At a glance
- Found in
- Calendars & Timekeeping
- Related
- Mean Solar Day, Sidereal Day, Calendar, Julian Date
An Apparent Solar Day runs from one true-solar noon to the next.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Apparent Solar Day
Before accurate clocks, local apparent solar time was a primary practical time reference.
Why Apparent Solar Day matters
Apparent Solar Day’s length changes through the year, explaining why sundial time does not advance uniformly against clock time.
How to read Apparent Solar Day
Apparent Solar Day speaks to sundials, true solar time, and daily solar-transit calculations.
Apparent Solar Day in practice: an example
The interval from one observed solar noon to the next can differ slightly from exactly 24 mean hours.
Where readings of Apparent Solar Day go wrong
The usual misreading is to equate apparent solar noon with 12:00 civil time.
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.
- NOAA Global Monitoring Laboratory: Solar Calculator Glossary - NOAA definitions for solar geometry, atmospheric refraction, and civil, nautical, and astronomical twilight.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Observe true solar transit
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More in Calendars & Timekeeping
- Calendar Year
- Double Dating
- Era
- Intercalary Month
- Local Apparent Solar Time
- Old Style Date
- Universal Time
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Common questions
What is an Apparent Solar Day?
An Apparent Solar Day is the interval between successive meridian transits of the actual Sun.
What do people get wrong about an Apparent Solar Day?
The usual misreading is to equate apparent solar noon with 12:00 civil time.
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