The Hebrew perpetual calendar can rightly be considered to be one of the greatest contributions to modern mechanical watchmaking. From a mathematical and technical point of view it is a most complex system.

Hebrew perpetual calendar and 19-year cycle

Making its very first appearance as a watchmaking mechanism, the Hebrew perpetual calendar in the Vacheron Constantin Reference 57260 watch is an unprecedented creation. It calculates the date based on the 19-year Metonic lunisolar cycle.
In the 5th century B.C. the Greek astronomer Meton observed that 19 solar years are made up of almost exactly 235 lunar months. The Metonic cycle therefore comprises 12 simple years and seven years to which an extra month is added to align the Hebrew calendar year with the sidereal year. While these cycles ensure extreme precision over the very long term, compensatory systems must be used to synchronize them with the solar calendar.

Hebrew day number

To the left of the Hebrew date, the day numbers in Hebrew are displayed in an aperture

Hebrew month number

To the right of the Hebrew date, the months in Hebrew are displayed in an aperture

Hebrew date

In the Hebrew calendar there are fixed lunar months alternating between 29 and 30 days. The months of Cheshvan and Kislev can comprise 29 or 30 days, depending on the type of year.

Hebrew secular calendar Hebrew century, decade and year Age of Hebrew year (12 or 13 months)

In order to synchronize the year of 12 lunar months with the solar year, a thirteenth intercalary month (Adar I) must be added seven times over the 19-year cycle.
In the Reference 57260 timepiece, the ingenious mechanism performs this synchronization by specifying whether the current year has 12 months (common year) or 13 months (embolismic year), via a 12/13 indicator with concentric hand on the minutes totalizer of the chronograph at 9 o'clock.

Golden number (19 years)

The golden number in astronomy is an indication of the 19 years of the Metonic cycle. Vacheron Constantin presents a totally unique complication that integrates the golden number into a highly complex Hebrew perpetual calendar.
The 19-year Metonic cycle —named after Athenian astronomer Meton who lived in the 5th century B.C.— is an almost perfect multiple of the solar year and the lunar month.
These 19 years, which add up to 6,940 days, correspond to approximately 235 lunar months, with only a few hours' difference between the two periods. The Metonic cycle may also be used to predict eclipses or calculate the date of Easter.
As a year represents 1/19th of the cycle of 6,940 days, it lasts 365 + 1/4 + 1/76 days, or a little over 12 lunar months. Over the period of 19 solar years, in order to correct the loss in lunar years a 13th lunar month is added seven times.

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