Before revision: Derivation For a right triangle, where is the parallax, is approximately the average distance from the Sun to Earth, and is the distance to the star.Using small-angle approximations (valid when the angle is small compared to 1 radian), so the parallax, measured in arcseconds, is If the parallax is 1", then the distance is This defines the parsec, a convenient unit for measuring distance using parallax.
After revision:   This defines the parsec, a convenient unit for measuring distance using parallax.