A little-known memorial rises on the western face of the Hoover Dam, commissioned by the US Bureau of Reclamation when dam building began in 1931. The large winged bronze sculptures and central flagpole that are frequently photographed by visitors are the most conspicuous features of this portion of the dam, now known as Monument Plaza. However, the most astonishing element of this plaza is right under their feet while they take those photos.
The terrazzo floor of the plaza is actually a celestial map that indicates the time of the dam’s construction based on the earth’s 25,772-year axial precession.
This monument piqued my curiosity because axial precession is also the slowest cycle tracked by Long Now’s 10,000 Year Clock. Surprisingly, little to no documentation of this installation seems to be available, with the exception of a few vacation photos on Flickr. So, the last time I went in Las Vegas, I made a point of visiting Hoover Dam to learn more about this obscure 26,000-year-old monument.
On a hot day approaching 100 degrees, I parked my rental car on the Nevada side of the dam. I easily discovered Monument Plaza, which is directly across from the visitor center where dam tours are available.
While the plaza is easy to find, it is distinct from all of the main dam tours and stories. With the exception of the lettering on the plaza floor, the only information I could locate came from a speaker that was broadcasting a rudimentary description of the monument while tourists strolled around the area on a loop. When I inquired about it with my tour guide, he suggested that there might be some historical documentation and sent me to Emme Woodward, the dam’s historian.
I was able to contact her after returning home. As she emailed me certain materials, I realized why the Bureau of Reclamation doesn’t discuss much about the monument’s history. The first item she emailed me was a description of the plaza written by the artist himself, Oskar J. W. Hansen, which I assumed would tell me all I needed to know. While some of it was useful, the artist’s declaration of intent was also quite complicated and obscure. An authority:
These [human] postures can be linked to their corresponding responses in terms of angle and degree, similar to how cams in a worm-gear drive are joined. There is an angle for doubt, sadness, hatred, joy, contemplation, and devotion. There are as many as there are transitory emotions within each individual’s brain who inhabits the Earth. Who does not know all of these mental postures if he stops to consider them as elements in determining character proclivities? It is a wisdom passed down to us from the collective experience of the entire human species.
It’s difficult to envision the US Bureau of Reclamation interpreting the monument with this style of writing… Neither do they. And so it stands, a sort of 26,000-year clock for all to see, but still shrouded in mystery.
While I may never fully comprehend the monument’s designer’s inner reasons, I did wish to comprehend it on a technical level. How did Hansen create a heavenly clock face preserved in time that we can interpret as the dam’s completion date? The earth’s axial precession is a very obscure element of astronomy, and our grasp of it has been patchy at best throughout history.
The fact that this important engineering feat was commemorated by this axial precession monument piqued my interest, and I wanted to learn more about it.
When I asked for more evidence, the historian offered me instructions on how to use the Bureau of Reclamation’s photographic archive site, as well as some keywords to search for. This resource provided the black and white photographs you see here. Using the complicated website was tough, and at first I couldn’t discover any images of the plaza before or during development. The issue, as I realized, was that I was searching for “Monument Plaza,” a name given to it only after its completion in 01936. To uncover photographs from the dam’s construction, I had to search for “Safety Island,” so named because it was an island in the road where employees could stand behind a berm to shield themselves from the never-ending onslaught of cement trucks at the time.
I now had some historical literature and images, but I still needed a detailed design of the plaza to really comprehend it. I called the historian once more, and she was able to acquire authorization from her superiors to reveal the real building drawings. I imagine they don’t like to reveal technical dam blueprints for security reasons, but it appears they assessed my request a minimal security risk because the monument is not part of the dam’s structure. The historian sent me a tube full of enormous blueprints as well as a CD with the identical prints that had already been scanned. With everything in hand, I was finally able to reconstruct the plaza’s technical intent and how it operates.
To comprehend how the plaza marks the dam’s construction date in the almost 26,000-year cycle of the earth’s precession, it is necessary to first define axial precession. To put it simply, the earth is “wobbling” on its tilted axis like a gyroscope – but very, very slowly. This tremor effectively shifts what we view as the central point around which stars appear to spin each evening.
This center point is currently extremely close to the very bright star Polaris. We have always paid close attention to this celestial center, or North Star, because it is the star that remains stationary throughout the night. This single unchanging point in the sky serves as the foundation for all celestial navigation.
However, that point near Polaris, known as the North Star, is actually moving slowly and tracing a circle over the night sky. While Polaris is our North Star, Hansen’s terrazzo floor informs out that Thuban was the ancient Egyptians’ North Star when they erected the vast pyramids. And Vega will be our North Star in around 12,000 years. The operation of this precession is best illustrated with an animation, as seen in figure 1. Over the span of 25,772 years, the axis of the earth traces a circle in the sky.
Unfortunately, the inlaid surface in Monument Plaza makes it difficult to observe how this all works. The perspective of the plaza that you truly want to have is from above. This view of the real object would require a crane, but by utilizing the original technical drawing as an overlay, I was able to make a diagram that perhaps simplifies it.
In this diagram, the center of the circle traced by the axial precession is actually the large flag pole in the plaza’s center. This axial circle is boldly highlighted around the pole, and Polaris’ angle was portrayed as precisely as possible to show where it would have been on the date of the dam’s completion. Hansen used the rest of the plaza floor to depict the planets visible that evening, as well as many of the bright stars visible in the night sky at that position.
We can pinpoint the dam’s completion date to within a day by using planet placements and the angle of precession. We are currently developing a comparable technology, but with moving elements, for the dials of the 10,000 Year Clock. Major components of the Hoover Dam are likely to remain in situ hundreds of thousands of years from today. Hopefully, the Clock will continue to tick and Hansen’s terrazzo floor will remain, even if it continues to perplex tourists.
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