apparent solar day

apparent solar day

[ə′pa·rənt ¦sō·lər ′dā] (astronomy) The duration of one rotation of the earth on its axis with respect to the apparent sun. Also known as true solar day.

apparent solar day

The period between two successive transits of a particular meridian on the earth by the apparent sun. Because of the elliptical shape of the earth's orbit, the apparent angular velocity of the sun as seen is not constant, or the apparent sun's (also called true sun velocity is not constant; hence, the days as indicated by the apparent travel of the true or apparent sun are not of uniform length. To make solar days of uniform length, astronomers have invented the mean sun, a fictitious body imagined to move at a uniform rate along the celestial equator, thus making a complete circuit from west to east in one year.