What is the Equation of Time?

Solar time is based upon the apparent solar day, which is the time between two successive local noons at the same longitude. The length of the solar day varies, partly because the earth’s orbit is an ellipse, and also because the earth’s axis is tilted to the celestial equator.

The mean solar day is fixed at 24 hours, but the actual solar day can be nearly 22 seconds shorter or almost 29 seconds longer. These differences can build up to around 17 minutes early or 14 minutes late. The difference between apparent and mean solar time is called the equation of time:

Mean Solar Time = Apparent Solar Time +/− The Equation of Time

If you want greater accuracy and if you have a pocket calculator, the Equation of Time for any day can be found (approximately) using the equation:

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