What this calculator does
Sidereal time works out local sidereal time for a date, UTC time and longitude, the star clock astronomers use. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
The inputs explained
| Field | What to enter |
|---|---|
| Date (UTC) | A calendar date. |
| Time (UTC, 24h) | A time of day, in 24-hour format. |
| Longitude (east positive, west negative) (°) | A number, measured in °. Starts at 0. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with longitude (east positive, west negative)
Every other input is held at the calculator’s starting values while longitude (east positive, west negative) varies. Select any row to load that scenario into the calculator.