Science & history · Seasons and clocks
Sundials and clocks disagree by up to 16 minutes
A sundial tells apparent solar time; a clock keeps mean time. The gap between them changes through the year.
Today's equation of time
Worked out in your browser for today's date.
The equation of time
USNO defines the equation of time as apparent solar time minus mean solar time. It can reach about 16 minutes. One cause is the tilt of Earth's axis, worth up to about 10 minutes; the other is Earth's elliptical orbit, which on its own puts the Sun about 7.5 minutes off in early April and early October.
The figure eight
Plot the equation of time against the Sun's noon height and the result is a figure-8 curve called the analemma. Photograph the Sun at the same clock time every week for a year and it traces the same shape. Never look at the Sun to do it.
Sources
- US Naval Observatory The Equation of Time (opens in a new tab) for apparent vs mean solar time, its two causes, the analemma · checked 2 October 2026