annual aberration

Annual aberration is due to the revolution of the Earth around the Sun.

Annual aberration is caused by the motion of an observer on the Earth revolving around the Sun.

De annusi fixarum aberrationibus (1742) (On the annual aberration fixed stars)

Its effect is much smaller than that of annual aberration, and is only 0'.32 in the case of an observer at the equator, where the rotational velocity is greatest.

Approximating the Earth's orbit as circular, the maximum displacement of a star due to annual aberration is known as the constant of aberration, conventionally represented by .

In practice, because the Earth is not an inertial rest frame but experiences centripetal acceleration towards the Sun, many aberrational effects such as annual aberration on Earth cannot be considered light-time corrections.

In the case of annual aberration of starlight, the direction of incoming starlight as seen in the Earth's moving frame is tilted relative to the angle observed in the Sun's frame.

A special case of annual aberration is the nearly constant deflection of the Sun from its true position by 'κ' towards the 'west' (as viewed from Earth), opposite to the apparent motion of the Sun along the ecliptic.

Assuming a circular orbit, annual aberration causes stars exactly on the ecliptic (the plane of the Earth's orbit) to appear to move back and forth along a straight line, varying by on either side of their position in the Sun's frame.

This is roughly 8 times the effect of annual aberration, as one would expect since the velocity of the Solar System relative to the Milky Way is about 8 times the velocity of the Earth relative to the Sun.