Equation of time complication of a watch

(Patek Philippe pocket watch caliber 89 created in 1989)

The equation of time complication of a watch shows a normal time i.e. the mean solar time and also the true solar time. As we talk about true solar time, then the question arises…

What is true solar time?

The time between noon to the noon of the next day when the Sun is exactly above the head is called a solar day.

(Tourbillon Équation Marchante)

The irregularity in the Earth’s path around Sun was known to Babylonians. The first tables to give the equation of time were published in 1665 by Christian Huygens. The sundials were a common time-telling device for a long time and even though, in the 1700s mechanical time-telling devices were there, sundials were considered to be telling the right time. Later when the watches became the main time-telling device the “meantime” became the accepted standard. 

(Marie-Antoinette pocket watch number 1160)

The Earth revolves around the Sun elliptically and its axis is tilted from perpendicular to the plane of the equator. Because of the above reasons the length of the interval of time between two noons, when the Sun is exactly above the head, is never the same throughout the year. It is never complete 24 hours in a day except for 4 days i.e. April 15th, June 14th, September 1st, and December 24th. The difference between the solar time and the mean time is usually seen in the minimum on 4th November less 16 minutes and 23 seconds, and the maximum on 11th February plus 14 minutes and 22 seconds.

(Breguet equation of time)

The Watch with an equation of time complication is an extremely complex watch. Horologists Have made this complication in different ways, in some watches there is a sub-dial for it, in some watches, there is an extra needle on the main dial representing this complication, and in some of them, there is a Retrograde sub Dial.

This beautiful astronomical complication is a classic addition to a very complex watch. In itself, demonstrating the true solar time is immense work, and then having it fit in small wrist watches is incredible.