For standard time, the earth is divided into 24 time zones. Within each time zone, all clocks are set to the same time. Time zones are described by their distance east or west of the zero meridian that passes through Greenwich, England. Because mean solar time is based on the motion of a hypothetical sun, a base position was established from which the mean time can be calculated. This base position is the vernal equinox, an imaginary point in the sky located by reference to star positions.
Solar time based on the position of the stars is called sidereal time.
Neither mean solar time nor mean sidereal time is precisely accurate, because the motion of the earth on its axis is not regular. This problem was solved with the introduction of ephemeris time, which is used chiefly by astronomers when the greatest degree of accuracy is required. Ephemeris time is based on the annual revolution of the earth around the sun. The scientific standard of time, the second, is measured by the atomic clock, which is tuned to the resonant frequency of the transition energy between two energy states of the cesium-133 atom. Yet time is not a physical constant.