Calculator inputs
Use UTC directly or enter a local civil time with its UTC offset. East longitudes are positive in the final calculation.
Formula used
This page calculates Greenwich mean sidereal time first, then shifts it by longitude to get local mean sidereal time. It also estimates apparent sidereal time by adding the equation of the equinoxes correction.
Longitude is entered in degrees, minutes, and seconds. East longitude stays positive, while west longitude is converted to a negative decimal value before the local sidereal adjustment.
How to use this calculator
- Enter the observation date and clock time.
- Select whether the typed time is already UTC or local civil time.
- If local time is used, enter the correct UTC offset.
- Fill in longitude degrees, minutes, seconds, and direction.
- Choose the number of decimals shown in the seconds field.
- Press the calculate button to place the result above the form.
- Review GMST, GAST, LMST, LAST, Julian Date, and correction terms.
- Download the result as CSV or PDF when needed.
Example data table
| Date | Time Basis | Time | Longitude | GMST | LMST | LAST |
|---|---|---|---|---|---|---|
| 2026-03-27 | UTC | 20:30:00 | 74° 00' 00" E | 8.8522 h | 13.7855 h | 13.7856 h |
FAQs
1. What is local sidereal time?
Local sidereal time is the right ascension currently crossing your local meridian. It helps observers know which celestial coordinates are highest in the sky at that moment.
2. Why does the calculator need longitude but not latitude?
Sidereal time depends on Earth rotation and your meridian position. Longitude shifts the local meridian east or west, while latitude affects altitude and visibility, not the sidereal clock itself.
3. What is the difference between mean and apparent sidereal time?
Mean sidereal time ignores the nutation correction. Apparent sidereal time adds the equation of the equinoxes, producing a slightly more refined value for high precision astronomical work.
4. Should I enter UTC or local civil time?
Enter whichever is easiest for your workflow. If you use local civil time, provide the correct UTC offset so the page converts it into UTC before computing sidereal values.
5. How should west longitude be entered?
Enter the absolute degrees, minutes, and seconds, then choose West. The script converts west longitude into a negative decimal number before applying the sidereal offset.
6. Why can this result differ slightly from observatory software?
Professional tools may use UT1 feeds, higher order nutation models, polar motion, and observatory standards. This page uses a practical approximation suitable for most planning and educational cases.
7. Can I use this for telescope alignment planning?
Yes. Sidereal time is useful for mount setup, target transit estimation, and checking when a right ascension value reaches your meridian during an observing session.
8. What does the graph show?
The chart compares Greenwich and local sidereal values in decimal hours. It gives a quick visual check of how longitude changes the local result and how small the apparent correction is.