Dictionary term
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.
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.Quick reference
- Use when
- Use Mean Solar Day when explaining Universal Time, civil hours, and the equation of time.
- Do not use it for
- Do not assume Earth rotation is perfectly uniform; UT1 day length varies slightly.
- Reference category
- Calendars & Timekeeping
- Related terms
- Calendar Month, Apparent Solar Day, Calendar, Julian Date
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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.
Mean Solar Day in spiritual tradition
Spiritually, the mean solar day shows how averaging can make a changing natural rhythm useful for community life.
Mean Solar Day: fact and faith, kept distinct
This is a factual calendar, astronomy, Earth-science, cultural-history, or observational reference. It can support wonder and practical planning without becoming a claim of fate, spiritual causation, or private revelation.
Mean Solar Day in numerology
Mean Solar Day is not a numerology claim. Any numbers attached to it should retain their ordinary scientific, calendrical, navigational, or historical meaning unless a separate symbolic interpretation is clearly labeled.
Why Mean Solar Day matters
Averaging removes the seasonal variation of apparent solar days caused by orbital eccentricity and axial tilt.
Mean Solar Day at work: an example
Twenty-four mean solar hours define the familiar day even when the true Sun crosses the meridian early or late.
Where interpretation stops
Mean Solar Day is presented as sourced astronomy, calendrical, Earth-science, cultural, or instrument-history reference. Definitions should distinguish measured phenomena from civil convention and historical interpretation, preserve location and time dependencies, and avoid turning factual sky data into a deterministic personal claim.
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
- Also appears in
- Calendars & Timekeeping
- Aliases
- None listed
- Last updated
- 2026-07-16
Compare mean and apparent days
Mean Solar Day in a book is one thing; Mean Solar Day in your own birth chart is your life. The Oracle casts your real chart and tells you what it means for your love, your work, and your year ahead, in conversation, free.
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More in Calendars & Timekeeping
Common questions
What is Mean Solar Day?
A Mean Solar Day is the average interval between successive meridian passages of an imaginary uniformly moving mean Sun.
How is Mean Solar Day used in an almanac?
Use Mean Solar Day when explaining Universal Time, civil hours, and the equation of time.
What should I avoid when interpreting Mean Solar Day?
Do not assume Earth rotation is perfectly uniform; UT1 day length varies slightly.
