The western flank of the Hoover Dam holds a celestial map that marks the time of the dam’s creation based on the 25,772-year axial precession of the earth.
On the western flank of the Hoover Dam stands a little-understood monument, commissioned by the US Bureau of Reclamation when construction of the dam began in 01931. The most noticeable parts of this corner of the dam, now known as Monument Plaza, are the massive winged bronze sculptures and central flagpole which are often photographed by visitors. The most amazing feature of this plaza, however, is under their feet as they take those pictures.
The plaza’s terrazzo floor is actually a celestial map that marks the time of the dam’s creation based on the 25,772-year axial precession of the earth.
I was especially interested in this monument because this axial precession is also the slowest cycle that we track in Long Now’s 10,000 Year Clock. Strangely, little to no documentation of this installation seemed to be available, except for a few vacation pictures on Flickr. So the last time I was in Las Vegas, I made a special trip out to Hoover Dam to see if I could learn more about this obscure 26,000-year monument.
I parked my rental car on the Nevada side of the dam on a day pushing 100 degrees. I quickly found Monument Plaza just opposite the visitor center where tours of the dam are offered. While the plaza is easy to find, it stands apart from all the main tours and stories about the dam. With the exception of the writing in the plaza floor itself, the only information I could find came from a speaker running on loop, broadcasting a basic description of the monument while visitors walked around the area. When I asked my tour guide about it, he suggested that there might potentially be some historical documentation and directed me to Emme Woodward, the dam’s historian.
I was able to get in touch with her after returning home. As she sent me a few items, I began to see why the Bureau of Reclamation doesn’t explain very much about the monument’s background. The first thing she sent me was a description of the plaza by Oskar J. W. Hansen, the artist himself, which I thought would tell me everything I wanted to know. While parts of it were helpful, the artist’s statement of intention was also highly convoluted and opaque. An excerpt:
These [human] postures may be matched to their corresponding reflexes in terms of angle and degree much as one would join cams in a worm-gear drive. There is an angle for doubt, for sorrow, for hate, for joy, for contemplation, and for devotion. There are as many others as there are fleeting emotions within the brain of each individual who inhabits the Earth. Who knows not all these postures of the mind if he would but stop to think of them as usable factors for determining proclivities of character? It is a knowledge bred down to us through the past experience of the whole race of men.
It is indeed pretty hard to imagine the US Bureau of Reclamation using this type of write-up to interpret the monument… and they don’t. And so there it stands, a 26,000-year clock of sorts, for all the world to see, and yet still mired in obscurity.
While I may never totally understand the inner motivations of the monument’s designer, I did want to understand it on a technical level. How did Hansen create a celestial clock face frozen in time that we can interpret and understand as the date of the dam’s completion? The earth’s axial precession is a rather obscure piece of astronomy, and our understanding of it through history has been spotty at best.That this major engineering feat was celebrated through this monument to the axial precession still held great interest to me, and I wanted to understand it better.
I pressed for more documentation, and the historian sent me instructions for using the Bureau of Reclamation’s image archive site as well as some keywords to search for. The black and white images you see here come from this resource. Using the convoluted web site was a challenge, and at first I had difficulty finding any photos of the plaza before or during it’s construction. As I discovered, the problem was that I was searching with the term “Monument Plaza,” a name only given to it after its completion in 01936. To find images during its construction, I had to search for “Safety Island,” so named because at the time of the dam’s construction, it was an island in the road where workers could stand behind a berm to protect themselves from the never-ending onslaught of cement trucks.
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 effectively 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 indeed 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.
One complete wobble, or precession cycle, takes approximately 25,771.5 years. Around 3000 BCE, the star Thuban in the constellation Draco was the North Star. As of 2026, Polaris is the North Star, but it won’t remain so indefinitely. In approximately 12,000 years, Vega in the constellation Lyra will become the North Star. this cyclical shift has been known and observed by various cultures for centuries,influencing mythology and astronomical understanding.
