It should be stressed that the Mayas knew perfectly well that the Haab calendar of 365 days was not a good measure of the solar cycle. Indeed, they normally measured astronomical cycles with great accuracy and, as far as the calendar is concerned, there is evidence that the value of the solar year calculated by the Mayas at Copan was 365.2420 days, which is actually a more accurate value than that of the Gregorian calendar (365.2425 days) we use today. Apart from being interested in the movement of the sun, the Mayas studied assiduously the cycle of the moon (on which they perceived the outline of a rabbit), with the aim of predicting eclipses. They were also interested in the movement of Venus and the other planets. Venus was the planet that preoccupied them the most, with its sometimes evasive behavior, but always somehow alluding to the activities of the sun (see Appendix 1).
Our knowledge of the Mayas' study of Venus is fortunately considerable, as some of their astronomical data have miraculously been conserved.
In 1566, in Izamal, in the Yucatan, a terrible assault on the culture of humanity occurred. Bishop Diego de Landa, convinced that Mayan books were none other than confirmation of the satanic religion prevailing in Mexico before the conquest, ordered them to be rounded up and consigned to flames. In one day, hundreds of volumes of astronomical observations recorded by Mayan scientists over many centuries went up in smoke. However, by a curious and fortunate twist of history, the plan aimed to completely annihilate the scientific knowledge of an entire civilization failed: four volumes survived (and possibly others that may yet be found). These are the so-called Dresden, Paris, and Madrid codices (certainly authentic, and named from the libraries where they are kept), and the Grollier codex (probably authentic). They were written on sheets of bark a few centuries before the conquest, but the data contained in them often refer to earlier times.
It is no easy task to explain what a Mayan codex is (or at least what the codices that have come down to us are; undoubtedly there existed other historical/literary genres that did not survive). Perhaps a better description of them would be "manuals," as they contain extremely accurate astronomical recordings together with pages of descriptions of rituals and predictions, divined from heavenly events. The most analyzed and comprehensible codex is the Dresden codex, which is 3.5 meters long and divided into thirty-nine 8.5- x 20.5-centimeter folios. It discusses the prediction of the eclipses and the cycles of Venus, Mars, and Mercury. The Grollier codex contains a "Venus Chart'' comparable to that of the Dresden codex, but simpler (some are of the opinion that the Grollier is false). The significance of the codices of Madrid and Paris has not yet been fully grasped. The Paris contains an astronomical chart that is very important for studying Mayan constellations, as we shall shortly see.
The precision of the astronomers who drew up the table of eclipses in the Dresden codex is quite astounding. The codex contains observations on lunar phases over a span of 11,960 days, that is, about 32 years; 405 new moons were observed, which means that the Dresden codex contains the following estimate of the cycle of lunar phases: 11,960/405 = 29.53086 days— a better estimate than Ptolemy's. Of course, there is nothing astonishing in the fact that the Mayas were better astronomers than Ptolemy. What enabled the drafters of the codex to attain such a high degree of accuracy was the purpose behind it: studying the comparability of various cycles, seeking the smallest common multiple. For example, the period of 11,960 days was not chosen randomly; in fact, 405 lunations are the minimum number required
to correspond to a integer number of Tzolkin years of 260 days—46 to be exact. In turn, it is often said that this desire for the periods to be commensurate shows that the Mayas did not possess a scientific mentality, thus "rescuing" poor Ptolemy (from what?). I do not agree at all, however.
As I have already said, the question of deciding who performed the most accurate measurements is not of any real concern. Having said that, the idea of estimating different cycles with the same unit of measurement (the day and its decimals, in this case) is typical of our way of reasoning and handling experimental data. Another way—equally valid—is the Mayan one. Actually, I think we might understand the lunar cycle better if we say, as the Mayas did, that there are 405 new moons every 11,960 days rather than if we say that there is a new moon every 29.530836 days.
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