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No. 2203:
The Chankillo Observatory

Today, we learn the time of the year. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

I'm always forgetting the date. So I just pass my mouse over the lower corner of the screen and it reminds me. I don't even have to turn my head to look at the calendar! It's so easy that I have to kick myself to realize how hard it once was to know just where we were in the course of a year.

Chankillo SiteSo let us go back and meet the people of Chankillo. These were pre-Incans who, 2300 years ago, lived ten miles from the northwestern Peruvian coast. There, against the western foothills of the Cordillera Negras, they'd built a large fortified temple. 

This region holds many ancient ruins. But only recently did anthropologists from Yale and the University of Leicester learn the meaning of another site, a kilometer east of the temple. These ruins are divided by a low ridge. Along the spine of the ridge are thirteen square towers between six and twenty feet high. They extend over an eighth of a mile, and make the ridge look a little like the back of a stegosaurus. All rather mysterious!

Now the scientists find observation posts on the flats to either side of the ridge. They define an imaginary east-west line through the ridge. So hold that thought while we come back to the problem of knowing the time of year. 

It's easy enough to observe lengthening and shortening days. But when, in that cycle, do we plant crops? How do we know just where we are in the year? If we can locate sunset and sunrise on a horizon, then we can mark where the summer and winter solstice occur, and we can fill in the middle. That's what went on here.

These pre-Incans built an artificial horizon along the ridge. Observers in the western observation post looked eastward to trace sunrises; those in the eastern one looked west to trace sunsets. The towers on either end marked winter and summer solstices. The rest of the towers marked points in the passage of each half year. 

These people probably broke time into blocks of a certain number of days. How many, would depend on just how they used the towers and the gaps between them. It's really very neat -- an annual sundial instead of the familiar daily sundial. We know that the later Inca people practiced a highly developed worship of a sun god. And here we surely see the roots of that worship.

At first, we might not realize just how early this was. History is compressed in the Americas, because Paleolithic humans arrived here so late in time. That meant they had to be late in inventing technologies like farming, writing, and calendar-keeping. 

So: Here is the earliest known western calendar, and how clueless it makes me feel. Each evening for 26 years, I've driven the same road home toward the setting sun. Sometimes I think, "What a beautiful sunset!" Sometimes I just lower my visor and squint. My car's dashboard tells me the date, and I've never noticed just where the sun sets in December and where it sets in June.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

I. Ghezzi and C. Ruggles, Chankillo: A 2300-Year-Old Solar Observatory in Coastal Peru. Science, Vol. 315, 2 March 2007, pp. 1239-1243. Or see these online summaries: or

For some Andean prehistory, see: or

Chankillo Observatory

(Images courtesy of Google Earth)