Equation of Time

The equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer on Earth. The equation of time can be found in tables (for example, The Astronomical Almanac) or estimated with formulas given below.

Apparent (or true) solar time can be obtained for example by measurement of the current position (hour angle) of the Sun, or indicated (with limited accuracy) by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time average to zero (with zero net gain or loss over the year).

The equation of time is also the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth. The equation of time values for each day of the year, compiled by astronomical observatories, were widely listed in almanacs and ephemerides.

Read more about Equation Of Time:  The Concept, History, Secular Effects, Practical Use, Calculating The Equation of Time

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