Equation of Time

The equation of time is the discrepancy between apparent solar time, which tracks the actual position of the Sun in the sky, and mean solar time, which assumes the Sun moves at a constant rate; it is the correction that must be added to or subtracted from sundial time to get clock time. Two features of the motion of the Earth cause it: the elliptical orbit of the planet means it moves faster near the Sun at perihelion and slower near aphelion, so the apparent speed of the Sun across the sky varies through the year, and the axial tilt of the Earth of about 23.44 degrees means the daily motion of the Sun projects unevenly onto the celestial equator, where clock time is measured, with the effect strongest near the solstices and weakest near the equinoxes. Over the course of a year the equation of time swings from about plus 16 minutes 33 seconds around early November, when sundials run fast, to about minus 14 minutes 6 seconds around mid-February, when they run slow, with the exact dates shifting slightly from year to year because of leap years and slow changes in the orbit of the Earth.