Calendars and time
Mean Solar Day
Direct answer
Mean Solar Day: A Mean Solar Day is the average interval between successive meridian passages of an imaginary uniformly moving mean Sun.
A Mean Solar Day is the averaged solar day underlying the conventional 24 hours.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Calendar Month, Apparent Solar Day, Calendar, Julian Date
A Mean Solar Day is the averaged solar day underlying the conventional 24 hours.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Mean Solar Day
Mean-time clocks replaced sundial time as mechanical timekeeping demanded equal hours across the year.
Why Mean Solar Day matters
Averaging removes the seasonal variation of apparent solar days caused by orbital eccentricity and axial tilt.
Mean Solar Day in practice: an example
Twenty-four mean solar hours define the familiar day even when the true Sun crosses the meridian early or late.
Where readings of Mean Solar Day go wrong
A common error is to assume Earth rotation is perfectly uniform; UT1 day length varies slightly.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Universal Time - Official explanation of Earth-rotation time and the Universal Time family.
- U.S. Naval Observatory: Astronomical Almanac Glossary - Reference definitions for astronomical periods, coordinates, dates, and time scales.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Compare mean and apparent days
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Common questions
What is a Mean Solar Day?
A Mean Solar Day is the average interval between successive meridian passages of an imaginary uniformly moving mean Sun.
What do people get wrong about a Mean Solar Day?
A common error is to assume Earth rotation is perfectly uniform; UT1 day length varies slightly.
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