Glossary of watchmaking

Equation of time

Équation du temps | 均時差/キンジサ | 时差

Wristwatch - Equation of time

The equation of time is the difference between true solar time and mean time. True solar time, given by sundials, varies from day to day because of the Earth's elliptical orbit, and according to the longitude of the point of observation.

Mean time, given by watches, ignores these variations and for every day of the year mathematically divides time into equal hours. Four times a year, on April 15th, June 14th, September 1st and December 24th, true solar time and mean time coincide. On the other days, the difference ranges from minus 16 minutes and 23 seconds on November 4th, to plus 14 minutes and 22 seconds on February 11th.

In 2000, Audemars Piguet unveiled an equation of time watch which combines the equation of time, sunrise, sunset and perpetual calendar. The equation of time hand clearly indicates this daily difference.