What is sidereal time?

Sidereal time is a time-keeping system based on Earth's rotation relative to distant stars rather than the Sun. A sidereal day is approximately 23 hours 56 minutes 4 seconds, making it about 4 minutes shorter than a solar day. This sidereal calculator helps you calculate sidereal time for astronomical and astrological purposes.

How do I calculate sidereal time?

To calculate sidereal time, use our sidereal calculator by entering your location coordinates and the desired date/time. The calculator computes Greenwich Sidereal Time (GMST) first, then converts it to Local Sidereal Time (LST) based on your longitude.

What is a sidereal astrology chart calculator?

A sidereal astrology chart calculator creates birth charts and horoscope charts using the sidereal zodiac system, which accounts for the precession of the equinoxes. Our sidereal birth chart calculator uses accurate sidereal time calculations to generate true sidereal astrology charts.

How does a sidereal day calculator work?

A sidereal day calculator determines the length of Earth's rotation relative to distant stars. Our sidereal day calculator shows that a sidereal day is 23 hours 56 minutes 4.0905 seconds.

Sidereal Time Calculator

Calculate local and Greenwich sidereal time for astronomical observations.

Current Time

Local: 9/29/2025, 2:54:47 AM

Date & Time

Select date and time for calculation

Hemisphere

Select hemisphere for longitude

Longitude (Optional)

Degrees (°)

Minutes (')

Seconds (")

Enter longitude in degrees, minutes, and seconds (e.g., 100° 3' 30") for local sidereal time calculation

Methodology

This calculator uses standard astronomical algorithms based on the J2000.0 epoch. GMST is calculated using the IAU polynomial formula, and GAST includes nutation corrections via the equation of equinoxes. Local sidereal times are computed by adding the longitude correction in hours.

Important Disclaimers:

This calculator provides accuracy suitable for amateur astronomy applications
Professional astronomical observations may require more precise nutation models
Results are based on standard IAU algorithms and J2000.0 epoch
Local sidereal time requires accurate longitude coordinates
Atmospheric refraction and other local effects are not included

About This Calculator

1. What is the difference between mean and apparent sidereal time?

Mean sidereal time assumes uniform Earth rotation, while apparent sidereal time includes corrections for nutation (small wobbles in Earth's axis caused by gravitational effects of the Sun and Moon). The difference is typically less than 1 second.

2. Why is sidereal time important for astronomy?

Sidereal time directly relates to the position of stars in the sky. Combined with a celestial object's right ascension, it determines the object's hour angle, which tells astronomers when and where to find objects in the sky.

3. How accurate are these calculations?

The calculations use standard astronomical algorithms with accuracy suitable for most amateur astronomy applications. For professional precision, more complex nutation models and atmospheric corrections would be needed.

4. What is the equation of equinoxes?

The equation of equinoxes is a correction that accounts for nutation effects on the position of the vernal equinox. It's the difference between apparent and mean sidereal time, typically ranging from -1.2 to +1.2 seconds.