distances to stars and other astronomical objects. Measuring a star's parallax is a way to find its distance. This method takes advantage of the apparent shift in position of a nearby star as it is observed from different positions as the earth orbits the Sun. Because the parallax effect depends upon the earth's motion about the Sun, it is often referred to as the heliocentric parallax.