The present clock was deamed so important by Tardy that he chose to put this piece on the cover of La Pendule Française dans le Monde, part II, devoted pp. 208-209 for the clock. A virtually identical clock, attributed to Jean-Louis Bouchet, Clockmaker to the King, was in the Time Museum inv. 461 and was sold at Sotheby?s on December 2, 1999, lot 64, for US $ 96,000. The association with Bouchet was based on the existence of a very similar clock signed by Bouchet see Les Ouvriers du Temps, by Jean-Dominique Augarde, Antiquorum Editions, p. 155. There is no doubt that all three clocks were made by the same maker; all have the same layout, the same fixed gridiron frame, and the same remontoire system. The clock is unusual not only on the account of employing remontoir d'égalité making the isochronism almost perfect but also it deals with the matter of temperature compensation in unorthodox way by fixing the gridiron frame to the back plate and not to the pendulum as customary. It makes no difference if the gridiron acts on the bob or on the suspension, as in this clock, but it gives us an idea of the extensive horological understandings of its maker. Equation of time Equation of time indicates the time difference between the true solar day and the mean solar day or time shown by a sundial and a clock or watch. It has two major causes. The first is that the plane of the Earth's Equator is inclined to Earth?s orbital plane. The second is that the orbit of the Earth around the Sun is an ellipse and not a circle. Equation of Time due to Obliquity the Earth's tilt. If the Earth's rotational axis were not tilted with respect to its orbit around the Sun, the apparent motion of the Sun along the Ecliptic would fall directly on the Equator, covering the same angles along the Equator in equal time. However, this is not the case, since the angular movement is not linear in terms of time because it changes as the Sun moves above and below the Equator. The projection of the Sun's motion onto the Equator will be at a maximum when its motion along the Ecliptic is parallel to the Equator at the summer and winter solstices and will be at a minimum at the equinoxes. Equation of Time due to Unequal Motion the Earth's elliptical orbit. The orbit of the Earth around the Sun is an ellipse. The distance between the Earth and the Sun is at a minimum around December 31st and is greatest around July 1st. The Sun's apparent longitude changes fastest when the Earth is closest to the Sun. The Sun will appear on the meridian at noon on these two dates and so the Equation of Time due to Unequal Motion will then be zero.