Sidereal Time Calculator

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Calculates the local sidereal time (LST) based on Greenwich Mean Sidereal Time (GMST) and the observer's longitude.

How To Use

How to use this calculator

Start with your values, or try a preset.

Step 1

Enter Greenwich Mean Sidereal Time (GMST) (hours) and Observer's Longitude (degrees East is positive) in the units shown.

Step 2

Read Local Sidereal Time (LST) (hours) in the results panel. Results update as you change the inputs.

Step 3

Choose a preset to load an example, then adjust the values for your situation.

Worked Examples

See the inputs and their calculated results.

Greenwich Mean Sidereal Time (GMST) (hours): 10.5 hours; Observer's Longitude (degrees East is positive): -74 degrees

Local Sidereal Time (LST) (hours): 5.566666667 hours

Greenwich Mean Sidereal Time (GMST) (hours): 20 hours; Observer's Longitude (degrees East is positive): 0 degrees

Local Sidereal Time (LST) (hours): 20 hours

Greenwich Mean Sidereal Time (GMST) (hours): 5 hours; Observer's Longitude (degrees East is positive): 100 degrees

Local Sidereal Time (LST) (hours): 11.66666667 hours

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