is the period
for one orbit of the Earth
around the Sun
. And is measured at the Vernal Equinox
(the spring-time) which equals 365.2422 mean solar
days (a tropical
year). If the Synodic months (lunar
) are used to count a year, 12 synodic months equal 354.36706 days. This is almost 11 days shorter than the tropical year. And if used it throws the seasons out of sync. Neither tropical
(Sun) or synodic
(Moon) has a complete number of days. So to compile a calendar
that keeps in step with the moon’s phases or with the Sun’s seasons it is necessary to insert days into the calendar which is called "Intercalations".
Sidereal time ( Star Time) - Whenever the Earth has completed 366.2422 rotations with reference to the Stars, one year has past. When the Earth has completed 365.2422 rotations with reference to the Sun, one year has past. Mathematical tables are used to derive mean solar time from mean sidereal time. Neither mean solar nor mean sidereal time is precisely accurate, because the motion of the earth on its axis is not regular.
Ephemeris time is used by astronomers for the greatest degree of accuracy. Ephemeris are tables giving the computed positions of celestial bodies for every day of a given period. It is an astronomical almanac containing positional tables. The base position is the vernal equinox. Through the use of mathematical tables, Ephemeris time is converted into mean solar time.