The equation of time

A sundial and a clock rarely agree. The gap between them, called the equation of time, swings from about 14 minutes late in February to about 16 minutes early in November.

Minutes the Sun runs early (above zero) or late (below) against the clock in 2026. Faint curves: the share from Earth’s elliptical orbit and from its tilted axis.

Why it happens

Clocks keep mean time: every day exactly 24 hours. The real Sun is not that steady, for two reasons.

The orbit is an ellipse

Earth moves fastest near perihelion in early January and slowest near aphelion in July. When Earth moves faster, the Sun seems to slide faster eastward against the stars, so Earth has to turn a little further each day to bring it back to noon. This alone puts the sundial up to about 7.7 minutes off, in one swing a year.

The axis is tilted

The Sun’s yearly path, the ecliptic, is tilted 23.4° to the equator. Clocks measure time along the equator. Near the solstices the Sun’s motion runs parallel to the equator, and because the Sun is then far north or south, where the hour lines crowd together, it counts for a little more than in full. Near the equinoxes part of it is north-south and does not. This alone puts the sundial up to about 9.9 minutes off, in two swings a year.

Added together, the two give the uneven curve above, with four peaks and four zero crossings.

The turning points in 2026

Feb 11	Great Latesun	The Sun is 14 min 11 s late
Mar 26	Minor Shortsol	Noon to noon: 24 h − 18.1 s
Apr 12	Northbound Echosun	Declination +8.92°, 40 s late
Apr 15	Vernal Evensun	The Sun is exactly on time
May 13	Lesser Earlysun	The Sun is 3 min 41 s early
Jun 13	Estival Evensun	The Sun is exactly on time
Jun 19	Minor Longsol	Noon to noon: 24 h + 13.1 s
Jul 26	Lesser Latesun	The Sun is 6 min 34 s late
Aug 30	Southbound Echosun	Declination +8.92°, 40 s late
Sep 1	Autumnal Evensun	The Sun is exactly on time
Sep 17	Major Shortsol	Noon to noon: 24 h − 21.5 s
Nov 3	Great Earlysun	The Sun is 16 min 27 s early
Dec 22	Major Longsol	Noon to noon: 24 h + 29.8 s
Dec 25	Hibernal Evensun	The Sun is exactly on time

Reading a sundial

To turn sundial time into clock time, take away the equation of time: when the Sun is early, the sundial reads ahead. Then correct for where you are in your time zone, four minutes for each degree of longitude from the zone’s central meridian, and for daylight saving, which the Sun has never observed.