light time

The light time is the time taken for light, traveling at 299,793 kilometers per second, to reach Earth from a distant object. A correction for the effect of light time has to be made when calculating rotation periods of planets, eclipses, and transits. The observed times of maxima and minima of variable stars also need a light-time correction depending upon the position of Earth in its orbit at the time of observation, as periods of variation are stated for the Earth at mean distance from the star.