Once again this is the year for the perpetual calendar to shine. Leap year is when we distinguish the good from the not so good in watches. In two ways actually. Not only do we need the watches to show us the extra day in February, we also demand that they switch to March 1st on the correct day.
It may sound simple, but in real life this is af ormidable technical and mechanical challenge. A four-year interval need a program that differentiate a 48 month cycle. As a rule, this task is handled by a wheel with 48 teeth in which the durations each month during a four-year period are permanently stored in the form of gaps that have different depths.
All displays of the perpetual calendar switch forward instantaneously, so they deliver precise readings at all times. The force needed for the switching cycles is gradually built up during a period of 24 hours. For this reason, the switching process does not affect rate accuracy. This requires two mechanical energy storage devices – one for advancing the date, the day-of-the week, and the moon-phase display on a daily basis, and the second one to switch the month ring and the leap-year disc.
The mechanism is designed such that a first one-day correction is not needed until the year 2100. In 2100, the centennial rule of the Gregorian calendar omits the leap year and skips 29 February as an exception.
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