The solstices and equinoxes have been important calendar markers over the centuries. To track them, many cultures built special observing sites. Perhaps the most impressive was in the desert along the coast of Peru. Known as Chankillo, it was designated as a UNESCO World Heritage site last year.

Chankillo was a complex of three sites: a hilltop fortress, a plaza, and a row of blocks known as the Thirteen Towers. The complex was built more than 2300 years ago, and was abandoned a couple of centuries later — perhaps as the result of a war.

The towers were the centerpiece of a solar observatory. They were built along the ridge of a small hill, making the site look like the spiny back of a dinosaur. The towers are up to 20 feet high, and are spaced about 15 feet apart.

Viewing sites flank the towers — one to the east, the other to the west. From these sites, observers could watch the Sun rise and set above or between the towers across the year. The northernmost tower marked sunrise at the winter solstice, in June. The southernmost tower marked sunrise at the summer solstice, in December. So the Sun would spend half the year moving in one direction across the towers, and the other half moving the opposite direction.

The site would have allowed its users to know the date with an accuracy of two or three days. That makes Chankillo unique in the ancient world — a year-long calendar that tracked the motions of the Sun.

Script by Damond Benningfield

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