Parallax in astronomy

In astronomy, parallax is the apparent shift in position of a nearby celestial object relative to distant background objects which is caused by a change in the observer's point of view. This effect is most commonly used to measure the distance to nearby stars from two different positions in Earth's orbital cycle, usually six months apart. By measuring the parallax angle, the measure of change in a star's position from one point of measurement to another, astronomers can use trigonometry to calculate how far away the star is.

The concept hinges on the geometry of a triangle formed between the Earth at two different points in its orbit at one end and a star at the other. The parallax angle is half the angle (α) formed at the star between those two lines of sight. The closer the star is to the observer, the larger the angle would be.

Parallax is a foundational method in the cosmic distance ladder, a series of techniques astronomers use to measure distances in the universe. While parallax is only effective at measuring distances of nearby stars, space telescopes like Gaia have significantly expanded its effectiveness. Parallax remains the most direct and reliable method for measuring stellar distances, forming the basis for calibrating more indirect methods to measure distances to galaxies and beyond.