The small apparent difference between the observed direction of a star and its true direction (see diagram). It is due to the combined effect of the observer’s motion across the path of incoming starlight and the finite velocity of light. The actual amount of displacement and its direction depend on the observer’s speed and direction of motion. Aberration of starlight resulting from the Earth’s orbital motion is termed annual aberration; the much smaller effect resulting from the Earth’s rotation is diurnal aberration. Planetary aberration is a combined result of the observer’s motion and the time taken for light to travel from a body in the Solar System to the observer.

Aberration of starlight:

The Earth’s orbital motion alters the apparent direction of incoming starlight. In a similar effect, vertically falling raindrops appear to be travelling at an angle as seen from a moving vehicle.