aberration of starlight

The aberration of starlight is an optical illusion caused by the motion of the Earth. Rain falling appears vertical when seen from the window of a stationary train; when seen from the window of a moving train, the rain appears to follow a sloping path. In the same way, light from a star ‘falling’ down a telescope seems to follow a sloping path because the Earth is moving. This causes an apparent displacement, or aberration, in the position of the star.

Apparent displacement of a star from its true position, due to the combined effects of the speed of light and the speed of the Earth in orbit around the Sun (about 30 kps/18.5 mps).

During a year the apparent position of a star describes a curve around its true position. This curve is an ellipse except when the star lies on the ecliptic (when it is a line along which the star appears to move backwards and forwards) or when it is in the pole of the ecliptic (when it is a circle). As aberration depends upon the ratio between the velocity of light and the velocity of the Earth, the determination of this ‘constant of aberration’ provides a method of calculating an approximate value for one of these velocities if the other is known.

Aberration, discovered in 1728 by English astronomer James Bradley, was the first observational proof that the Earth orbits the Sun.