Local Sidereal Time Calculator
Fill in the values and the answer updates as you type.
How the Local Sidereal Time Calculator works
Sidereal time is the right ascension of the stars crossing your meridian right now, so objects with that right ascension are highest in the sky. Greenwich mean sidereal time in degrees = 280.46061837 + 360.98564736629 × (Julian date − 2451545). Local sidereal time adds your east longitude. Enter the date and time in UTC, not local time. An object is best placed for observing when its right ascension is close to the local sidereal time. Example: with Date (UTC) 2026-10-07, Time (UTC) 20:00, and Longitude (east positive) 100.5 °, the result is Local sidereal time (hh:mm:ss) = 03:47:55; Local sidereal time (decimal hours) = 3.8; Greenwich sidereal time (hh:mm:ss) = 21:05:55.
Inputs and results
The Local Sidereal Time Calculator takes 3 inputs and gives 3 results.
What you enter
- Date (UTC) — starts at 2026-10-07.
- Time (UTC) — starts at 20:00.
- Longitude (east positive) (°) — starts at 100.5.
What you get
- Local sidereal time (hh:mm:ss)
- Local sidereal time (decimal hours)
- Greenwich sidereal time (hh:mm:ss)
Local Sidereal Time Calculator FAQ
How do I use the Local Sidereal Time Calculator?
Enter date (utc), time (utc) and longitude (east positive) in the fields, then read local sidereal time (hh:mm:ss), local sidereal time (decimal hours) and greenwich sidereal time (hh:mm:ss) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Local Sidereal Time Calculator calculate?
Sidereal time at your longitude, the right ascension on your meridian. It returns local sidereal time (hh:mm:ss), local sidereal time (decimal hours) and greenwich sidereal time (hh:mm:ss) from the 3 values you provide.
What values does the Local Sidereal Time Calculator start with?
It opens with date (utc) 2026-10-07, time (utc) 20:00 and longitude (east positive) 100.5 ° so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Local Sidereal Time Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Local Sidereal Time Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.