The
aberration of light (also referred to as
astronomical aberration or
stellar aberration) is an
astronomical phenomenon which produces an
apparent motion of celestial objects about their locations dependent on the velocity of the observer. Aberration causes objects to appear to be angled or tilted towards the direction of motion of the observer compared to when the observer is stationary. The change in angle is typically very small, on the order of
v/c where
c is the speed of light and
v the velocity of the observer. In the case of "stellar" or "annual" aberration, the apparent position of a star to an observer on Earth varies periodically over the course of a year as the Earth's velocity changes as it
revolves around the Sun, by a maximum angle of approximately 20
arcseconds in
right ascension or
declination.