And they say it does not predict — contrary to popular interpretations of other calendars by the Maya civilization that once occupied Central America — that the world will end this year.

According to experts from several US universities, the astronomical computations are 1200 years old — four centuries older than the Maya calendar previously regarded as the oldest.

They were found on the walls of a room unearthed amid Mayan ruins in the lost city of Xultun in northern Guatemala.

While many of the notations have been damaged by looters, a number of them are said to be still recognizable. Some are calculations of phases of the moon. Others are more puzzling but seem to describe the movements of Mars, Mercury and Venus.

The tiny, millimetre-thick red and black hieroglyphs are said to be unlike any seen before at Mayan sites. According to archaeologist William Saturno of Boston University, they appear to represent various astronomical cycles of the Maya: the 260-day ceremonial calendar, the 365-day solar calendar, and even the 584-day cycle of Venus and 780-day cycle of Mars.

“For the first time we get to see what may be actual records kept by a scribe, whose job was to be official record keeper of a Maya community ... and they’re painting it on the wall. They seem to be using it like a blackboard,” Saturno said.

The painted walls, crumbling and overgrown with vegetation, were discovered two years ago by one of Saturno’s students, who had followed a trench dug by looters. Backed by grants from the Washington-based National Geographic Society, the young man returned with Saturno and other scientists to excavate the room.

The ruined city of Xultun, once home to tens of thousands of people, was first described by archaeologists about 100 years ago.

The Maya calendar has attracted a lot of attention recently because some people have come to believe that it points to doomsday this December. The scientists say, however, that while one calendar cycle does end this December, another one begins right afterwards.

“It’s like the odometer of a car, with the Maya calendar rolling over from the 120000s to 130000,” remarked Anthony Aveni, a professor of astronomy and anthropology at Colgate University, in the US state of New York.

“The car gets a step closer to the junkyard as the numbers turn over; the Maya just start over.”

The scientists’ findings are detailed in the journal Science and the June issue of National Geographic magazine.