Sidereal Time Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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The Sidereal Time Calculator turns a specific everyday question into a transparent calculation or transformation. It is designed to estimate local mean sidereal time from UTC date, UTC hour, and east-positive longitude, using the fields shown on the page so the result can be checked, repeated, and adjusted when your situation changes.

What this calculator does

For the Sidereal Time Calculator, the primary output is the page’s local mean sidereal time estimate. Supporting values include Angle, GMST. Every value shown by the Sidereal Time Calculator comes from the visible fields used by this rule. In practical terms, it is best used to estimate local mean sidereal time from UTC date, UTC hour, and east-positive longitude.

How to use it

Enter UTC date, UTC hour and Longitude (east +). For the Sidereal Time Calculator, use the exact calendar dates you want to compare; keep each measurement in the unit selected beside that field. When comparing two Sidereal Time Calculator scenarios, change one input at a time and keep the other entries unchanged so the source of the difference stays clear.

How the calculation works

For the Sidereal Time Calculator, the page converts the entered UTC date/hour to Julian date, estimates Greenwich mean sidereal time with a standard polynomial, adds east-positive longitude, normalizes to 0–360°, and divides by 15 for sidereal hours.

Example

For the Sidereal Time Calculator, for September 25, 2026 at 12:00 UTC and longitude 0°, the page estimates about 12.288 sidereal hours, equivalent to roughly 184.322°. Changing east-positive longitude shifts the local sidereal angle accordingly.

How to interpret the result

For the Sidereal Time Calculator, read the local mean sidereal time estimate as the result of the specific inputs and rule described above. It is most useful when you want to estimate local mean sidereal time from UTC date, UTC hour, and east-positive longitude. For a Sidereal Time Calculator comparison, keep the unchanged fields identical and vary only the value you actually want to test; that prevents a second assumption from being mistaken for the effect you are measuring.

Limitations and notes

For the Sidereal Time Calculator, the expression is suitable for general estimation but not precision astrometry. For the Sidereal Time Calculator, uT1, precession/nutation, polar motion, and modern Earth-orientation data are outside this page’s scope.