
Fig7_4 Annual parallax
Fig. 7.4 . When a distant and nearby star are observed at sixmonth 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 pA, which can be used to determine the distance, D, of the nearby star. From trigonometry, sin pA = AU/D » pA for small angles, where 1 AU is the mean distance between the Earth and the Sun. The distance D to the star in units of parsecs is given by 1/pA, if the parallax angle is measured in seconds of arc. This angle is greatly exaggerated in the figure, for all stars have a parallax of less than one second of arc or less than 1/3,600 of a degree. The German astronomer Friedrich Wilhelm Bessel (17841846) announced the first reliable measurement of the annual parallax of a star in 1838.
Copyright 2010, Professor Kenneth R. Lang, Tufts University

