The Calendrical Influence Of The Moon

The Moon also affects us in sundry ways. Much has been made by numerous authors of the apparent fact that the human female menstrual cycle has an average duration of about 29.5 days, which is indistinguishable from the cycle of lunar phases. One might consider this to be a coincidence. On the other hand it might be evidence of a causal relationship, perhaps amenable to scientific analysis; for example it may be that repeated high fertility near new moon, when it is dark and dangerous to roam at night, favored reproductive success in early humans.

Quite apart from this physiological cycle, it is indisputable that our natural satellite exerts control over our affairs. First, we should look at the calendar (or perhaps I should write calendars, because different nations and religions use calendars other than the familiar calendar to which we in the Western world are habituated) . In the Western calendar the months, despite the etymology of that word, are no longer linked to the lunar phases, which is why I differentiate between lunar and calendar months. That divorce between the calendar month and the Moon, in the evolutionary history of the Western calendar, occurred before 400 B.C., in the Roman republican era before Julius and Augustus Caesar instigated Imperial Rome. The influence of learned Egyptians upon Julius Caesar ensured that his reformed calendar post-46 B.C. was an exclusively solar calendar.

Other calendars retain the influence of the Moon. While the civil Western calendar is solar, the ecclesiastical calendar endorsed by Pope Gregory XIII in A.D. 1582 (called the Gregorian calendar) is luni-solar. That is, the Moon defines the dates of all move-able feasts in the liturgical year, reckoned from Easter, which is based on the full moon after the spring, or vernal, equinox. (Beware, though, that the days of the full moon and the equinox used to derive the date of Easter Sunday are founded upon ecclesiastical rules, rather than the real Moon and Sun in the sky.) The Hebrew calendar is also luni-solar, with the Moon affecting the dates on the Western calendar for Passover, Rosh Hashanah, and Hanuk-kah. Similarly Chinese New Year is often celebrated at the second full moon after the winter solstice, although again the precise rules are complicated. In such calendars the Sun still defines the average (mean) length of the year, because the holidays are simply phased according to the lunar cycle following some solar-defined juncture (one of the equinoxes or solstices).

The Islamic calendar contrasts with this, the year being defined exclusively by the Moon, the annual round containing 12

lunar cycles. Because each month starts with an observed new moon, and there is only one chance a day to witness this (after sundown), the months and the years must contain a discrete number of days. On average, the Islamic year lasts for 354.37 days, but particular years generally contain either 354 or 355 days, and more extreme lengths are possible because of vagaries in spotting the new moon owing to atmospheric conditions. The Islamic lunar year is thus 10 or 11 days short of a solar year, and the calendar slips through the seasons on a cycle of almost 34 years.

More information about the astronomical bases of different calendars is given in the Appendix.

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