The Month

The month as a practical unit of time derives naturally from the cycle of phases of the Moon, the synodic period or synodic month. In §2.3.5, the several different periods of the Moon or months were defined. The two calendrical months were the sidereal and synodic months.

Sidereal months were used in a number of cultures, from India to California, but for various purposes and in different ways. The passage of the Moon through a particular asterism marks the length of a sidereal month. Sidereal intervals were used across Micronesia and Polynesia and in the southwestern United States. In India, the usage was in connection with the nakshatras, or lunar mansions, a series of 28 asterisms (sometimes 27, due to the omission of Abhijit, Vega). The purpose of the nakshatra system was to reconcile sidereal and lunar motions. The interval of 13 sidereal months, each of an assumed length of 28d, gave a 364d year in Southeast Asia.

The use of synodic months was much more widespread. There is evidence of the measurement of days using lunar phases from early human history. Marshack (1972a) has cited an elaborate serpentine pattern of crescent-like markings on the Blanchard bone as the earliest example of human notation and has argued that it seems to record the changing phases of the Moon over a two-month period during an Ice Age, more than 25,000-30,000 years ago. The reindeer bone was excavated in 1911 from the Blanchard rock shelter near the southwestern French village of Les Eyzies, central among the known sites of Cro-Magnon man. Marshack has suggested several other possible Ice Age examples as well (see §6.1).

A much more recent possible record is that on a well-crafted pyrite mosaic excavated from an Olmec site at Las Bocas, Mexico, and dated ~1000 b.c. Marshack (1975) has argued that the mosaic patterns themselves constitute a (basically lunar) calendar, but the argument is not complete because part of the pendant is apparently missing. This is unfortunate because the pyrite is relatively well preserved. Although the Olmec heartland, including the major sites of La Venta and San Lorenzo, was in the northern half of the Isthmus of Mexico, this site is in western Puebla in central Mexico. The mosaic may have been worn on the chest as a pendant or pectoral, as were the pectoral mirrors of later Mesoamerican groups. More recently, Spackman (1996) has attempted a much more complex mathematical, calendrical, and astronomical analysis of the pendant, comparing it with other Olmec artifacts and with Mesoamerican mythology. Spackman argues that the ornament was designed for a headdress rather than a pectoral and that it was prepared for a particular ceremony of 21 March 1083 b.c. (JDN 1325947). The date is reached by an amplification of Marshack's techniques but rests on a substantial number of additional premises. At present, Spackman's analysis seems untestable.

Lunar calendars were widely used in the ancient world. In Mesopotamia, the civil calendar was lunar. Each month began with the first sighting of the waxing crescent. According to Neugebauer (1983, p. 1), the main goal of Babylonian lunar theory was the accurate prediction of the dates of such occurrences. To this end, ephemerides or tables of the lunar positions, much like those in the Astronomical Almanac of today, were produced. An example is shown in Figure 7.4 and Table 7.11. The tables contained such information as the year (in the Seleucid era14) and the month, the longitude (in zodiacal signs and degrees) and latitude (in units of barleycorns = 1/72 degree) of the Moon, the daily motion of the Moon (in °/d), the date and time of new or full Moons, and sometimes, the time between settings of the Sun and Moon at the start of the month and the time between risings of the Moon and Sun on the last day of the month. The tables also contained information related to eclipses, the duration of daylight, the previous month's duration, and corrections to the latter to permit a calculation of the current month's duration. See Table 7.3 for a list of Babylonian months. An average day-length, 1/30 of a mean synodic month, was also used.

Unlike the Babylonian months, the Egyptian months were not strictly lunar. There were 12 months, each of exactly 30 days, with 5 extra or "epagomenal" days added to make up a year of exactly 365d (the Egyptian Year). It is clear that

14 A year y in the Seleucid era is (to within a year) y - 311 in the Gregorian calendar. Because most relevant dates taken with respect to this era base are before Christ, we may also write the equation: b.c. date = 312 - y (see §4.1.5). A precise equivalent from Neugebauer (1955/1983,

I, p. 7) is 188 b.c. July 17 = S. E. 124III28, where "III" is the third month.

with such a fixed length, these months are no longer closely tied to any astronomical phenomenon. Because of the ease of use in calculation, Ptolemy used the Egyptian months to describe the dates of his observations, even the very old ones. The Egyptian months appear on a part of the Antikythera mechanism and were widely used throughout the Mediterranean world. The names of the months are given in Table 8.2. Even in the Middle Ages, the Egyptian year was called the Astronomers' Year, and it served the purpose of calculating intervals between astronomical events. In this respect, it provided a function similar to the current Julian Day Numbers.

Among the Yuman groups of the southwestern United States and adjacent Mexico, a combination of synodic lunar months and sidereal intervals approximating a month was in use. The six summer months were true lunations, whereas the six winter months, which repeated the summer month names, each began with the heliacal rising of stars. The stars were so selected that the heliacal rising intervals approximated the length of a lunar month. Further details about calendars can be found in Kelley and Stewart (in preparation).

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