1. What is Greenwich Sidereal Time?

Greenwich Sidereal Time (GST) is the hour angle of the vernal equinox at the prime meridian in Greenwich. It represents the right ascension that is currently on the local meridian and is used in astronomy for celestial navigation and telescope pointing.

2. How Does the Calculator Work?

The calculator uses the GST equation:

Where:

• \( JD \) — Julian Date (days since January 1, 4713 BC)
• 2451545 — Julian Date for J2000.0 epoch (January 1, 2000, 12:00 TT)
• 6.6974 — GST at J2000.0 epoch (hours)
• 0.065709824 — Conversion factor (hours/day)

Explanation: The equation accounts for the Earth's rotation rate and converts Julian Date to sidereal time at Greenwich.

3. Importance of GST Calculation

Details: GST is fundamental for astronomical observations, satellite tracking, and celestial navigation. It provides the reference frame for locating celestial objects from Earth.

4. Using the Calculator

Tips: Enter the Julian Date (JD) in days. The result will be in hours (0-24) formatted as HH:MM:SS.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between sidereal time and solar time?
A: Sidereal time is based on Earth's rotation relative to distant stars (23h56m), while solar time is based on rotation relative to the Sun (24h).

Q2: How accurate is this calculation?
A: This provides mean sidereal time. For apparent sidereal time, nutation corrections would be needed (accuracy ~0.1 seconds).

Q3: What's the relationship between GST and Local Sidereal Time (LST)?
A: LST = GST + observer's longitude (east positive), converted to 0-24 hour range.

Q4: Why use Julian Date instead of calendar date?
A: Julian Date provides a continuous count of days, making astronomical calculations simpler.

Q5: How often does GST repeat?
A: GST completes a full cycle every sidereal day (about 23 hours 56 minutes 4 seconds).