The equation of time is the difference between apparent solar time and mean solar time. Apparent 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 (Northern or Southern hemisphere).

Mean solar time, given by watches, ignores these variations and for every day of the year mathematically divides time into 24 equal hours.

Apparent solar time can be ahead (fast) by as much as 16 minutes 33 seconds (around November 3rd), or behind (slow) by as much as 14 minutes 6 seconds (around February 12th).

The Spring and Fall Equinoxes are when the amount of day and night are virtually equal, which is also when Mean and Apparent time are equal.