Of all the problems dealt with by Maya scholars, the so-called correlation problem is the most fundamental. It is only a solution to this problem which allows us to locate the phenomenon of the Maya civilization in a time-frame that relates to our own. The correlation problem is the problem of finding a particular day which is identified both by a Western date and by a Maya date, allowing the Western date to be equated with the Maya date. Once this is done, conversion from any date in one system to a date in the other becomes possible. Without a solution to this problem we may be able to determine that the great ruler of Palenque, Lord Pacal, died on 9.12.11.5.18  6 Edznab 11 Yax, but whether this was before or after the fall of Rome or the coronation of Charlemagne, and by how much, would remain unknown.

The "correlation number", C, is the number which will allow us to correlate the Maya and Western calendrical systems. It can be defined as the Julian day number of the most recent day denoted by the Maya long count 0.0.0.0.0. Then we can say that the day denoted by a given long count is the day with Julian day number L + C, where L is the long count. For example, if Smiley [62] is correct then the correlation number is 482,699, and so the long count 9.16.4.10.8 (= 1,412,848 days since 0.0.0.0.0) corresponds to the day with Julian day number 1,412,848 + 482,699 = 1,895,547, which is 477-09-22 in the Julian calendar (and 477-09-23 in the Gregorian).

As noted above, most long count dates occur in the inscriptions together with a tzolkin/haab date, e.g. the date occurring on a stela at Quirigua given as 9.14.13.4.17 12 Caban 5 Kayab (see Morley [39], pp. 218-219). This implies that the preceding day, with long count 9.14.13.4.16, was the tzolkin/haab date 11 Cib 4 Kayab, and so on. We may thus work backwards through the system of long counts and the system of tzolkin/haab dates to find what tzolkin/haab is (or would have been) associated with the long count 0.0.0.0.0. The answer is 4 Ahau 8 Cumku. The first day of the current Maya era (of 13 baktuns) is thus 0.0.0.0.0 4 Ahau 8 Cumku, the base date of the Maya calendar.

It follows that the Julian day number of the day denoted by any given long count date is simply the sum of the long count and the Julian day number of 0.0.0.0.0 4 Ahau 8 Cumku, which is (by definition) the correlation number.

An adequate correlation number should be such as to accord with:

• data from the Venus table in the Dresden Codex
• most of the post-Conquest survivals of the tzolkin among the Maya peoples
• Bishop Landa's 16th-Century records on the subject
• the Aztec records of the arrival of Cortes and
• the lunar information found on the stellae

As noted above, numerous scholars have suggested various values for the correlation number (see the list given in Severin [61]). The answer usually accepted is 584,283, the number suggested by Sir J. Eric S. Thompson. It is thus called the Thompson correlation (a.k.a the modified Thompson 2 correlation). Other scholars have suggested other correlations, from 482,699 (Smiley) through 774,078 (Weitzel). These correlations imply a date for 0.0.0.0.0 4 Ahau 8 Cumku ranging from -3391-06-26 (G) to -2593-04-03 (G), and a date for the end of the current Maya era, 13.0.0.0.0, ranging from 1734-11-05 (G) to 12 August 2532 (G). Thus according to some scholars the current Maya era ended over 200 years ago whereas according to others it still has over 500 years to run. According to the Thompson correlation 0.0.0.0.0 4 Ahau 8 Cumku occurred on -3113-08-11 (August 11th, 3114 B.C.) and 13.0.0.0.0 occurs on 2012-12-21 (December 21st, 2012).

Strangely, scholars seem particularly prone to reporting the date of 4 Ahau 8 Cumku incorrectly. Some give the year as 3113 B.C., but this may be due to a misunderstanding of the astronomical system of denoting years; the year -3113 is the year 3114 B.C.

Tedlock [65] has reported that, for the contemporary Mayas with whom she worked, 1977-03-02, was the tzolkin date 4 Ik. These and other tzolkin dates that she gives are consistent with the Thompson correlation. Severin [61], after a survey of evidence for and against the various suggested correlations, concludes that the Thompson correlation seems to have the most support. The matter, however, still cannot be said to have been established beyond doubt, and some anthropologists still adhere to a correlation differing from the Thompson correlation by a day or two.

There is one way to settle the matter once and for all. This is to discover in the Mayan codices a clear reference to an eclipse (complete with long count date) which can also be determined by astronomers as having occurred on a certain date in the European calendar. Solar eclipses are promising, because they are visible only in a restricted area (unlike lunar eclipses which can be seen over most of the Earth's night hemisphere), and thus are not common in any one region such as the Mayas occupied. Several scholars (e.g. Owen [46] and Smiley [62]) have sought an event which can be identified in both Maya and Western calendrical systems among the dates of known or calculated astronomical phenomena but despite heroic attempts their work has not been convincing.

Although 0.0.0.0.0 of the current Maya era coincides with a tzolkin/haab date of 4 Ahau 8 Cumku, this is not true of every Maya era. The tzolkin date will always be 4 Ahau, since there are 1,872,000 days in a Maya era and this number is divisible by 260, the number of days in the tzolkin cycle. But 1,872,000 is not divisible by 365, the number of days in the haab cycle, so the haab date of the first day of a Maya era will vary. The tzolkin/haab dates for 0.0.0.0.0 of the current, next and previous Maya eras are given below together with (assuming the Thompson correlation) the corresponding Western dates in the Gregorian calendar:

    next        4 Ahau  3 Kankin     2012-12-21
    current     4 Ahau  8 Cumku     -3113-08-11
    previous    4 Ahau  8 Zodz      -8238-04-01
    previous    4 Ahau 13 Mol       -13364-11-20
    previous    4 Ahau 18 Ceh       -18489-07-12
    previous    4 Ahau  3 Kayab     -23614-03-01
    previous    4 Ahau  3 Zip       -28740-10-19

Even if we can determine the Western date of 0.0.0.0.0 4 Ahau 8 Cumku, this still leaves open the question of why the Mayas used this day as the base date for the current Maya era. In discussing the Hindu calendar Aveni mentions "a conjunction of all the visible planets in the constellation of Aries, an event calculated to have occurred at midnight on the night of 17-18 February 3100 B.C." ([5], p.130). This is surprisingly close to the year 3114 B.C. of 0.0.0.0.0 as dated according to the Thompson correlation. Could it be that the Mayas regarded this same group conjunction as marking the beginning of the current era? Since all of the visible planets never line up exactly, it is difficult to give a precise date to when they might all be said to be in conjunction. Thus a difference of 14 years from the result of some particular method of calculation is explicable.

Since presumably there were no Maya astronomers in 3114 B.C. (because presumably there were no Mayas) if this hypothesis is true then the Mayas had sufficient astronomical knowledge to be able to calculate planetary positions in the distant past. Severin [61] has claimed that the Mayas did indeed have this ability, even to the point of being able to calculate the period of the precession of the equinoxes (which is somewhat in the neighborhood of 25,000 years, a period roughly equal to five Maya eras).

All such claims that the Mayas possessed an advanced astronomy, especially those that present the Mayas as familiar with the size of the galaxy, and so on, should be subjected to close examination. It is tempting to view the Mayas as in possession of esoteric secrets, and perhaps they were, but it is another question as to whether we can ever know that they were.

Consider, for example, the fact that light travels at a finite velocity. This fact was discovered in Europe only in the 17th Century, by Galileo, using one of the earliest optical telescopes to observe the moons of Jupiter. Did the Maya have optical telescopes? Or telescopes of any sort? No evidence has thus far been found that they did. If not, how could they have been aware that light travels with a finite velocity? And if they did not know this then they certainly could not have known that it takes light 100,000 years to travel across the galaxy.

Even though the Mayas apparently lacked the precision technology that has made possible the most advanced discoveries in Western science, there was one source of knowledge readily available to them, namely, the psilocybin mushroom. Such esoteric knowledge as the Mayas possessed was perhaps obtained with its help. It is thus not necessary to elevate the Mayas to the status of a galactic race, unique on Earth, since the sources of their knowledge of the cosmos, whether it be by observation of the heavens or by observation of interior psychic landscapes, are readily available even to us.

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"Linda Schele and David Freidel, unlike most Mayanists, continue to support the work of Floyd Lounsbury in promoting the 584285 correlation. This is 2 days off from the Thompson correlation that I use. The decisive factor in supporting the Thompson correlation of 584283 is the fact that it corresponds with the tzolkin count still followed in the highlands of Guatemala. To account for this discrepency in his correlation, Lounsbury claims that the count was shifted back two days sometime before the conquest (not likely), thus explaining its present placement. This means that either correlation will give the December 21st end date. Nevertheless, Schele and Freidel still report that the end date is December 23rd, 2012 rather than Dec. 21st, an unfortunate faux pas understandable only because they aren't particularly interested in the specifics of the correlation debate. For a detailed discussion of this topic, refer to my book Tzolkin: Visionary Perspectives and Calendar Studies." John Major Jenkins, footnote 1 in The How and Why of the Mayan End Date in 2012 A.D.