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Sidereal Time Calculator

Work out sidereal time instantly with clear inputs, formula shown and shareable results.

About Sidereal Time Calculator

Sidereal time measures Earth's rotation against the stars rather than the Sun, so a sidereal day is 23h 56m 4s - about four minutes short. Local sidereal time equals the right ascension currently on the meridian, which is exactly what an observer needs to know which objects are highest.

Formulas

Greenwich sidereal time

GST = 18.697374558 + 24.06570982441908 x days since J2000

Local sidereal time

LST = GST + longitude / 15

Worked example

With the default values below, Sidereal Time Calculator returns Local Sidereal Time of 4.6710. Change any input above and every figure here is recalculated in your browser.

Inputs used

Date
2026-01-01
Universal timeaccepts 0–23.999 h
22 h
Longitudeaccepts -180–180 deg
-1.5 deg

Results

Local Sidereal Time
4.6710 h
Local Sidereal Time As hh:mm
04:40
Greenwich Sidereal Time
4.7710 h
Length Of A Sidereal Day
23.93447 h

Step by step

  1. 1Values used
  2. 2Greenwich sidereal time
  3. 3Local sidereal time
  4. 4Local Sidereal Time= 4.6710 h
  5. 5Local Sidereal Time As hh:mm= 04:40
  6. 6Greenwich Sidereal Time= 4.7710 h
  7. 7Length Of A Sidereal Day= 23.93447 h

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is the sidereal day shorter?

Because Earth also advances along its orbit, so it must turn slightly more than 360 degrees to face the Sun again - about four extra minutes.

What is sidereal time used for?

Pointing telescopes. An object culminates when the local sidereal time equals its right ascension.

Related Calculators

Sidereal Time Calculator – Calculate Local Sidereal Time