Calendars and time
Equation of Time
Direct answer
Equation of Time: The Equation of Time is the varying difference between apparent solar time and mean solar time through the year.
The Equation of Time is the seasonal offset between sundial time and mean clock time.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Local Mean Solar Time, Greenwich Mean Time, Calendar, Julian Date
The Equation of Time is the seasonal offset between sundial time and mean clock time.
A cycle or timeline schematic for comparing the named timekeeping concept.
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Why Equation of Time matters
Earth orbital eccentricity and axial tilt make the true Sun run alternately ahead of and behind the mean Sun.
Equation of Time in practice: an example
Depending on the date, apparent noon can differ from mean noon by more than a quarter hour.
Where readings of Equation of Time go wrong
State the sign convention because some tables compute apparent minus mean while others reverse it.
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 Tides and Currents Glossary - Operational definitions for tides, tidal currents, ranges, datums, bores, and amphidromic systems.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Correct sundial and clock time
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More in Calendars & Timekeeping
- Intercalary Day
- Leap Second
- Modified Julian Date
- Sidereal Day
- Tropical Year
- Anomalistic Year
- Calendar Month
- Dominical Letter
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Common questions
What is the Equation of Time?
The Equation of Time is the varying difference between apparent solar time and mean solar time through the year.
What do people get wrong about the Equation of Time?
State the sign convention because some tables compute apparent minus mean while others reverse it.
Why does the Equation of Time matter?
Earth orbital eccentricity and axial tilt make the true Sun run alternately ahead of and behind the mean Sun.
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