Sidereal time is the hour angle of the vernal equinox, the ascending node of the ecliptic on the celestial equator. The daily motion of this point provides a measure of the rotation of the Earth with respect to the stars, rather than the Sun. Local mean sidereal time is computed from the current Greenwich Mean Sidereal Time plus an input offset in longitude (converted to a sidereal offset by the ratio 1.00273790935 of the mean solar day to the mean sidereal day.) Applying the equation of equinoxes, or nutation of the mean pole of the Earth from mean to true position, yields local apparent sidereal time.
Astronomers use local sidereal time because it corresponds to the coordinate right ascension of a celestial body that is presently on the local meridian. North Americans tell time on a 12-hour clock cycle. Hours before noon are ante-meridian (am), hours after noon postmeridian (p.m.). Due to the vastness of the North American continent, the U.S.A is divided into four time zones, going from East to West. · Eastern Time zone · Central Time Zone · Mountain Time Zone · Pacific Time Zone Alaska, Hawaii and the Aleutian Islands have their own time zones.
As we move west wards from noontime at GMT (00 Longitude) towards U.S.A., after every change of 150 Longitude 1 hour is subtracted from the GMT noontime (12 O’clock) and if we move eastward from GMT is every 150 Longitude, 1 hour is added. Most of the US and all of Canada observe Daylight Savings Time from the last Sunday in April to the last Sunday in October by advancing clocks ahead by one hour.