The Bases and Functions of Calendars

In preceding chapters and sections, the bases of most calendars are the motions of the Sun and Moon, even if there are some calendars that are based primarily on the seasonal appearances of stars. The reason for using the Sun is fairly clear: The seasonal variations of climate, involving temperature changes in higher latitudes and mainly precipitation changes in lower latitudes, lead to changes in vegetation and in wildlife behavior. Such changes are usually seasonal, i.e., connected to the tropical year. With these variations, there are changes in the stellar asterisms that are visible at any particular time of night, hence, "the rainy Pleiades," which rose as the Sun set at the beginning of the rainy season. The Moon has played a strong calendrical role due to the obvious changes in phase and the useful light provided by a gibbous or full Moon and its tidal effects. There are also ample cultural connections between the Moon and rain, and between the Moon and woman (the approximation of the synodic month to the menstrual cycle). Its early (perhaps Ice Age) use may be surrounded by magic and cultural associations.

European calendars—from Julius Caesar's time to the present—are solar calendars, retaining the use of intervals called months, but paying no attention (in a calendrical sense) to the phases of the Moon at which the month begins. Caesar's reform was introduced on Jan 1,45 b.c. and marked a clear departure from the luni-solar calendar in place until that time. As the inheritors of the European calendar, North Americans have also foregone the lunar calendar. The religious calendar of Islam, which is also the civil calendar in certain Moslem countries, is a pure lunar calendar. The year ends on the 12th lunar month so that the Feast of Ramadan cycles through the solar year. A lunar calendar that has been the subject of recent studies is the calendar of the Borana people of Ethiopia (Ruggles 1987).

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