Aberration of Light

Aberration of Light, the angular discrepancy between the apparent position of a star and its true position, arising from the motion of an observer relative to the path of the beam of light observed. This motion is the resultant of such motions as the diurnal rotation of the Earth, its orbital revolution around the Sun, and the motion of the solar system through space. Although the resultant velocity of the observer is small (only about 0.2 per cent of the velocity of light), it is enough to cause an apparent displacement of the rays of light coming from a celestial object, just as a drop of rain, falling vertically, leaves a diagonal trace on the window of a moving car. This displacement reaches a maximum of 20.47 seconds of arc, called the constant of aberration. The discovery of the aberration of light, by the British astronomer James Bradley (pub. 1729), was one of the most important discoveries in physical science—initiating a series of investigations that led to the formulation of the theory of relativity.