light-timeThe time taken for light, radio waves, or other electromagnetic radiation to travel at the speed of light (3 × 105 km s–1) between two points. The light-time along the mean distance between Sun and Earth is 499 seconds. More precisely, the light-time for unit distance is a constant, τA, equal to 499.004 78 seconds. Since the light-time between celestial bodies and Earth varies as the Earth revolves in its orbit, a correction – the equation of light – must be made to calculations involving measurements of time. Light-time produces a delay between transmission and reception of command signals and data between Earth and a spacecraft.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006