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“Apparent solar time,” time measured by Earth’s rotation relative to the Sun can be measured by direct observation of the Sun or by a sundial. It varies from day to day because of the Earth’s elliptical orbit which is not exactly circular, and because the Earth moves at slightly different speeds in different seasons. Therefore, solar time can be off by up to 16 minutes per day relative to “mean solar time.” Mean solar time is what is used by most clocks and watches.

The difference between apparent solar time and mean solar time is called the “equation of time.”