Sidereal Time Calculator
Convert civil time ↔ Local Sidereal Time (LST) & Greenwich Sidereal Time (GST) — instantly, in your browser
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What Is Sidereal Time?
Sidereal vs. Solar Day
A solar day is 24 hours. A sidereal day is ~23 hours 56 minutes 4 seconds — the true rotation period of Earth relative to distant stars, not the Sun.
Greenwich Sidereal Time (GST)
GST is the hour angle of the vernal equinox as measured from Greenwich. It is the baseline from which Local Sidereal Time is derived for any longitude.
Local Sidereal Time (LST) for Astronomers
LST tells you which part of the sky is currently on your meridian. When an object's Right Ascension equals your LST, it is at its highest point in the sky — ideal for observation.
Julian Date — The Astronomer's Universal Timestamp
Julian Date (JD) is a continuous count of days since noon on January 1, 4713 BCE. It eliminates calendar ambiguity and is the foundation for all sidereal time calculations.
Frequently Asked Questions
- What is sidereal time used for?
- Sidereal time is the backbone of amateur and professional astronomy. It lets you know which stars and deep-sky objects are currently observable from your location.
- How is Local Sidereal Time calculated?
- LST = GST + (longitude in hours). The calculation is based on the Julian Date of the given UTC time, using the standard IAU formula for Greenwich Apparent Sidereal Time.
- Why does sidereal time differ from clock time?
- Solar time is based on the Sun's position, which shifts slightly each day because Earth orbits the Sun. Sidereal time ignores the Sun and measures Earth's rotation against fixed stars.
- Is the result accurate enough for telescope alignment?
- Yes, for visual observation and most GoTo mount alignment. For sub-arcsecond astrometry, use a dedicated application with atmospheric refraction correction.