Parallax

Parallax, also called parallactic shift, the apparent displacement of the position of a celestial object on the celestial sphere when viewed from two different positions. For example, the lunar parallax is defined as the difference in position of the moon on the celestial sphere when observed from a particular point on the earth's surface, as compared to its position if observed at the same time from a point then at the center of the earth's disk facing the moon. The diurnal parallax is the parallactic shift of a celestial object when viewed from two points that are one earth diameter apart; it can be determined by measuring the position of the object at an interval of 12 hr, when the observation point has moved from one edge of the earth's disk facing the object to the opposite edge.

The most important parallax in astronomy is stellar parallax. It is defined as the apparent change in position of a nearby star when observed from opposite sides of the earth's orbit—that is, at a six-month interval. The more distant the star, the smaller is its parallax. Stellar parallax is measured in arc seconds. The German astronomer Friedrich Wilhelm Bessel was the first to determine the parallax of a fixed star, 61 Cygni. The parallax of the nearest known star, Proxima Centauri, is 0.762 arc seconds, placing the star at a distance of 4.3 light-years. The unit of distance, the parsec, is defined as the distance of a star having a parallax of 1 arc second.