There’s a little-known monument located at the site of the Hoover Dam that shows the progression of “North Stars” as the Earth moves through its 25,772-year change of rotational axis. Alexander Rose of the Long Now Foundation couldn’t find much public documentation related to this celestial map, so he did some research.
I now had some historical text and photos, but I was still missing a complete diagram of the plaza that would allow me to really understand it. I contacted the historian again, and she obtained permission from her superiors to release the actual building plans. I suspect that they generally don’t like to release technical plans of the dam for security reasons, but it seems they deemed my request a low security risk as the monument is not part of the structure of the dam. The historian sent me a tube full of large blueprints and a CD of the same prints already scanned. With this in hand I was finally able to re-construct the technical intent of the plaza and how it works.
In order to understand how the plaza marks the date of the dam’s construction in the nearly 26,000-year cycle of the earth’s precession, it is worth explaining what exactly axial precession is. In the simplest terms, it is the earth “wobbling” on its tilted axis like a gyroscope — but very, very slowly. This wobbling effectively moves what we see as the center point that stars appear to revolve around each evening.
Presently, this center point lies very close to the conveniently bright star Polaris. The reason we have historically paid so much attention to this celestial center, or North Star, is because it is the star that stays put all through the course of the night. Having this one fixed point in the sky is the foundation of all celestial navigation.
Here are some explanatory notes that Rose wrote over the blueprints of the monument showing how to read the map:
from kottke.org http://bit.ly/2TNXD3U
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