The equation of time is the difference between true solar time and mean time. True solar time, given by sundials, varies from day to day because of the Earth’s elliptical orbit, and according to the longitude of the point of observation. Mean time, given by watches, ignores these variations and for every day of the year mathematically divides time into equal hours. Apparent solar time, can be ahead (fast) by as much as 16 minutes 33 seconds (around 3 November), or behind (slow) by as much as 14 minutes 6 seconds (around 12 February).

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