The Lunar calendar and solar eclipses

Although lunar calendars may well have been kept across Mesoamerica, the only recorded ones survive in Maya inscriptions, appended to the Initial Series as part of what has been called the Supplementary Series, and in the series of lunations counted in the Postclassic Dresden Codex. In the Supplementary Series, ages of lunations on a given date are generally reckoned from the first appearance of the new MOON, counted by their position in the six-month lunar half-year, and tallied for total days, either 29 or 30. Eventually, the Maya came to recognize that 149 lunations = 4,400 days, or 29.53020 days per month in decimal terms, a number very close to the 29.53059 used by modern astronomers.

In and of itself, a lunar calendar may have been of intrinsic interest, but careful lunar calculations were also necessary in order to produce ECLIPSE warning dates. Eclipses were believed to threaten disaster for Mesoamerican people, so their prediction would have been of great use. Solar eclipses take place only during the dark of the moon, and within 18 days of when the moon's path crosses the apparent path of the suN. The lunar tables of the Dresden Codex calibrated such coincidences in order to generate eclipse warning dates. Late in the 8th c., a total eclipse did occur during the dry season in the Maya lowlands, and the phenomenon was recorded in the Supplementary Series of a stela at Santa Elena Poco Uinic.