A leap year indicator is a perpetual-calendar display that shows whether the present year is a leap year. This lets the watch handle 29 February without correction. Most perpetual calendar watches have one. Setting it is the trickiest part of setting a perpetual calendar.
How does a leap year indicator work?
The indicator moves on by one step every 1 January. It makes one full turn every four years, in step with the calendar’s main program cam. That cam encodes all 48 months of the four-year cycle: three common years of 365 days and one leap year of 366 days. The indicator’s reading shows the cam’s position in that cycle. On some movements the indicator is connected to the month cam. The usual mechanism follows only a four-year cycle, so it has to be corrected by hand once a century, when the Gregorian calendar skips a leap year.
How is the leap year shown?
Many perpetual calendars mark the four positions 1, 2, 3 and L. The numbers stand for the three common years, and L stands for the leap year. Patek Philippe marks the leap year with a Roman IV or an Arabic 4 instead. Not every maker puts the display on the dial. H. Moser & Cie. places it on the movement side, where it can be seen only from the back of the watch.
Why is it hard to set?
Of all the settings on a perpetual calendar, this one causes the most trouble. The indicator has to match the real year’s place in the four-year cycle. Its reading reflects the position of the cam that encodes all 48 months of that cycle. If it is off by just one position, the month-end jumps will be wrong. The mistake does not stay in one year. Errors at the end of the month then appear in every following year.
Not to be confused with
An annual calendar differs from a perpetual calendar because it does not track leap years. It usually has to be corrected by hand at the start of March. A perpetual calendar uses its leap year indicator and four-year cam to handle 29 February without correction. It needs hand correction only once a century, when the Gregorian calendar skips a leap year.
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