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.

The Venus calendar seems to have taken form in the sixth century B. C. on the basis of heliacal risings of the planet as morning star in sets of five making an eight year cycle. The dates in the Mayan calendar especially emphasized in connection with Venus are 19 Xul, 18 Kayab, 12 Yax, 6 Zip, and 5 Kankin standing exactly 584 days apart, while the corresponding dates in the Gregorian calendar are April 12, November 17, June 24, January 29, and September 5. When these sets of dates, one in a fixed and the other in a vague calendar are carried back to a common focus they are found to correspond very closely with the proper astronomical phase of Venus. The maximum difference of the true positions of Venus from the positions in the Venus calendar is then only two days, plus or minus.

The coincidences of the 8 day period of obscuration of Venus at inferior conjunction with the following round numbers in the day count was memorialized by important monuments:—

9.14.0.0.0,	6 Ahau 13 Muan,	Feb. 4, 452 A. D.	Venus rises as morning star
9.17.0.0.0,	13 Ahau 18 Cumhu,	Mar. 27, 511 A. D.	Venus invisible during conjunction
10.0.0.0.0,	7 Ahau 18 Zip,	May 17, 570 A. D.	Venus invisible during conjunction
10.3.0.0.0,	1 Ahau 3 Yaxkin,	July 6, 629 A. D.	Venus about to set as evening star

The Venus table in the Dresden Codex, the introductory page of which has been explained in an earlier section (see [Plate XXIV]) emphasizes the same Mayan and Gregorian positions of Venus as the ancient monuments but this table was evidently intended to be used between the Tenth and Thirteenth centuries A. D. The point of departure for the table is 9.9.9.16.0, 1 Ahau 18 Kayab, April 12, 363 A. D., which does not coincide with an heliacal rising of the planet, although April 12 and 18 Kayab occur in other connections at the time of the inauguration of the Venus calendar in the Sixth century B. C. But in the Lunar table we find 10.19.6.1.0, 4 Ahau 18 Kayab, November 20, 950, which does reach an heliacal rising of Venus as morning star.