Space · Sky
Sidereal time calculator.
Enter a UT date, time and your longitude to get the sidereal time that tells you which stars are overhead.
Inputs
Sidereal time
Enter a UT date and time.
How sidereal time is worked out
Sidereal time is the hour angle of the vernal equinox. In practice it tells you which right ascension is crossing your meridian: a star whose right ascension equals your local sidereal time is highest in your sky. A sidereal day is 23 h 56 min 4.0905 s of solar time (Wikipedia, Sidereal time), so sidereal time gains about 3 min 56.6 s on the clock each day.
The tool uses the US Naval Observatory formula GMST = 6.697375 + 0.065709824279·D + 1.0027379·H + 0.0000258·T² hours, reduced to 0 to 24 h. D is days from J2000.0 to the previous 0h UT, H the hours of UT since then and T the Julian centuries since J2000.0. Local sidereal time adds longitude divided by 15 (east positive). The tool treats your UTC entry as UT1; UTC is kept within 0.9 seconds of UT1 (Wikipedia, UT1).
Worked example. For 1 January 2017 at 0h UT, the Astronomical Almanac value quoted by Wikipedia is GMST = 6 h 43 m 21.1060 s. The tool returns 6 h 43 m 21.1066 s, within a thousandth of a second.
Frequently asked questions
What is sidereal time?
Time measured by Earth's rotation against the stars rather than the Sun. Local sidereal time equals the right ascension of objects crossing your meridian.
How long is a sidereal day?
23 hours 56 minutes 4.0905 seconds of ordinary time, about 3 minutes 56 seconds shorter than a solar day.
How do I get local sidereal time from Greenwich time?
Add your longitude in hours: longitude in degrees divided by 15, east positive. Places west of Greenwich subtract.
Which time zone should I enter?
Universal Time (UTC). Convert your clock time to UTC first. The tool does not read your time zone.
Sources
Checked
- US Naval Observatory: Approximate Sidereal Time
- Wikipedia: Sidereal time
- Wikipedia: UT1 (UTC kept within 0.9 s)
- US Naval Observatory: Julian Date Converter
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Scope
Mean sidereal time from the USNO approximate formula, accepted for years 1900 to 2100. Apparent sidereal time adds a small nutation correction that this tool leaves out.