Aberrationoflight

Aberration of light, the name given to the apparent alteration in the true direction of the rays of light from any heavenly body, due to the earth's own motion. Raindrops falling vertically, when viewed from a moving railway carriage, have apparently an oblique motion. The faster the carriage moves, or the slower the raindrops fall, the more oblique will the motion appear. So also with light, the obliquity of the rays of light from any star depending on the velocity of the earth as compared with that of the light itself. Thus a star is never seen in its true position, but always a little distance away in the direction of the earth's motion. The aberration is greatest when the earth's velocity is a maximum, i.e. in mid-winter. Thus a knowledge of the earth's speed enables us to determine the velocity of light. The phenomenon was discovered by Bradley in 1727, and received full mathematical treatment first by Bessel.