Aberration (astronomy)

Aberration (astronomy), 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 velocities as the speed of the diurnal rotation of the earth, its orbital speed in revolving 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 percent of the velocity of light), it is enough to cause an apparent displacement of the rays of light 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 sec of an arc, called the constant of aberration. The discovery of the aberration of light, by the British astronomer James Bradley (pub. in 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.