Here’s what you’ll learn when you read this story:

• A 12th century C.E. codex from Maya culture accurately predicts solar eclipses.
• The eclipse table in the Dresden Codex was a lunar calendar that helped track eclipses.
• The table helped matched eclipse events with ritual dates.

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Ancient Maya astrologers known as “daykeepers” were highly accurate when it came to eclipse prediction—even hundreds of years before an eclipse occurred. In a new study published in the journal Science Advances, researchers revised a century of interpretation to show how ancient Maya calendar specialists designed a predictive eclipse table. Not only was the wild combination of mathematics and observation needed to create this table impressive, it was also shockingly predictive.

The research team scoured the 12th century C.E. Dresden Codex—a rare, fully preserved Maya book known for its eclipse table. This codex, which was created by a society that marked celestial events with dedication, has long been misunderstood, according to the study’s authors.

Central to the codex is a 405-month cycle, the sole purpose of which scholars had long believed was to predict eclipses. The study challenges that notion. The authors wrote that the cycle was in fact a lunar calendar, which could combine with the 260-day ritual calendar to better pair up eclipses with key ritual dates and events.

“Within a few passes through this lunar calendar, intervals among observed eclipses could have stimulated an approximation of the series of lunar intervals that were later compiled as stations of an eclipse table,” the authors wrote. Combining observation of eclipse events and mathematics, the ancient astrologers then created a “sequence of tables that would anticipate every solar eclipse observed in Maya territory from a century or two after the first evidence of the Maya lunar calendar to at least the era of the extant eclipse table, 700 years later.”

As the authors explained, the 405 lunar months—a total of 11,960 days—also line up with 46 cycles of the 260-day ritual calendar, helping to mark and predict when an eclipse would align with a ritual event. “Fifty-five of these dates were intended to anticipate dates on which a solar eclipse might take place,” the authors wrote.

The deeper study of the 405-month cycle also uncovered how the codex uses an eight-page eclipse table to delineate each new moon cycle (there are 69 of them recorded in the tables). Prior to this discovery, it had long been presumed that the Maya table in the codex simply repeated itself as needed to equal 405 months. The researchers discovered that the resetting of tables at 223 and 358 months lined up with both the saros and inex eclipse cycles, allowing the astronomers to clean up slight timing variations that crept into the table and keep the predictive analysis accurate for over 700 years.

By comparing the predictive tables in the Dresden Codex to actual eclipse data from 350 to 1150 C.E., the researchers found that the codex produced incredibly accurate predictions.

This exacting record keeping—done, obviously, without the technologies astronomers use today—and predictive tabling was critically important to a culture for whom an eclipse of the Sun was a religious event indicative of a god showing emotion or signaling a change. The codex was good for 700 years, but the researchers said that the method is still accurate, even today.

Tim Newcomb is a journalist based in the Pacific Northwest. He covers stadiums, sneakers, gear, infrastructure, and more for a variety of publications, including Popular Mechanics. His favorite interviews have included sit-downs with Roger Federer in Switzerland, Kobe Bryant in Los Angeles, and Tinker Hatfield in Portland.