Astrophysics (Index)About

solar time

(time based upon the position of the Sun in the Earth's sky)

The term solar time is used for time systems based upon the Sun's position in the Earth's sky, or equivalently, the Earth's rotation compared to the direction to the Sun. The day is the basic unit.

Pre time-zone time that defined noon as the moment when the Sun was at its zenith (solar noon) is referred to as solar time, resulting in the moment termed noon varying by locality. The faster communication and transportation of modern times encouraging coordination between distant locations made this less practical and time zones were adopted. The periods between noons defined as the Sun's moment at its zenith varies a bit throughout the year due to the Earth's orbital eccentricity and axial tilt. A time system defining the length of a day by averaging of these periods is also termed solar time, being still based upon the position of the Sun in the sky, if less directly. The time we use for general purposes uses such a day.

Astronomy is used to define solar time, but astronomical observation also uses sidereal time, a non-solar-time system based upon the Earth's rotation compared to the position stars in the celestial sphere. Its unit of a sidereal day is roughly four minutes shorter than the day we know. It has the advantage that any given star appears in the same place in the sky at the same sidereal time.

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