solar time

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 (aka solar day) is the basic unit. The term generally means time scaled so that noon (solar noon) is the moment when the Sun is at it zenith, which differs from the standard time we use for general purposes.

Approximations of solar times were generally used in the past, i.e., a city set its clocks according to the solar time of its location, solar time varying by longitude: walking either east or west constantly changes your precise solar time. With the advent of modern higher-speed transportation and communication, the necessary time-coordination between distant locations made such solar time less practical and time zones were adopted, and times used in nearby localities were generally the same, and when they weren't, they were exactly an hour apart.

The length of solar days, the periods between solar noons, actually varies a small amount over the course of a 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 may still be termed solar time if it approximates coordination with solar noon. The time-zone-based time we use for general purposes also uses such a day.

Astronomy is used to define solar time, but astronomical observation also uses sidereal time, a system counting days and time-of-day 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. Within the context of discussions including sidereal time, ordinary time-zone based time might be referred to as a variation on solar time.