The present instrument is a highly rare and unusual "cadran d'equation" indicating month, date and equation of time, impressing by the complex construction and highest quality of its movement. To accurately adjust their timepieces, watchmakers in the past had no alternative than using the true noon, meaning to determine when the sun is at its highest point. A convenient way of defining this is with a sundial - or a device like the present, in use until the second half of the nineteenth century, when electricity took over.
The equation of time results from the 23 degree tilt of the Earth's rotational axis and the fact that the Earth moves around the Sun in an elliptical orbit. 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: 15 April, 14 June, 1 September and 24 December. In an unchanging cycle, all other days are either longer or shorter. This difference, which ranges from less 16 minutes and 23 seconds on 4 November to plus 14 minutes and 22 seconds on 11 February, is the "equation of time".
The most common indication is by means of a subsidiary dial or arc, graduated from -16 to +14 minutes, requiring some mental arithmetic by the wearer by adding or subtracting the relevant difference from mean time.
The accomplishment of the "equation running" display required the invention of another mechanical marvel. By means of extremely precise calculations based on the irregular path of the earth around the sun, watchmakers have developed a cam shaped like the figure-8 path made by the sun in the sky called "analemma", thus mechanically mimicking the curve derived by astronomers. Turning once per year, this cam is used to drive a hand for the plus/minus display of the difference between solar time and civil time.