In honor of Friday the 13th, let’s talk about lunar cycles and the number 13. The moon’s sidereal period is 27.3 days—this is the time it takes the moon to complete one orbit relative to the stars. When one divides 365.24 (the number of days in a year) by 27.3, the result is 13.38. One could argue, that based on this, a lunar calendar should have at least 13 mo(o)nths. (Oh, what to do with that fraction of a month?!) However, the moon’s synodic period (the time it takes to complete all of its phases—i.e., waxing, full, waning, new) is 29.53 days. If one divides the years by this factor, the result is 12.37. So, perhaps there should be 12 mo(o)nths. As one can see, the effort to integrate the lunar and solar cycles into a single calendar is inexact at best.
Posted on: Fri, 13 Sep 2013 15:59:19 +0000
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