Equation of time

The equation of time is the difference between apparent solar time, shown by a sundial, and mean solar time, kept by a clock. It arises from two effects: the Earth’s elliptical orbit, which changes its orbital speed, and the tilt of the ecliptic to the equator. The value swings through the year from about +16m33s in early November to about -14m20s in mid February, passing through zero four times. A navigator uses it to relate the instant of the Sun’s meridian passage to clock time, and the Nautical Almanac carries it implicitly through the tabulated GHA of the Sun.