The Petroglyph Calendar: An Archaeoastronomy Adventure
by Hubert A. Allen, Jr.


Book Review by Joe Orman

[Originally published in the newsletter of the East Valley Astronomy Club, December 2000]

One day when he was out for a walk, Hubert A. Allen, Jr. made an intriguing discovery. At base of the Sandia Mountains near Albuquerque, New Mexico, he found a near-perfect equilateral triangle deeply carved into the flat top of a granite boulder. The site afforded a clear view of the western horizon, and as the sun set that evening, Allen placed a pen upright in the center of the triangle. He watched as the shadow cast by the pen fell in perfect alignment with one point of the triangle. The date was on June 24, 1997 -- practically the summer solstice. Could this be an ancient astronomical calendar?

Thus began a research "adventure" in which Allen would seek celestial significance in the two other points of the triangle. Allen was in for a uphill battle, since he had, he admits, "no previous formal training in astronomy and just a smattering in archaeology." But after browsing the books at a museum gift shop, he declared himself an archaeoastronomer! He would eventually show that at the winter solstice, the sunset shadow is cast away from the second point, to the middle of the opposite side. And the third point of the triangle seems to point to the north star, Polaris.

His findings are intriguing, but some of the mathematical details make me skeptical. Allen's averaged measurements of the triangle points match both sunset directions to within 3, but the uncertainty in his measurements is fairly large -- which is not surprising given that the position of the gnomon (shadow-casting device) at the triangle's "center" is somewhat arbitrary and a hand compass was used (two different measurements of one point differ by 5). I was impressed that "... the petroglyph calendar in New Mexico is at the optimum latitude for its triangular geometry based on the two solar extremes," until I read that this "specific latitude range" is approximately 34 to 40 north latitude -- a swath around our earth almost 500 miles wide! And the 300-pound free-standing rock could have shifted over the centuries, only adding to the possibility that these alignments are mere coincidence. In any case, watching the sun's shadow progress along one half-edge of a 4-inch triangle does not seem a very practical way of measuring the year. Ancient use of a gnomon was fairly common, but wouldn't notches along the edge of the boulder be more accurate markers for the shadow?

Allen is on his most shaky mathematical ground when he theorizes that the third point of the triangle (which points to an azimuth of 5) aligned with Polaris at the date of the petroglyph's construction (presumed to be 1200 AD). Here he makes a basic error: The fact that Polaris appeared 5 from the celestial pole in the year 1200 due to precession does not mean that it would appear at a constant azimuth of 5; Polaris could appear anywhere on a 5 radius circle about the pole, depending on the season and time of night -- covering a 10 range in azimuth! But more subtly, Allen's use of this Polaris alignment to date the petroglyph to the year 1200 employs circular reasoning -- he presumes the third point was constructed to align with something, then he looks into the past to see when it actually did, and takes that as the date of its construction!

Besides the mathematical details, there are also cultural considerations which are just as important:

1. With its straight edges and sharp inside corners, such a deep triangle in solid granite would seem to suggest the use of metal tools not available to Pre-Columbian civilizations. Allen does not address tool-making capabilities at all.

2. Allen failed to find a record of a single other triangular petroglyph in the American Southwest which could function as an astronomical calendar. The depth of the petroglyph (about 1 1/2 inches) is also uncharacteristic; directly across the Rio Grande Valley, Petroglyph National Monument has over 17,000 petroglyphs, all with their designs shallowly pecked into the rock surface.

3. There are no other associated artifacts to indicate this site was actually used in ancient times, with the sole exception of a similar "heart-shaped" petroglyph nearby.

The casual reader might shrug off these shortcomings, but I wonder if they would withstand the scrutiny of scientific peer review. The closest Allen comes to independent verification of his findings is, "I took a couple of photographs and showed them to the noted archaeologist Dr. Frank Hibben, who pronounced that both were probably real petroglyphs, and oddities at that." Indeed, several statements like the following lead me to wonder if Allen let personal desire drive his interpretation of the data: "Instead of responding to these highly rational thoughts, I felt a burning, confident intuition that the petroglyph was important and that I should pursue it."

This, then, is the dilemma of archeoastronomy: What we see delights us and we feel a profound connection with those who came before. But when we look so far into the past, we must be sure our gaze is as reliable and impartial as possible. No matter how attractive a theory, if it is not supported by what we know of the culture, and if it has not been independently confirmed by others, then it must remain an intriguing mystery.

Revised: June 6, 2001
Copyright © 2001 Joe Orman
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