The Equation of Time explains why a sundial doesn't match your clock. It's the difference between apparent solar time (what a sundial shows) and mean solar time (what clocks keep).
The sun is not a reliable timekeeper—at least not by modern standards. Due to Earth's elliptical orbit and axial tilt, the time between successive solar noons varies throughout the year.
If you photograph the sun at the same clock time each day for a year, it traces a figure-8 called the analemma:
The vertical dimension shows the sun's altitude; the horizontal shows whether the sun is "running fast" or "running slow" compared to a perfect clock.
| Date | Sundial vs Clock | Note |
|---|---|---|
| Feb 12 | 14 min behind | Most behind—wait for the sun! |
| Apr 15 | 0 min | Sundial and clock agree |
| May 14 | 4 min ahead | Sun arrives early |
| Jun 13 | 0 min | Brief agreement |
| Jul 26 | 6 min behind | Summer slowdown |
| Sep 1 | 0 min | Autumn alignment |
| Nov 3 | 16 min ahead | Most ahead—sun rushes! |
| Dec 25 | 0 min | Christmas coincidence |
Two factors combine:
These two cycles, with different periods, combine to create the analemma's figure-8.
On day 146 of the year, a sundial reads approximately
1 minutes
compared to mean solar time