The equation of time shows the difference between the true solar time and the mathematical mean time. Due to the elliptical orbit, the two times match only on 15 April, 14 June, 1 September and 24 December. On all other days, the difference reaches from minus 16 minutes to 23 seconds (on 4 November) to plus 14 minutes 22 seconds (on 11 Februar).
If the hand at the dial center, which moves between the positive and negative values of the equation of time, lies just above the minutes hand, the hours hand shows the sun peak, namely at the geographical location requisited be the watch-owner and accordingly engraved on the bezel. These devices make it possible to read the moment of sun peak without calculations or astronomical tables. The equation of time mechanism is driven by a kidney-shaped control cam, which is manufactured with the highest precision and rotates once per year on its own axis.
Only at the equator days and nights have equal length all year. The more one moves away from the greatest latitude, the more vary the day and night hours, depending on the season. During the solstice at the poles the sun at times does not set at all (midnight sun) or is seen only on the horizon. All year round, the dial shows the sunrise time with the gradation 0 to 12 and the with the gradation 12 to 24 the sunset time, taking into account the latitude of the city which is engraved on the bezel. The hands of these dials are controlled by a cam which is manufactured with a accuracy of two thousandths of a millimeter.
All indications of the perpetual calendar are linearly arranged on an axis between the “12” and the “6”. By means of a wheel which rotates once in four years, each leap year it adds an additional day for February 29 and so requires no manual correction before 1 March 2100. By a novel catch-up mechanism which skips the extra days of the month, a few tenths of millimeters could be saved in height.
With a conventional perpetual calendar the moon phase indication has to be corrected every two years and seven months. The Jules Audemars Equation du Temps with its astronomical moon - provided the watch has been programmed for it in 2005 - only requires a manual correction in 2127 by using the corresponding corrector button. In contrast to the usual “horological” moon phases with 59 teeth, the gear transmission mechanism of the astronomical moon is equipped with 135 teeth. Therefore, the astronomical moon display differs by only 57.2 seconds from the real moon-time; the “horological” moon phases, however, show a deviation of 44 minutes 2.8 seconds.
While the time of the solar peak (“true noon position”) depends on the longitude of the observing site, the times of sunrise and sunset are determined by the latitude as well as the longitude. In order to individually adjust each individual watch on the situation at the location selected by the owner, there are special cams for the display of sunrise and sunset times at any place on earth - with the exception of those lying beyond the 55th parallel in the north (Novosibirsk and Edmonton) or south (Auckland and Falkland Islands). The chosen site is engraved next to the equation graduation on the bezel. On request, Audemars Piguet produces also bezels and cams calibrated to other cities.