In 1976, having moved from Middlebury College to Dartmouth College in the
middle of the academic year, I was prevented from returning to Mexico to continue
my field work during the winter dry season. However, I decided instead to avail
myself of Dartmouth's excellent computer facilities and write a computer program
which would not only allow me to "run back" both (1) the 365-day Mesoamerican
secular calendar and (2) the 260-day sacred almanac to the dates of their beginnings
but would also tell me (3) when the Mayas had reformed their calendar by changing
their "New Year's Day" to July 26 and (4) when the ingenious device known as the
"Long Count" had been devised. (My book and my 1978 article in the Journal for the
History of Astronomy detail the parameters which I used in developing my program.)

I inserted four "flags" in the program to alert me when each of the critical
dates had been reached. Here I am concerned with only one of these -- the most
recent. My computer program revealed that the Maya calendar reform most likely
occurred in the year A.D. 48 and that geographically it would have taken place at a
site situated at latitude 19.5º N. -- the latitude at which the zenithal sun passes overhead
on July 26 -- the event and date which Bishop Landa specified as having served to
begin the Mayan New Year. Such a parallel of latitude neatly bisects the
Yucatán peninsula and thus would have served the Mayas well, but the only
ceremonial center located along this line was a place called Edzná. In the
archaeological literature available in 1976, Edzná was identified as a "Late
Classic" site -- in other words, its origins had been dated to between A.D. 600
and A.D. 900.

Needless to say, I made no mention of Edzná in my journal article described
above. However, at the very time I was carrying out my computer reconstruction
of the Mesoamerican calendars, Prof. Ray Matheny of Brigham Young University
was initiating a thorough investigation of the site of Edzná and two years later, by
pure chance, our paths crossed at the headquarters of the New World Archaeo-
logical Foundation in San Cristóbal de Las Casas. Almost apologetically I told him
of my "find" of a calendar reform which supposedly had taken place 600 years before
Edzná had been founded, but he quickly assured me that his excavations conclusively
proved that Edzná was in existence by 150 B.C. and that it in fact represented the
oldest major Maya city yet discovered! When I suggested to him that I expected to
find that Edzná was also a major Maya astronomical center, he informed me that an
astronomer he had already engaged had found nothing of significance there, but he
invited me to explore the site for myself -- which I did within the week.

The "discoveries" which I made at Edzná in 1978 were as exciting as any that
I have made anywhere. The site itself lies on the edge of the largest "aguada",
or alluvial basin, in all of the Yucatán -- certainly the premium agricultural location
in that otherwise stony and water-deficient peninsula. Its dominating structure is the
five-story pyramid -- called simply "Cinco Pisos" by the archaeologists -- seen above,
visible across the featureless plain for almost 50 km (30 mi.).

My first "discovery" was the recognition of a gnomon at the base of Cinco Pisos so
ingenious in design that it could calibrate the passage of the zenithal sun at the Maya
New Year with absolute precision. Second came the recognition that the entire ceremonial
center had exactly the same orientation as Teotihuacán -- its contemporary 1000 km (600
miles) away on the Mexican plateau -- in that it was oriented to the setting sun on
August 13th. And finally, from the top of Cinco Pisos I realized that the lofty ruined
pyramid which intersected the featureless northwestern horizon did so precisely in line
with the northernmost stillstand of the moon, making it most likely the oldest lunar
observatory in the New World! Although Edzna had been denied a place in my 1978
article because of its unseemly "date of origin", it became the theme of several of my
subsequent research papers and recently has been recognized by the Mexican government
as one of the key potential tourist attractions of the Yucatán.

From the base of Cinco Pisos, one looks across the plaza to a great mound erected as
an "artificial horizon". In the middle of the mound is a "notch" through which one can
sight from the doorway of the Cinco Pisos courtyard to the top of a small pyramid beyond
the mound. This three-point sight-line marks the azimuth of 285.5º, the sunset position
on August 13. The top of the ruined pyramid ("La Vieja") in the right-background intersects
the true horizon at an azimuth of 300º, the northernmost setting position of the moon.

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