The largest amount (annual aberration), in theory, by which a star's apparent direction varies during the year solely because of the Earth's revolution about the Sun. It is commonly denoted by κ and is given by the formula κ = 2π a/(cT (1 - e²)½) , in which a is the length of the semi major axis of the Earth's orbit, e is the eccentricity of that orbit, T is the length of the sidereal year and c is the speed of light in space. For the epoch 2000 A. D. , κ has the value 20. 495 52 (I. A. U. 1977).