An excavation of an archaeological site in Guatemala has uncovered Mayan astronomical records dating to the ninth century A.D. The tabulated numbers, which predate existing Mayan astronomical documents by several hundred years, chart the motion of the moon and also seem to relate to the orbits of Mars and Venus. (And good news: they do not predict the world will end this year—in fact, some of the numbers appear to refer to dates far in the future.)

Archaeologists stumbled onto the astronomical tables, inscribed on the walls of a small building, while excavating part of the Xultun ruins, a large, heavily looted archaeological site in northern Guatemala, near its borders with Mexico and Belize. William Saturno, an archaeologist at Boston University (B.U.), recalls that an undergraduate student noticed the remains of a mural on one of the walls, triggering an excavation of the room, which had been partly exposed by looters. On three of the walls the researchers found figural paintings, along with a series of glyphs and numerals.

The presence of lunar glyphs in one of the numerical tables raised the possibility that the table related to astronomy. After all, evidence from later centuries has proved that the Maya kept highly accurate records of astronomical phenomena. But the context of the numerals, many of which have deteriorated beyond recognition, was not immediately clear. "It took some decoding," Saturno says.

The numbers on the table, arranged in columns of three numerals each, looked like calendrical entries in well-studied Mayan manuscripts, written on bark paper, that survive from sometime around the 13th to 15th centuries. So the researchers took the numbers to be days tallied in units of the culture's Long Count calendar—the three numerals in each column representing multiples of a 360-day "tun," a 20-day "winal" and a one-day "k'in," respectively. The number column 13/5/4, then, would equal 4,784 days (13 x 360 + 5 x 20 + 4). The dates of the final two columns, which are the most legible, are separated by 178 days. The date in the third-to-last column, which is mostly legible, looks to be separated from that in the penultimate column by 177, 178 or 179 days, pointing to a common pattern.

The Maya clustered lunar months into sixes, making lunar "semesters" lasting 177 or 178 days. The variation accounts for the calendar's whole-number approximation of a messy decimal number, in much the same way that the modern calendar uses 366-day leap years to keep the months in sync with Earth's orbit around the sun. The researchers suggest that the Xultun table marks a series of lunar semesters over some 13 years. Saturno and his colleagues from B.U., the University of Texas at Austin and Colgate University report their findings in the May 11 issue of Science.

Another table contains four much larger numbers whose meaning is less clear. But all four numbers are divisible by 18,980, the number of days that makes up what is known as the Calendar Round, a combined cycle of the solar year and the Mayan ritual year. "It turns out these numbers are really important anniversaries," Saturno says. "They’re essentially numbers that represent multiples of Maya calendrical periods."

Intriguingly, the tables also hint at planetary motions. All four numbers are multiples of 780, the number of days it takes Mars to return to the same location in the sky, a tally known as the synodic period of Mars. The tallies are also closely related to the position of Venus: all four are whole- or half-number multiples of Venus’s synodic period. If the numbers indeed represent days, the largest entry in the table, 2,448,420, lies thousands of years in the future.

The smaller numbers on the lunar table might also relate to the planets, notes Susan Milbrath, curator of Latin American art and archaeology at the Florida Museum of Natural History. That table ends with the number 4,784, which is nearly equal to the days elapsed in 12 synodic periods of Jupiter. "I think they were actually looking at the relationship of the lunar cycle to Jupiter, which is really intriguing," Milbrath says.

The researchers say that they are still peeling back the layers of meaning in the new find. "We're currently expanding the study of the possible astronomical implications of the Xultun inscriptions, and I think it is entirely possible that the Jovian period might have been involved as one of the periods of commensuration," says study co-author Anthony Aveni, a professor of astronomy, anthropology and Native American studies at Colgate. "There isn't much more we can say at this time, as there are no other inscriptions at Xultun that might relate to Jupiter, at least as far as we know."

"What we’re looking at in a couple examples now are different types of astronomical tables similar to the type that we know in bark paper books," Saturno says. Those manuscripts, or codices, which survive from later centuries, include dates going back to the Classic period, the era of Maya history from approximately A.D. 200 to 900. So it is no great surprise that the Maya were keeping astronomical records so early—the evidence of those records simply did not survive colonization or centuries of exposure in a tropical climate. Somehow the Xultun paintings and tables escaped destruction from weathering—and avoided drawing the attention of looters.

"These are our first records that surely show astronomical tables in the Classic period, which is a major discovery, I think," Milbrath says. "It's just an amazing find—it really is."