Sidereal Time
This data service calculates Greenwich
sidereal time
,
both mean
(GMST)
and apparent
(GAST)
,
local sidereal time
, both mean and
apparent, and the
Equation of the Equinoxes
for specified time(s).
Data will be provided for the years 1800 through 2050.
Use the buttons under Location to find coordinates of cities or towns in the U.S. or its territories,
or to convert between Degrees-Minutes-Seconds (DMS) and Decimal Degrees.
Be sure to read the Notes section
(at the bottom of this page) for additional details on the
data.
Notes
UT1
UT1 is a form of Universal Time
affected by irregularities in the Earth's rotation, and is the modern version of mean
solar time on the Greenwich meridian.
Sidereal Time
Sidereal time is the same for all locations with the same longitude; the latitude of a place is not used in the calculation.
Sidereal time is the
hour angle
of the
equinox
. If the mean equinox is used, the result is mean sidereal time; if the true equinox is used, the result is apparent sidereal time. The hour angle can be measured with respect to the local
meridian
or
Greenwich meridian
, yielding, respectively, local or Greenwich (mean or apparent) sidereal times. The meridian is the north-south line on the celestial sphere at any given location on Earth; it is a great circle passing through the zenith and the celestial poles. The hour angle is a specific measure of how far east or west of the meridian a celestial object (or celestial reference point) is at any given time.
An equivalent definition—one that is more useful to observational astronomers—is that sidereal time is the right ascension of celestial objects transiting (crossing) the meridian as the Earth rotates.
Right ascension
is one of two angular coordinates on the celestial sphere; it is an angle that is measured in time units (where 24h = 360°). It may be specified as mean right ascension or apparent right ascension, resulting in mean or apparent sidereal time, respectively. As in the first definition of sidereal time, the meridian may be the local meridian or the Greenwich meridian, yielding local or Greenwich (mean or apparent) sidereal time. Because every celestial object has a right ascension coordinate, the sidereal time indicates which ones are at the highest point in their daily arcs across the sky.
The fundamental reference point that is relevant to both definitions is the equinox, which serves as the zero point for both right ascension and sidereal time (the distinction between the mean and true equinox is discussed in the next paragraph). There are two equinoxes, located at two points on the celestial sphere where two great circles intersect. The two great circles are the celestial equator, which is the projection of the Earth’s equatorial plane onto the celestial sphere, and the ecliptic, which is the projection of the Earth’s orbital plane onto the celestial sphere. The specific point referred to here is the vernal equinox, the point in the constellation Pisces that the Sun appears to cross on or about March 21 of each year. When, as the Earth rotates, the vernal equinox crosses a meridian (actually, of course, it is the meridian sweeping past the equinox), the sidereal time is zero on that meridian, and a star transiting the meridian at that time has a right ascension of zero. As the Earth rotates, sidereal time advances, and objects of increasing right ascension cross the meridian.
Due to the various motions of the Earth, the equinox is not a fixed point on the celestial sphere. The location of the equinox at a given time may be computed in two slightly different ways. When a complete model of the orientation of the Earth’s equator in space is used, including both precession and nutation, the true equinox results, which is the origin of apparent (or true) right ascension and apparent sidereal time. If only precession is used in the calculation (neglecting nutation), the mean equinox results, which is the origin of mean right ascension and mean sidereal time. The difference apparent sidereal time minus mean sidereal time is the
equation of the equinoxes
, which never exceeds 1.2 seconds. These distinctions arose at a time when the computation of nutation, which consists of a large number of periodic components, was difficult. The mean quantities are still useful surrogates for the more accurate ones for some purposes.
The interval between two crossings of the equinox (mean or true) past a given meridian is about 23h 56m 04s of solar (civil) time and exactly 24h 00m 00s of sidereal time. Sidereal time increases by about 24h 03m 57s in each solar (civil) day. A given star crosses the meridian at the same sidereal time each day (assuming its
proper motion
is negligible), but 3m 56s earlier in solar time each day. Because the equinox moves slowly with respect to the stars, the mean sidereal day is shorter than the rotation period of the Earth by about 0.008 second.
Application Programming Interface
This data service uses one of our Application Programming Interfaces (APIs). The API returns data in JavaScript Object Notation (JSON) format for users who wish to manipulate data into a customized format. For more information on the API, please see the documentation page.
