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Local Sidereal Time Calculator — result sheet
Calculate Julian date, approximate Greenwich mean sidereal time, and local mean sidereal time from a UTC date, time, and longitude.
Inputs used
Results
Visual chart
Breakdown
Calculation steps
Returned data table
Formula and methodology
Formula: Convert the UTC instant to Julian date. Using the USNO approximate GMST relation, GMST = 6.697375 + 0.065709824279D + 1.0027379H + 0.0000258T² modulo 24 hours; local mean sidereal time = GMST + longitude ÷ 15 hours modulo 24.
Sidereal time tracks Earth’s rotation relative to the stars. This page calculates mean sidereal time for an entered longitude and date; it does not include a full apparent-sidereal-time nutation model or replace an observatory ephemeris.
This result follows the calculator's declared inputs, precision, validation boundaries, and model limits.
Input contract
- UTC date
- UTC time — 24-hour UTC time from 00:00 through 23:59.
- Longitude — degrees east positive; minimum -180; maximum 180
Worked example
| Input | Value |
|---|---|
| UTC date | 2026-01-01 |
| UTC time | 00:00 |
| Longitude | 50.55 |
The page returns the UTC Julian date, approximate GMST, and a local mean sidereal time shifted eastward by 50.55° ÷ 15.
Assumptions and limits
- The date and clock time are UTC and use a proleptic Gregorian calendar in the interface range.
- Longitude is geodetic-style decimal degrees with east positive and west negative.
- The approximation treats UTC as close enough to UT1 for an educational or planning result.
- Julian date is reported from the Unix-epoch conversion with the standard Julian-day offset.
- GMST is the mean sidereal time approximation, not GAST or apparent sidereal time.
- Latitude is not used for local mean sidereal time.
- The result is normalized to 0 through less than 24 sidereal hours.
- Atmospheric refraction, polar motion, nutation, precession model choice, and DUT1 are not modeled.
- The page is suitable for learning and first-pass scheduling, not precision telescope control.
- The longitude must describe the meridian where the local sidereal time is wanted.
Calculator note
Source and methodology
Use the official WorldCalculate methodology policy for the source, formula, precision, and boundary standards behind this calculator.
Planning estimate, not financial, medical, legal, or professional advice. © WorldCalculate — reuse with attribution. Built and curated by Hassan ALRowaie.
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