Solar time

On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). More simply, 1→2 is a complete rotation of the Earth, but because the revolution around the Sun affects the angle at which the Sun is seen from the Earth, 1→3 is how long it takes noon to return. [Note that in this diagram, the relative motion, and corresponding angles, are highly exaggerated for illustrative purposes.]

Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time (discussed in this article), and sidereal time, which is based on the apparent motions of stars other than the Sun.[1]

  1. ^ For the three kinds of time, see (for example) the explanatory section in the almanac Connaissance des Temps for 1902, page 759 Archived August 10, 2011, at the Wayback Machine.