The central meridian of each zone is precisely defined, however, to be an integral multiple of 15 degrees longitude. This is the basis for all civil timekeeping and is very close to the LMT at 0 degrees longitude at Greenwich Observatory. International atomic time is the time kept by atomic clocks. Sidereal time is measured relative to the stars and is based on the true rotation period of the Earth. Since the Sun appears to move relative to the stars, a sidereal day is 3 minutes 56 seconds shorter than a solar day.
Sidereal time is measured by the position of the vernal equinox relative to the meridian.
We use sidereal time to adjust our setting circles. As the name implies, this is the time upon which the ephemeris is based. It is reckoned by the orbital periods of the moon and the planets and, therefore, is not subject to the irregularities of the Earth's rotation. It is a uniform measure which forms the basis of the theories of celestial dynamics. It was chosen to be close to UT during the 19th century.