The small displacement in position of a star’s image during the year due to the motion of the Earth around the Sun. Annual aberration was discovered by J. Bradley in 1728 from observations of the changes in distance from the zenith of the star Gamma Draconis. The ratio of the Earth’s mean velocity to the speed of light gives the constant of aberration, 20″.5. This is the maximum amount by which a star can appear to be displaced from its mean position. During the course of a year, the star appears to move around its mean position in a shape that ranges from a circle for a star at the ecliptic pole, via a progressively flattened ellipse, to a straight line for a star on the ecliptic. See also E-Terms.