Mesoamerican Venus calendars

The Role of Venus in Mesoamerican Calendrical Origins
by Vincent H Malmström

Astronomical calculating Stone Rings at Zempoala

In earlier papers I have argued that the unique 260-day sacred almanac which was used throughout pre-Columbian Mesoamerica traces its origin to the interval between zenithal sun passages over the Formative site of Izapa in southernmost Mexico. I have argued further that, by employing the Goodman-Martínez-Thompson correlation, it is possible to date the beginning of that count very precisely to August 13, -1358 (or 1359 B.C.). (Because the present Gregorian calendar was not used in the Christian world before A.D. 1582, the equivalent date in the Julian calendar was August 26, -1358.) As I have also pointed out, each year the southward passage of the zenithal sun over Izapa is heralded by the Perseid meteor shower on the two preceding evenings, but on that specific morning in 1359 B C. another astronomical event of significance took place of which the Zoque priest who formulated the sacred almanac must surely have taken notice. That was the heliacal rising of the planet Venus (at 2:48 A.M. local time) over Volcán Tajumulco, the highest mountain in Central America.

We know from the later records of such peoples as the Maya and the Aztecs that the heliacal rising of Venus was attended with great awe, if not apprehension. Of course we can only speculate that the priest who set in motion the sacred almanac at Izapa was actually aware that this event would take place on this auspicious morning, for, if he wasn’t, it certainly was a most fortuitous coincidence.

We likewise know that, despite Venus’s mysterious “disappearances into the underworld” as it alternated between being a morning star and an evening star, the Mesoamericans managed to fix the length of its complete cycle as 584 days. In doing so, they must have also been intrigued to discover that with the completion of each cycle, the day-number of the sacred almanac decreased by one whereas the day-name advanced by four.

(See Table 1. Please note that Maya day names are used in the absence of their Zoque equivalents.)

Table 1 — Heliacal Risings of Venus at Izapa, -1358 to -1342

Source: Voyager 1.2 Program, Carina Software, San Leandro, CA 94557

From the above table, the calendrical regularity of Venus’s heliacal rising is readily apparent, as is also the fact that with every fifth completion of its cycle Venus once more rises in the vicinity of the Volcán Tajumulco. Nevertheless, even within the time-span examined in Table 1 it must have been clear to the priests in Izapa that the heliacal rising of Venus was getting out of phase with the zenithal passage of the sun, for in the year -1350 the latter did not pass overhead at Izapa until two days later (on 11 Akbal according to their count), and in -1342 its zenithal passage occurred five days later (on 8 Chicchan).

As we have seen, assigning a length of 584 days to the cycle of Venus allowed for a very convenient way to calibrate it calendrically. Although every fifth time its heliacal rise once again took place over Tajumulco, the interval between it and the zenithal passage of the sun steadily continued to widen. Had the priest chosen an interval of 584.4 days for the duration of the Venus cycle, the two events would have remained in phase with one another, but the fact that he did not strongly suggests that he was unable to conceive of any temporal unit shorter than an entire day — in other words, mathematically he was able to think in integers but not in fractions.

On the other hand, it must have been tempting for the priest to have believed that, because of the calendrical regularity of the Venus count, the real problem of calibrating the two events lay in defining the interval between the zenithal passages of the sun. In effect, he was recognizing that although the sun spent 260 days south of Izapa and 105 days north of it, these two values did not equate to the length of its total cycle — in other words, to the true length of the solar year. This problem was far more serious than the growing discrepancy between the heliacal rising of Venus and the zenithal passage of the sun, for already within four years a lack of calendrical regularity began to manifest itself. Rather than passing overhead at Izapa on 1 Imix, the sun now passed overhead on 6 Ik instead, and in four more years its passage occurred on 11 Akbal, and so on. Clearly, if the original intent of his time-count had been to help define the beginning of the rainy season, for example, then the 260-day sacred almanac was already quickly becoming less and less useful.

To redefine the length of the solar year required a new and hopefully more precise starting point than the zenithal passage of the sun. Through continued observations of its movements it gradually became apparent that, as viewed from Soconusco, the sun reached its northernmost turning point in the sky (i.e. the summer solstice) somewhere behind the great wall of volcanoes that loomed up on the northeastern horizon. Therefore, by careful planning it should be possible to find a site where the zenithal passage of the sun on August 13 and the June 22 sunrise over the highest mountain in Central America could both be calibrated at the same place. As I have explained in earlier writings, it was this fortuitous combination of circumstances that was responsible for the choice of Izapa’s location.

With the priest now thinking in terms of the summer solstice and the Volcán Tajumulco, it was perhaps only natural that the earlier correspondence of the heliacal rising of Venus over the mountain and the zenithal sun passage should once again come to mind. In previous papers I have demonstrated that the internal structure of the 365-day secular calendar strongly suggests that
it came into use during the period between -1324 and -1321, and that it may in fact have been the product of the same mind which had developed the sacred almanac just over thirty years earlier.

By employing the Goodman-Martínez-Thompson correlation, we find that the summer solstice (June 22 in the Gregorian calendar, or July 3 in the Julian calendar) coincided with the date of 0 Pop during only those four years. It is also interesting that there were heliacal risings of Venus in two of those years — one at 4:12 a.m. in -1324 when the planet rose at an azimuth of 67º 35′, or nearly over Tajumulco, and one at 3:10 a.m. in -1321 when its rise took place at an azimuth of 72º. The date of the first occurrence according to the sacred almanac was 4 Manik and of the second, 7 Ik. One is tempted to conclude, therefore, that of the two possible dates for the secular calendar’s beginning it was the former on which it was set in motion. Indeed, it may have been that the choice of 4 Manik for the initiation of the 365-day secular count gave rise to the notion held by later peoples, such as the Aztecs, that previous creations of the world likewise occurred on days numbered 4 and that our present world shall also end on a day numbered 4.

(It may be of interest to note that the next heliacal rising of Venus on August 13th will take place in the year 2001 at 3:15 a.m. On that occasion, its azimuth will be 67º 21′, or still very close to Volcán Tajumulco. In the year 2003 a heliacal rising of Venus will occur on the summer solstice (June 22) at 4:37 a.m. closer yet to Volcán Tajumulco, namely at an azimuth of 67º 02′.)

Vincent H Malmström Dartmouth.edu 2000

One Comment on “Mesoamerican Venus calendars”


  1. […] (solstices, equinoxes), the Governor’s Palace at Uxmal is precisely aligned to view the heliacal rising of Venus at its southernmost point in an 8-year […]


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