Stars that are members of binaries further complicate the picture. In practice stars with significant proper motions require at least three epochs of observation to accurately separate their proper motions from their parallax. If they were this would complicate the picture as presented here. It is important to note that in this example we assume that both the Sun and star are not moving with a transverse velocity with respect to each other. If the parallax angle, p, is measured in arcseconds (arcsec), then the distance to the star, d in parsecs ( pc) is given by: Note how the orange star moves from the right to the left compared to the more distant ‘fixed’ stars.

Two images of a nearby star taken with the Earth at positions A and B in the diagram above. The definition of the parallax angle may be determined from the diagram below: The position of your finger will appear move compared to more distant objects.īy measuring the amount of the shift of the object’s position (relative to a fixed background, such as the very distant stars) with observations made from the ends of a known baseline, the distance to the object can be calculated.Ī conveniently long baseline for measuring the parallax of stars (stellar parallax) is the diameter of the Earth’s orbit, where observations are made 6 months apart. A simple demonstration is to hold your finger up in front of your face and look at it with your left eye closed and then your right eye. One such method is trigonometric parallax, which depends on the apparent motion of nearby stars compared to more distant stars, using observations made six months apart.Ī nearby object viewed from two different positions will appear to move with respect to a more distant background.

Instead, a number of techniques have been developed that enable us to measure distances to stars without needing to leave the Solar System. Measuring distances to objects within our Galaxy is not always a straightforward task – we cannot simply stretch out a measuring tape between two objects and read off the distance.

Parallax method