Solar time

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 synodic day. Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time, and sidereal time, which is based on the apparent motions of stars other than the Sun.

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). Or more simply, 1-2 is a complete rotation of the Earth. However because the Earth moving around the Sun affects the angle the Sun hits a position on the Earth, 1-3 is how long it takes noon to happen again.

Related pages

• Sidereal time
• Solar calendar