Astronomical events, such as eclipses, were central to Maya culture, reflected in the meticulous keeping of calendars for astronomical predictions. Among the few surviving Maya texts is the so-called Dresden Codex, which contains an eclipse table. Researchers concluded that this table was created based on earlier tables of lunar months, rather than developed exclusively for predicting eclipses, according to an article published in Science Advances. They also figured out the mechanism by which the Maya ensured the accuracy of this table over a very long period. This is reported by Arstechnica, writes UNN.
Details
The Maya used three main calendars: the count of days, known as the Long Count; the 260-day astrological calendar, called the Tzolkin; and the 365-day calendar year, the Haab'. Previous researchers have speculated how impressive solar or lunar eclipses must have been for the Maya, yet our understanding of their astronomical knowledge is limited. Most Maya books were burned by Spanish conquistadors and Catholic priests. Only four hieroglyphic codices survived: the Dresden, Madrid, Paris, and Grolier Codices.
The Dresden Codex dates from the 11th–12th centuries, likely originating from the Chichen Itza region. It can be folded like an accordion, and when unfolded, it is 12 feet long. The text was deciphered in the early 20th century and describes local history, as well as astronomical tables for the Moon and Venus.
In their research, co-authors John Justeson of the University at Albany and Justin Lowry of SUNY Plattsburgh focused primarily on pages 51 and 58, which contain eclipse tables covering all solar and most lunar eclipses. They are accurate enough to operate from a start date in the 8th century all the way to the 18th century. (The Madrid Codex also contains an eclipse almanac, but it mainly concerns the correspondence of agricultural cycles with eclipses.)
Maya calendars were maintained by specialists known as "daykeepers," a cultural tradition that persists to this day. There is general agreement that eclipses were important to the Maya. "They tracked them, had rituals associated with eclipses, and it was built into their belief system," Lowry told Ars. "So we know that the eclipse table is part of the cultural knowledge of the time. We were just trying to understand how that table came to be in its current form."
How the prediction worked
Lowry and Justeson's analysis involved mathematical modeling of eclipse predictions from the Dresden Codex table and comparing the results with NASA's historical database. They focused on 145 solar eclipses that were visible in the Maya region between 350 and 1150 AD.
They concluded that the codex's eclipse tables evolved from a more conventional table of successive lunar months. The duration of the 405-month cycle (11,960 days) aligned significantly better with the 260-day calendar (46 × 260 = 11,960) than with solar or lunar eclipse cycles. This suggests that Maya daykeepers discovered that 405 new moons almost always equaled 46 periods of 260 days—knowledge they used to accurately predict the dates of full and new moons over 405 consecutive lunar cycles.
Daykeepers also noticed that solar eclipses seemed to repeat on the same or nearby days of their 260-day calendar, and over time learned to predict the days when a solar eclipse might occur locally. "An eclipse only happens during a new moon," Lowry said. "If you can accurately predict a new moon, you can pinpoint one of seven possibilities for an eclipse. That's why it makes sense that the Maya refined their moon prediction models for more accurate eclipse forecasts, because they didn't need to predict the moon's position relative to the ecliptic."
The Maya also understood that their tables needed occasional adjustments to compensate for accumulated error. "When we talk about accuracy, we sometimes think about predicting to the microsecond," Lowry said, referring to NASA records. "The Maya had a very accurate calendar, but they predicted events by the day, not by the second."
However, the Maya did not start tables from scratch at the same position, which would have made them increasingly inaccurate. Instead, they used a series of overlapping tables. Lowry and Justeson concluded that the tables were likely restarted at one of two specific early points before the completion of the previous one: at the 358th new moon (the most accurate overestimation of the total length of the eclipse cycle) and at the 223rd new moon (the most accurate underestimation).
"The traditional interpretation was that you go through the eclipse table eclipse by eclipse, and then you rebuild it every iteration," Lowry said. "We found that if you do that, you'll miss eclipses, but we know they didn't. They made internal adjustments. We think they restarted the table somewhere in the middle. If you do that, you go from missing eclipses to covering them completely. You'll never miss an eclipse. So it's not just a calculation table for predictions, but a calculation table with adjustments based on empirical observations over time."
"This is the basis of true science—empirical data collection and constant adjustment of expectations, built into a system of understanding the movement of celestial bodies that allows for predicting events," Lowry said. "But here it is deeply encoded in a religious system. Their rituals were fundamentally linked to astronomy and astrology. There was a group of people for over 1000 years—through wars, collapses, famine, and external conquests—who kept observational records every five to six months. They weren't making their calendar more accurate—they were ensuring it remained accurate. And that's very cool."