Distances, Absolute Magnitudes and Radii of the Stars
As a result of the revolution of the Earth round the Sun, in the course of a year a nearer star must describe a small ellipse in the sky relative to the much more distant fainter stars (Fig. 14.1: to be distinguished from aberration, Fig. 5.7). Its semi-major axis, which is the angle that the radius of the Earth’s orbit would subtend at the star, is called the heliocentric or annual parallax p of the star to and fro motion).
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