A large pile of stones are shown with a square opening and a passageway.

UNESCO adds an ancient Peruvian solar observatory to its World Heritage Sites

Iván Ghezzi, an archeologist and the director of the Chankillo project that was awarded the honor, discusses how the centuries-old system operated, and how it's still precise to this day.

The World

View of the Chankillo solar observatory near Casma, Peru, July 22, 2021.

Janine Costa/AFP via Getty/File photo

UNESCO has announced its newest World Heritage Sites. One of them is the oldest solar observatory in the Americas, found in the deserts of northern Peru. The site is called Chankillo, and it dates back to the year 230 BC.

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The physical structure functioned as a calendar, using the rising and setting arcs of the sun to define the time of year to within one or two days.

It is the third site in Peru to be added to the UNESCO World Heritage Sites list in just over a decade.

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Iván Ghezzi, an archeologist and the director of the Chankillo project, and joined The World's host Marco Werman, from Casma, the Peruvian town closest to the site.

Marco Werman: Iván, congratulations on this recognition from UNESCO. How do you feel about the honor?
Iván Ghezzi: I'm really overwhelmed with emotion. We have been working toward this goal for a number of years, and I am truly blessed to be able to announce this to Peru on the day of its national holiday.
Yeah, that's very cool. So, take us there. What does a Chankillo solar observatory look like today? If I visit it, what would I see?
It is an amazing set of 13 towers on a monumental scale that were built atop a relatively small hill. But when you stand at the buildings that we call the observing points and watch the towers, what you will see is that the position of this giant ruler matches precisely the range of movements of the sun across the horizon throughout the year. The southernmost tower, Tower #13, follows exactly the position of the sun during the December solstice sunrise. Across the line of towers, if you look at Tower #1, it is perfectly aligned with the sunrise on the June solstice. And so all the dates in between, throughout the year, mean that every single day, the sun rises or sets in relationship to the towers, and thus, providing the means to determine the date with the precision of one or two days.
So, those 13 towers, are they still there and still functioning?
They're still functioning today, twice a day, because there is an observing point for sunrise and there is an observing point for sunset. And they have been working like this with minimal disruption for 2,300 years. So, the time that has transpired between the construction of this positioning instrument, and today, means less than half of a degree of displacement. So today, you can see almost exactly what the ancient Peruvians saw when they were using the solar observatory. Very few places in the world can really offer such an immersive experience.
That is really amazing. So, this solar calendar ... was the calendar the same as our modern calendar, 24 hour days and 365 and a quarter days a year?
There are some differences. We can see that the ancient builders of Chankillo, these ancient astronomers, clearly used the sun to determine the year. It is likely that they used the moon to determine a period similar to our months. Other subdivisions, we are not so sure about. The time that it takes from the sun, typically, to move from one tower to the next, it's about 11 to 12 days. So, that is very different from a solar month, and very different from a week. So, clearly we are seeing evidence of a native calendar that has a completely different origin and that predates the calendars that we used today, that were developed in the Roman era.
What do we know about the ancient people who may have used the Chankillo solar observatory?
We don't know enough, but we do know that they clearly worshipped the sun. But, we need to do more research to learn about what language they spoke, what they ate, how they buried their dead or how they dressed. So, we still have to do much more archeological research to give us a greater picture of the society of astronomers and warriors that built Chankillo.

This interview has been lightly edited and condensed for clarity.