A fine mid-19th century French gilt bronze mantel clock with perpetual calendar, equation of time and Moonphase by Brocot & Delettrez, Paris. The engraved and pierced case modelled as a pair of columns surmounted by a scrolling dome, raised on an ebonised plinth with glass dome. The white enamelled dial with Roman numerals and visable Brocot escapement and moon hands, over the calendar dial with equation of time, moonphase, day and date, the twin train movement is signed 'A. Brocot & Delettrez PARIS', striking on a bell, linked via a Cam to the calendar mechanism, the pendulum with similarly engraved bob, the clock is 42cm high, 50cm high overall. Achille Brocot and J.B. Delettrez were based at Rue Charlot, Paris.

I will try to explain The Equation of Time for those who may not know or understand in its entirety, upon reading you will realise just how advanced the mechanism to this clock is.

Achille Brocot was a pioneer of many different applications to clock movements and to this day is respected as one of the finest French clockmakers of the 19th Century.

The equation of time describes the discrepancy between two kinds of solar time, True and Mean.

The word equation is used in the medieval sense of "reconcile a difference".

An equation of time shows the difference between "true" solar time (that of Nature) and "mean" solar time (that of Man). This rare complexed dial was nearly always shown combined with other astronomical indicators such as moon-phase, Day,Date,Month and sometimes Bissextile (leap year status).

The Earth makes an elliptical orbit around the Sun, also, its axis is tilted from perpendicular to the plane of the equator. For these two reasons, a "true" solar day, which is the interval of time between two "true" noons when the Sun is at its highest point in the sky, is never the same length over the course of the year. It is exactly twenty-four hours long on just four days:

April 15th
June 14th
September 1st
December 24th

(As shown in the image of the clock dial)

In an unchanging cycle, all the other days are either longer or shorter.

This difference, which ranges from less 16 minutes 23 seconds on November 4th to plus 14 minutes 22 seconds on February 11th, is the "equation of time".

Clockmakers have always tried to find ingenious ways to convey these celestial mechanics. Because these variations occur identically on the same dates, they found it could be "programmed as such" by means of a cam making a rotation each year. This extremely sophisticated complication first appeared on longcase clocks, then to mantle and desk clocks before it was eventually miniaturised to fit inside a pocket watch and later, in the twentieth century, a wristwatch.

Rarely seen in the early to mid 19th Century the equation of time was a very complicated feature of a clock.

There are different ways to show the equation of time. Most clocks prefer a hand sweeping a subsidiary dial or arc, graduated from -16 to +14 minutes. This requires a little mental arithmetic by the the owner by adding or subtracting to calculate true time from mean time. On the four matched dates shown above, the dial would show +/- Zero.

Simpler to use but more complex to make, the “running equation” (équation marchante) had two coaxial minute hands, one to show mean solar time and the other true solar time at a glance.