To the Limits
by David Dearborn

Few would doubt that Native American groups employed lunar calenders recognizing that some years contained 12 full moons while others had 13. This requires a recognition of the synodic month, 29.5 days, and the length of the year, 365 days. The Maya, with their interest in astronomy, determined a great deal more. The eclipse table of the Dresden Codex contains the number 177 in a long series, with an occasional 148 and 178. Simply averaging this series results in a value of 173.31, the number of days between eclipse seasons, or half the Eclipse year. As eclipses occur only at new or full moon, the 177's (and occasional 178) are easily understood as the number of days between 6 full (or new) moons, or the time between a pair of eclipses. As this period is 4 days longer than the 173 days separating the eclipse seasons, a series of eclipses beginning near the start of an eclipse season will advance through subsequent seasons until it occurs near the end. When this happens, the next eclipse will occur after only 5 synodic months (148 days), at the beginning of the next eclipse season.

The time between eclipse seasons results from the 18.61 year precession of the moon's orbit. This precession advances the locations where the moon's orbit crosses the ecliptic (the nodes). Eclipses will occur on the new or full moon nearest the time that the sun passes one of the nodes. Because of precession, the time for the sun to move from one node to another is only 173.31 days.

One must consider how the Maya determined this 173.31 day period. It can be derived from a long-term record of eclipse data, but such data would certainly be incomplete, as not all eclipses would have been visible to the Maya. A directly related observation that could have provided the Maya with a clue to the precession of the moon's orbit is the dramatic change in the moon's monthly rise (or set) point. As the moon's orbit precesses, its monthly declination range changes from +/- 18 degrees at minor standstill to +/- 28 degrees at major standstill, 9.3 years later. This causes a variation in the azimuth limits of moon rises (and sets) of about 10 degrees in tropical regions (more in temperate ones).

Evidence is accumulating that Native Americans living in what is now the United States monitored this 18.61 year precession period of the moon. Foremost among this is the work of J. McKim Malville at Chimney Rock published in Archaeoastronomy, no 16 (JHA, xxii(1991). The Chimney Rock Pueblo, 5AA83, is a Chaco outlier located such that the major northern standstill of the moon was observable between the prominent double chimney feature on the horizon. Dendro-dates show occupation and construction activity coincident with standstill dates in 1076 and 1093. This supports the suggestion by Anna Sofaer, Rolf Sinclair, and LeRoy Doggett for a lunar standstill marker at Fajada Butte (in Archaeoastronomy in the New World, 1981, ed. Aveni), and recent work by Anna on the orientation of major Chacoan structures reported at Oxford 3, and as yet unpublished. There would be little question of the significance of these alignments if they were corroborated with ethnographic data from Pueblo cultures. Such ethnographic evidence has been sought, but not found. Steve Mclusky (in World Archaeoastronomy, p. 362) has noted that certain irregularities in the Zuni scheduling of Shalako can be understood if they were influenced by the position of moon rise, but this data do not resolve the question of whether or not they were aware of lunar standstills.

Additional evidence suggesting Native American interest in lunar standstills has been gathered by Bill Romain (see articles listed in new publications). He proposes alignments to lunar standstills in a number of Hopewell earthworks. Unfortunately the current state of most Hopewell sites does not permit great accuracy for individual measurements, but perhaps the growing data base on these sites will permit a statistical investigation to test the hypothesis that the alignments are intentional.

While the present evidence that Native American cultures of the United States were monitoring the 18.61 year cycle of the moon by means of the lunar standstills is not compelling, it is reasonably suggestive. The existence of contact between the Southwest and Mexico adds plausibility to the hypothesis that Southwestern groups knew of lunar standstills, or at least the existence of long-term cycles of the moon. This appears to be a developing area of inquery, that could benefit from a survey of Mexican and Mayan sites to determine the nature of the lunar observations that they must have made.