The Supplementary Series in Long Count dates is probably to be interpreted as the statement of the day reached by the Initial Series in a lunar calendar with an accumulated error; that is, the Mayas had an uncorrected lunar count as well as an uncorrected calendar year. Glyph C records a number of complete lunations which is never in excess of six; Glyph D gives the number of days in the current lunation when these are 19 or less and Glyph E, which has the basic value of 20, finishes the count of a current lunation. There is some evidence that the Mayan lunar calendar in the fifth century A. D. had receded about four days from the true positions of the moon, the count being made from the new or conjunctional phase. When, however, a new or full phase actually coincided with an important round number in the day count special record of the fact was made.
Fig. 50. Representations of the Moon: a, sun and moon hieroglyphs; b, moon from a “celestial band”; c, moon hieroglyph used for 20 in codices.
Fig. 51. The Last Glyph of the Supplementary Series: a, moon glyph; combined with the numeral 9 or 10 to indicate a 29 or a 30 day lunar month.
The lunar table in the Dresden Codex does not apply precisely to records of the First Empire but possibly may be adjusted to the times of the Second Empire. The indications are, however, too complicated to be examined in detail.
Venus Calendar.
Mayan astronomers reached a remarkable knowledge of the movements of the planet Venus and evolved a Venus calendar based essentially on the correspondence between 8 calendar years of 365 days each and 5 apparent or synodical revolutions of Venus of 584 days each. Venus whirling on an inside orbit actually makes thirteen revolutions around the sun in very nearly the same time that the earth makes eight revolutions and therefore passes between the earth and the sun five times (the difference between 13 and 8) during the course of this astronomical period of 2920 days. Just before inferior conjunction the planet disappears as evening star and a few days later emerges as morning star. The mean length of the synodical revolution of Venus is 583.92 days and the actual length may vary about four days from this mean. While the Mayas standardized the Earth year at 365 days and the Venus year at 584 days, they were fully aware of the amount of error in each case, and made proper correction for it without resorting to the devices of intercalation or excision.
We have seen that the Mayas manipulated the year and the lunation in combination with the tzolkin or permutation of 20 days and 13 numbers. They also found a round of these elements in combination with the phases of Venus. Since the period of 2920 days is divisible by 20 but not by 13 it had to be taken 13 times before the round of the Venus calendar was reached.
In the Dresden Codex five pages are devoted to this round of the Venus calendar. Each Venus year of 584 days is divided into four parts of 236 days for the phase of morning star, 90 days (superior conjunction), 250 days (evening star) and 8 days (inferior conjunction). These divisions agree closely enough with actual appearance. But we must remember that the observations were made without instruments and that the planet cannot be seen by the naked eye when close to the sun. Moreover we must expect beliefs as to the nature of this planet, personified as a god, to supplement the knowledge gained from actual observations. The obscuration of Venus at inferior conjunction seems to have been greatly dreaded especially when a round number in the day count fell within the eight days of its duration. A grotesque two-headed monster apparently ruled this fatal period: on the front head is seen the symbol of Venus and on the rear head the symbol of the sun, both associated with elements of death.