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.
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