Astronomers believe they may have found humanity's first stargazing tools. At the National Astronomy Meeting held this week in Nottingham, England, researchers presented evidence that simple apertures like tunnels and doorways might have been used to observe stars more effectively by our prehistoric ancestors. They suggest that 6,000-year-old tombs in Portugal could be an example of this, which would make the structures our oldest known astronomical observatories.

It's difficult to decipher the intentions of men and women who lived six millennia ago. And since the research has yet to be peer reviewed or published in a journal, it must be taken with a grain of salt — other experts in the field may not have seen enough of the data to agree or disagree with the conclusions.

But it's a fascinating hypothesis: According to the team's analysis, the narrow entrances of so-called passage graves can increase the visibility of specific stars to the human eye. The passage grave would have kept stargazers cloaked in darkness save for its narrow opening, which would make stars in that field more visible.

At the Seven-Stone Antas in central Portugal, they say, this optical trick can be used to better observe Aldebaran, the brightest star in the constellation of Taurus.

“This first rising of Aldebaran occurred at the end of April or beginning of May 6,000 years ago, so it would be a very good, very precise calendrical marker for them to know when it was time to move into the higher grounds,” Fabio Silva of the University of Wales Trinity Saint David told the Guardian.

“The first seeing of this star would have indicated to the civilization that it was time to move to the mountains, as these civilizations spent the winters on low ground near the river and the summers on higher grounds in the mountain area,” Kieran Simcox, an undergraduate student at Nottingham Trent University, told Discover Magazine. The Portuguese passage grave could have made the star visible days earlier than it would be when observed outside — especially if the stargazer in question first spent a night inside the pitch-black tomb as part of a ritual.

The researchers will continue to study this passage grave and others for more evidence. They may never be able to prove that ancient peoples used these structures to study the sky. But their work is a reminder that humans have likely looked to the heavens for knowledge for thousands and thousands of years.

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