Long ago, when Venus was coursing through the sky at night, an ancient royal Mayan astronomer captured those moments in a text, called the Dresden Codex. It contains detailed, painstakingly documented measurements of Venus rising and setting. They enabled modern historians to locate the astronomer in a 25-year span in the first half of the 10th century.

"We can see the moment when this person puts it all together," said Gerardo Aldana, a science historian in the Department of Chicano Studies at the University of California, Santa Barbara and a co-author of a new study describing the findings.

The Dresden Codex, a fascinating Mayan text of 39 double-sided pages, is traced back to the Yucatan Peninsula. It landed in the Royal Library in Dresden, Germany by the 1730s, say the Foundation for the Advancement of Mesoamerican Studies.

However, the text around the table had not been analysed much. Aldana examined the wording around the Venus table, and found the Maya measuring the phases of Venus so that they could time their ceremonial events more precisely. Hence, the first anchor event was a real measurement.

The Maya "had a really elaborate ritual set of events that were tied to the calendar," Aldana said. "They were probably doing large-scale ritual activity connected to the different phases of Venus."

The Maya made certain corrections in the map to reduce inaccuracies. But their text as well as the table shows that it was indeed a "sophisticated scientific method of observation at a specific point in time in the "Terminal Classic Period," or the 10th century."

"There's this 25-year period, a window, when an astronomer could have been making these records," Aldana said.

Hence, there was probably a Mayan astronomer who was examining the stars from his observatory called El Caracol at Chichen Itza. He was probably engaged in the task by an ancient personality called K'ak' U Pakal K'awiil, said Aldana.

"There's this transition that occurs in the post-Classic [period]," Aldana said. "Some scholars have argued there's closer to a pan-Mesoamerican religion tied to this Quetzalcoatl figure who is very tied to Venus."

The report was published in the current issue of the Journal of Astronomy in Culture.