Annual parallax

Annual parallax

Fig. 1.20 . When a distant and nearby star are observed at six-month intervals, on opposite sides of the Earth’s orbit around the Sun, astronomers measure the angular displacement between the two stars. It is twice the annual parallax, designated by πA, which can be used to determine the distance, D, of the nearby star. The German astronomer Friedrich Wilhelm Bessel (1784-1846) announced the first reliable measurement of the annual parallax of a star in 1838.

Copyright 2010, Professor Kenneth R. Lang, Tufts University