Abstract

The lunisolar Jewish calendar is an example par excellence of "time engineering." It is based on the relation between the mean period of the lunar motion about the earth and the mean period of the earth about the sun (the tropical year). This relation is that, to six places of decimals, the number of days in 19 tropical years is equal to the number of days in 235 lunar cycles and is known as the metonic cycle. Moreover, the Jewish calendar, unlike our civil calendar, for example, is intertwined with the days of the week by virtue of the first day of Tishri (Rosh Hashanah) being allowed—for religious reasons—only to occur on Monday, Tuesday, Thursday, and Saturday. Because the metonic-cycle relation is not exact, there will eventually be a significant seasonal drift in holidays such as Passover unless the calendar construction is modified appropriately.

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