Astronomical Observatories.
One of the most interesting pieces of evidence in support of the correlation explained above has to do with a giant sun dial at Copan. Two stelæ stand on opposite sides of the valley establishing a line which runs about 9 degrees north of west. When observation is made from the eastern marker the sun sets behind the western stone two times during the course of a year, once shortly after the vernal equinox and once shortly before the autumnal equinox. Now the Mayan chronicles state that the calendarial New Year was “counted in order” during a certain Katun 13 Ahau which extended from 491 A. D. to 511 A. D. Altar U at Copan was observed to record two New Year’s dates equaling April 9 in conjunction with another date, equaling September 2, 503 A. D., and falling in the required interval covered by Katun 13 Ahau. These dates were such as might be reached by just such a base line as exists at Copan and it was first believed that they were exactly reached by it. Careful reconsideration of the evidence in the inscriptions and a re-survey of the line of sight led to the interesting conclusion that the sun dial of Copan was originally set up in 392 A. D. to give sunset coincidences on April 5 and September 6. About 490 A. D. the stones were re-adjusted to give the April 9 and September 2 which are recorded on Altar U and still later a third and present arrangement was effected giving April 12 and August 30. Each pair of dates is “reciprocal” in the sense that one member marks the same interval after the Spring equinox that the other does before the Fall equinox. The shifting seems to have been decided upon by astronomical congresses, and the purpose was to fix propitious times of planting the crops.
Fig. 49. Diagram of the Astronomical Base Line at Copan giving readings at April 9 and September 2. Slight shifts were made in this line: at an early time it was arranged to read April 5 and September 6 and at a later time April 12 and August 30.
Other Mayan observatories at Uaxactun and Chichen Itza have lines of sight which mark exactly the positions of the sun (the summer solstice, etc.), and all in all the evidence deduced from these observatories is in complete agreement with the correlation of the Mayan and Christian time counts originally effected on the evidence in sixteenth century documents.