Local Sidereal Time: A Source-Reviewed Guide
Local sidereal time expresses the right ascension crossing the local meridian. Scope and limits are explicit.
Overview
Greenwich sidereal time, geographic longitude, UT1, and mean or apparent convention combine to locate the local meridian relative to the celestial sphere. The page shows exactly what to verify, how to repeat the method, where a plausible near miss fails, which variants change the result, and what the evidence cannot establish.
At a glance
- Direct scope
- Reviewed finding — Local sidereal time expresses the right ascension crossing the local meridian. It depends on UT1, Earth rotation convention, longitude sign, mean or apparent choice, and normalization around twenty-four hours.
- Evidence to verify
- Method checkpoint — Compute Greenwich sidereal angle from UT1 under a named formula, apply nutation only for apparent sidereal time, add east-positive geographic longitude, normalize once, and retain degrees plus clock notation.
- Repeatable method
- Worked-case result — For one UTC instant, Greenwich and New York share the same Greenwich angle but receive different local sidereal times after longitude is applied. The resulting meridian and Midheaven differ accordingly.
- Worked example
- Failure condition — Adding west longitude as positive when the formula expects east-positive shifts the sky by twice the location offset. A result still wraps into a valid clock range, so fixture cities are essential.
- Near miss
- Documented variant — Mean sidereal time omits the equation of the equinoxes; apparent sidereal time includes it. Either can be valid when the downstream angle algorithm expects the same convention.
- Limits and safety
- Use boundary — A correct sidereal clock supports orientation calculations but says nothing about personal meaning. Uncertain longitude or civil time directly limits the confidence of Ascendant and house results.
Definition and Scope
Greenwich sidereal time, geographic longitude, UT1, and mean or apparent convention combine to locate the local meridian relative to the celestial sphere.
Local sidereal time expresses the right ascension crossing the local meridian. It depends on UT1, Earth rotation convention, longitude sign, mean or apparent choice, and normalization around twenty-four hours.
Evidence and Repeatable Method
Compute Greenwich sidereal angle from UT1 under a named formula, apply nutation only for apparent sidereal time, add east-positive geographic longitude, normalize once, and retain degrees plus clock notation.
Worked Example and Near Miss
For one UTC instant, Greenwich and New York share the same Greenwich angle but receive different local sidereal times after longitude is applied. The resulting meridian and Midheaven differ accordingly.
Adding west longitude as positive when the formula expects east-positive shifts the sky by twice the location offset. A result still wraps into a valid clock range, so fixture cities are essential.
Variants and Disagreement
Mean sidereal time omits the equation of the equinoxes; apparent sidereal time includes it. Either can be valid when the downstream angle algorithm expects the same convention.
Limits and Safe Use
A correct sidereal clock supports orientation calculations but says nothing about personal meaning. Uncertain longitude or civil time directly limits the confidence of Ascendant and house results.
Sources and editorial basis
- US Naval Observatory: Sidereal TimeUSNO Sidereal Time service, “Data Services” description for Greenwich/local mean and apparent sidereal time and UT1 inputAn official service distinguishing Greenwich and local mean and apparent sidereal time and their required UT1 input.
- Swiss Ephemeris 2.10 DocumentationSwiss Ephemeris 2.10 manual, chapters 2 “The Ephemerides”, 6 “Sidereal Time, Ascendant, MC, Houses, Vertex”, and 7 “Delta T”, pages 5–64Public technical documentation for ephemerides, time conversions, sidereal time, apparent positions, houses, nodes, and Delta T; software policy and version must still be disclosed.