Sidereal Time Calculator

Frequently Asked Questions

Why is a sidereal day shorter than a solar day?

Because the Earth moves along its orbit while rotating. After one full rotation relative to the stars (one sidereal day), the Earth has moved a little along its orbit, so it needs about 4 more minutes to face the Sun again. That extra rotation makes the solar day about 4 minutes longer than the sidereal day.

What is the difference between GMST and LST?

GMST is the sidereal time at the Greenwich meridian (longitude 0). LST is adjusted for your specific longitude, telling you which right ascension is on your local meridian right now.

How do I enter my longitude?

East longitudes are positive, west are negative. Find your longitude from a map, GPS, or your phone's location settings. For example, Chicago is approximately −87.6°.

Is UTC the same as GMT?

For practical purposes yes - they differ by less than a second due to leap seconds. Enter your time in UTC (also labeled Z or Zulu time) to get the correct result.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Sidereal time measures Earth's rotation relative to the distant stars rather than the Sun. Because the Earth both spins and orbits the Sun, a sidereal day is about 4 minutes shorter than a solar day (23 hours 56 minutes 4 seconds vs. 24 hours). The calculator converts your date and time into a Julian Date, then applies the standard IAU formula for Greenwich Mean Sidereal Time (GMST) and adjusts for your longitude to give Local Sidereal Time (LST). Enter everything in UTC, not your local clock time.

Formula

D = JD − 2,451,545.0 (Julian Date minus J2000.0 epoch). GMST (in hours) = (18.697374558 + 24.06570982441908 × D) mod 24. Local Sidereal Time = (GMST + longitude ÷ 15) mod 24. Here longitude is in degrees, east positive and west negative, and dividing by 15 converts degrees to hours (360° = 24 h → 1 h = 15°).

Why Sidereal Time Matters

Local Sidereal Time (LST) tells an observer which celestial objects are currently crossing the meridian - the imaginary north-south line through the zenith. A star whose right ascension equals your current LST is at its highest point in the sky (transit or culmination), which is the ideal moment to observe it because it is least affected by atmospheric distortion near the horizon. For telescope users, LST is the natural clock for pointing and planning observation sequences.

Tips for Use

Always convert your local clock time to UTC before entering it. A common mistake is entering local time and getting a result that is off by several hours.

West longitudes are negative (for example, New York is about −74°, Los Angeles −118°). East longitudes are positive (London 0°, Tokyo +139°).

Use this result as a planning estimate. For high-precision pointing or coordinate transformation, use dedicated astronomy software that accounts for precession, nutation, and polar motion.

The sidereal day is 23h 56m 4.091s, so LST advances about 3m 56s faster each solar day compared to your clock.

Limitations

This tool computes apparent sidereal time at the level of accuracy adequate for amateur observing and planning. It does not apply corrections for polar wandering, nutation, or the equation of the equinoxes, which introduce errors on the order of a fraction of a second. For professional-grade accuracy use an ephemeris library such as SOFA or ERFA. Results are in hours:minutes:seconds format on the 0–24 hour scale.

Sidereal Time Calculator

Frequently Asked Questions

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