An Etymological Dictionary of Astronomy and Astrophysics

The second postulate in Einstein’s theory of → special relativity whereby the → velocity of light in → vacuum has the same absolute value in all → inertial reference frames irrespective of the direction and speed of propagation of the light source. It should be emphasized that this constancy of the speed of light holds among → reference frames moving uniformly with respect to each other. An observer accelerated with respect to a light source will measure a speed of light that is smaller than the speed of light measured in a → rest frame. See also → principle of relativity.

See also: → principle; constancy, noun related to → constant.