🤔How it works

For nearby stars, distance is determined directly from parallax by using trigonometry or calculus and the size of Earth's orbit. The trigonometric or stellar parallax angle equals one‐half the angle defined by a baseline that is the diameter of Earth's orbit. Because even the nearest stars are extremely distant, the parallax triangle is long and skinny as seen in the below graph.

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. Instead, a number of techniques have been developed that enable us to measure distances to stars without needing to leave the Solar System. 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.

A nearby object viewed from two different positions will appear to move with respect to a more distant background. This change is called parallax. 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. The position of your finger will appear move compared to more distant objects.

By 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.

A 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.

Two images of a nearby star taken with the Earth at positions A and B in the diagram above. Note how the orange star moves from the right to the left compared to the more distant ‘fixed’ stars.

Fig-1

By - Jahan Shah