Telescopes as we think of them date back 400 years to the Enlightenment. But astronomers studying huge tombs in Portugal believe ancient humans were making their own stargazing instruments 6,000 years ago, the Atlantic reports. Researchers, who presented their findings Wednesday at the National Astronomy Meeting in Britain, believe the tombs themselves were a type of massive lensless telescope, according to Live Science. The Guardian reports the long, narrow entrances to the tombs—which were the only source of light—would focus the eye on a single piece of sky, block out the sun's rays at dawn, and make the eye more sensitive to low light. "This would allow enhanced observing, especially in the twilight hours of dusk and dawn," astronomer Daniel Brown tells Live Science.
The tombs may "have been the first astronomical tools to support the watching of the skies, millennia before telescopes were invented,” the Royal Astronomical Society says in a statement. Ancient humans admitted inside the tombs may have been seen as having "secret knowledge or foresight" due to being able to see the rise of seasonal stars days before those outside the tombs could see them. They might use that information to declare when it was time to move herds to new grazing areas, for example. Researchers are now experimenting to see what stars might align with the entrances to the tombs. (Astronomers say this is the farthest galaxy ever spotted.)