A Hubble image of the star system XZ Tauri, located some 450 light-years away in the constellation of Taurus. ESA / Hubble / NASA

Telescopes as we know them today trace their origins back to the Enlightenment. The earliest such devices emerged about 400 years ago. But humankind has fashioned environments for stargazing for far longer than that.

Scholars have long speculated about the astronomical orientation of the Pyramids at Giza, for instance, and the possibility that Stonehenge was built to be a celestial observatory.

Now, there’s evidence of ancient telescopic structures that date back even farther, to about 6,000 years ago. Astronomers are exploring ancient tombs in Portugal that they believe may have been used by prehistoric humans to enhance specific views of the night skies. Researchers are focusing on the alignment of the stars with megalithic tombs—stone structures known as dolmens that feature long narrow entrances that act as apertures, essentially zooming in on stars and planets that wouldn’t always be visible from the outside. “These structures could therefore have been the first astronomical tools to support the watching of the skies, millennia before telescopes were invented,” the Royal Astronomical Society wrote in an statement announcing the research on Wednesday.

The megalithic cluster of Carregal do Sal, in Portugal, includes this dolmen, which may have been used for stargazing rituals. (F. Silva)

They also may have been used for ceremonious rites of passage, researchers say. “Similar suggestions have been made for the ritual use of caves in the Neolithic of the Mediterranean, for instance,” Daniel Brown, an astronomy lecturer at Nottingham Trent University, told me in an email. The idea Brown and his colleagues are exploring is whether stellar alignment was a key component of rituals in these ancient spaces. One theory, he says, is that the structures were designed to reveal a certain star to a person staying in the chamber—where the aperture would make the star visible days or even a week before it could be seen otherwise.

A “window of visibility” inside the tomb (F. Silva)

The structures in Portugal, Brown and his colleagues say, may have been focused on Aldebaran, the brightest star in the constellation Taurus, where it appears as the twinkly reddish eye of the bull. Millennia after these dolmen were built, spacecrafts enabled humans to see Aldebaran as it slipped behind Saturn’s rings. The star is now visible with astounding clarity through massive telescopes on Earth, too.

Telescopes have now become sacred structures in their own right. Today they’re built like temples—solitary monuments on towering mountaintops. But even in the age of great optical instruments, humans haven’t stopped building immersive lensless apertures. One example is Star Axis, an art installation in the New Mexico desert that’s been under construction for 40 years—a long time for a single work of art, perhaps, but barely a blip in the history of admiring the sky.

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