Calendars and time
Local Mean Solar Time
Direct answer
Local Mean Solar Time is uniform mean-solar time adjusted to a location's longitude.
Local Mean Solar Time is longitude-based clock time referenced to the mean Sun.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Local Apparent Solar Time, Equation of Time, Calendar, Julian Date
Local Mean Solar Time is longitude-based clock time referenced to the mean Sun.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Local Mean Solar Time
Nineteenth-century towns commonly kept local mean time until transportation and telegraph networks favored standard meridians.
Why Local Mean Solar Time matters
Local Mean Solar Time removes true-Sun irregularity while preserving each meridian own local noon.
How to read Local Mean Solar Time
Local Mean Solar Time speaks to historical time conversion and observations recorded before standardized zones.
Local Mean Solar Time in practice: an example
Two towns one degree of longitude apart have Local Mean Solar Times about four minutes apart.
Where readings of Local Mean Solar Time go wrong
A common error is to treat an old local-time record as modern zone time without knowing the observing longitude.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Astronomical Almanac Glossary - Reference definitions for astronomical periods, coordinates, dates, and time scales.
- U.S. Naval Observatory: Universal Time - Official explanation of Earth-rotation time and the Universal Time family.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Correct historical time by longitude
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More in Calendars & Timekeeping
- Ordinal Date
- Sidereal Year
- Unix Time
- Astronomical Day
- Civil Calendar
- Draconic Year
- Golden Number
- International Atomic Time
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Common questions
What is Local Mean Solar Time?
Local Mean Solar Time is uniform mean-solar time adjusted to a location's longitude.
What do people get wrong about Local Mean Solar Time?
A common error is to treat an old local-time record as modern zone time without knowing the observing longitude.
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