Sidereal Time
Astronomical Applications Dept. Astronomical Applications Dept.
 
Skip navigation

This data service provides the Greenwich mean and apparent sidereal time, local mean and apparent sidereal time, and the Equation of the Equinoxes for specified time(s).

Data will be provided for a three year period from 1 January of the preceding year through 31 December of the following year. For dates outside of this range, see MICA.

Use Form A for cities or towns in the U.S. or its territories. Use Form B for all other locations. Both forms are immediately below.

Be sure to read the Notes section (on this page beyond the two forms) for definitions and additional details on the data.

Form A - U.S. Cities or Towns

             

      Universal Time (UT1)


(1 to 9999)

  

  

The place name you enter above must be a city or town in the U.S. The place's location will be retrieved from a file with over 22,000 places listed. Either upper- or lower-case letters or a combination can be used. Spell out place name prefixes, as in "East Orange", "Fort Lauderdale", "Mount Vernon", etc. The only exception is "St.", which is entered as an abbreviation with a period, as in "St. Louis". You need only enter as many characters as will unambiguously identify the place.

Form B - Locations Worldwide

             

      Universal Time (UT1)


(1 to 9999)

 

The place name you enter above is merely a label for the table header; you can enter any identifier, or none (avoid using punctuation characters). The data will be calculated for the longitude and latitude you enter below.

Longitude:          
Latitude:          

Need coordinates?  Try NGA's GEOnet Names Server (GNS).
Need U.S. coordinates?  Try the USGS Geographic Names Information System (GNIS).

Notes

Sidereal Time

Sidereal time is equal to the hour angle of the catalog equinox and is a direct measure of the diurnal rotation of the Earth. Apparent sidereal time is defined by the intersection of the true equator of date with the ecliptic of date and is affected by the precession and nutation of the axis of the Earth. Mean sidereal time is measured against the mean equinox of date and is only affected by precession.

The tabulated sidereal times are given in hours, minutes and seconds, rounded to the nearest 10-4 second. The hours and minutes refer to both the mean and apparent sidereal times, since the two quantities differ by very little. The tabulated seconds may occasionally be greater than or equal to 60, which indicates that the associated sidereal time lies in the next whole minute greater than the one given in the table. For example, a tabulated apparent sidereal time of 6h 40m 60.1000s is equivalent to 6h 41m 00.1000s.

Equation of the Equinoxes

The Equation of the Equinoxes is equal to the apparent sidereal time minus the mean sidereal time.

Equation of Time

The Equation of Time is the difference apparent solar time minus mean solar time. To obtain this value for a specified time, calculate the apparent geocentric right ascension and declination of the Sun using the Geocentric Positions of Major Solar System Objects and Bright Stars service.