THE great Chaco civilization, trading partner of the Maya, established a far-reaching sphere of influence in the North American desert a millennium ago. Among the most remote and mysterious of their outposts was Chimney Rock, in what is now the very southwest corner of Colorado, 90 miles from Chaco Canyon in New Mexico, the center of the culture.
Why did the Chaco people — the Anasazi, or “ancestral Puebloans,” as their descendants prefer — build an enormous ceremonial Great House at Chimney Rock, so far from home, 1,000 feet above the nearest water supply and at the base of immense sandstone spires?
It was not until two decades ago that archaeologists arrived at an explanation that most now accept: the Chaco people built the Great House as a lunar observatory precisely aligned to a celestial event that occurs just once in a generation.
That rare event, a “major lunar standstill,” is happening now, and continues through 2007. To witness this extraordinary moonrise, some two dozen visitors, including me, arrived to climb the Chimney Rock mesa in the middle of an August night.
Every 18.6 years, the moon does something strange: it radically expands the voyage it makes each month across the sky and, at the northern and southernmost edges of that journey, appears to rise in the same spot for two or three nights in a row.
What archaeologists discovered at the last occurrence of this event — in the mid-1980’s — was that if you stood atop the Chacoan Great House at Chimney Rock (or as we did, on the modern fire watchtower that stands in front of the Great House and replicates its alignment), you saw the northernmost moonrise directly between the sandstone spires. Move anywhere else on the mesa, and the shift in perspective slipped the moon from its frame.