trigonometric parallax

Trigonometric parallax is a method used in astronomy to directly measure the distance to celestial bodies. It relies on geometry, similar to how our two eyes perceive depth. By observing a star from two different points in Earth's orbit, typically six months apart, astronomers can measure a slight angular shift of the star against a background of more distant, seemingly fixed stars.12

This apparent shift is called parallax. Using the known radius of Earth's orbit as the baseline of a large triangle, the distance to the star can be calculated from this parallax angle. A star with a parallax of one arc second is defined as being one parsec away, which is equivalent to 3.26 light-years. This method is reliable for stars within approximately 100 light-years. For more distant stars, other indirect methods are necessary.1234