In the pecking order of calendar complications, the perpetual calendar is unequivocally at the top of the list, besting such other variations as the annual calendar, the triple calendar, the day-date, and of course the simple date function. Such a movement displays the date, day, month, leap year, and sometimes the moon phase as well. But even among perpetual calendars, there are some which are more accurate than others.
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Enter the secular perpetual calendar. As we know, perpetual calendars take into account the leap year cycle, so every four years it correctly displays 29th February instead of skipping to 1st March. But because of the imperfect alignment between the earth against the orbital path of the moon, there is no leap year every 100 years, and a secular perpetual calendar takes that into account as well, thus making it accurate to about 1,000 years as opposed to the usual 122.
Perpetual calendars were invented in the late 1700s by the English watchmaker Thomas Mudge who also made the first lever escapement, the equation of time mechanism, and the minute repeater. The earliest examples were obviously pocket watches and not many of them had the leap year indication on the dial, because all calendar watches were presumed to be perpetual calendars.
There are only a handful of manufactures that have ever made a secular perpetual calendar and unsurprisingly Franck Muller is one of them. The watch in question is the Aeternitas Mega (above) and in addition to the secular perpetual calendar, it proffers three time zones, a flyback chronograph, a grande sonnerie with Westminster chimes, and a flying tourbillon. Other perpetual calendars made by this manufacture include the Cintrée Curvex Quantième Perpétual Tourbillon and the Cintrée Curvex Perpetual Calendar Retrograde Day and Date.