What does the term annual parallax refer to?

Annual parallax refers to the apparent shift in the position of a star when observed from different points in Earth's orbit around the Sun. This shift is measured as an angular displacement, specifically the angle formed at the star due to the movement of Earth over the course of a year. By observing the same star at two different times, six months apart (essentially on opposite sides of Earth's orbit), astronomers can measure this angle.

The smaller this angular displacement is, the more distant the star is from Earth. This method is crucial for determining distances to nearby stars, as it relies on the geometry of the situation. The concept of parallax forms the basis for one of the key techniques in astrometry, enabling astronomers to measure stellar distances more accurately.