equation of time

noun phrase

: the difference between apparent time and mean time usually expressed as a correction which is to be added to apparent time to give local mean time

The Equation of Time is the difference between your local apparent solar time, when the Sun crosses your meridian, and local mean solar time, which is the average rate of solar time used in clocks. Because the Earth travels at varying speeds as it traverses its elliptical orbit, the difference can be as much as 16 minutes.

—Kathleen Hill Zeller

Word History

First Known Use

1645, in the meaning defined above