Linked astronomy tools
Quick start (3 steps)
- Enter date, time, time zone, and observer longitude with east positive.
- Click Compute to get Greenwich mean sidereal time (GMST) and local sidereal time (LST).
- Optionally enter right ascension to compute hour angle for a target.
Inputs
Results
| GMST (hms) | — |
|---|---|
| GMST (deg) | — |
| LST (hms) | — |
| LST (deg) | — |
| Hour Angle (HA) (hms / deg) | — / — |
| Resolved TZ | — |
This tool runs in your browser. Inputs are not sent.
Interpretation & notes
- Uses a practical GMST approximation for education and planning.
- Longitude sign convention: east positive.
- If RA is omitted, HA is hidden.
How to use the sidereal time calculator effectively
Enter the date and time in UTC, then add observer longitude using the page's east-positive convention. GMST is the Greenwich reference; LST shifts that value by longitude so it can be compared with right ascension.
How it works
The calculator converts the UTC instant to a Julian date and then derives GMST and LST with deterministic astronomy formulas. Display values are rounded for readability, but the workflow is most reliable when you keep the original UTC time and longitude sign unchanged while checking another source.
When to use
Use this page to plan telescope sessions, verify an observing note, or connect Right Ascension and Declination (RA/Dec) coordinates with local sky timing. For high-precision astrometry or mount control, confirm with observatory-grade software and the exact time standard required by your equipment.
Common mistakes to avoid
- Entering local clock time instead of UTC.
- Reversing east/west longitude signs.
- Comparing LST to right ascension without matching the same instant.
- Copying a rounded sidereal time into a downstream calculation that needs higher precision.
Interpretation and worked example
If LST is close to an object's right ascension, the object is near the local meridian. When the result seems shifted by hours, check the timezone conversion first, then the longitude sign and date boundary around midnight UTC.
See also
FAQ
What is the difference between GMST and LST?
GMST is Greenwich sidereal time. LST is GMST adjusted by observer longitude.
Why does HA disappear sometimes?
HA is shown only when RA is entered because it requires the target right ascension.
What should I enter first?
Start with a UTC date and time, then enter observer longitude with the correct east/west sign. Use zero longitude first if you only need GMST.
How precise are the results?
The calculator keeps internal precision and rounds only for display. Small differences can still appear if another tool uses a different epoch convention, sidereal-time formula, or rounding policy.
Why can my result differ from another calculator?
The most common causes are local time entered as UTC, reversed longitude sign, or a date rollover near midnight. Align those inputs before comparing GMST or LST values.