What we might have seen from Carregal do Sal megalithic structure, at dawn in late April, circa 4,000 B.C. All images: Fabio Silva.

Prehistoric humans may have observed the sky via primitive lens-less “telescopes,” according to a team of British astronomers who have studied the long passageways of ancient megalithic tombs. The details were presented today by Kieran Simcox, a student at Nottingham Trent University, at a meeting of the Royal Astronomical Society.


Technically it’s not a telescope (with or without a lens), per se. According to Simcox, it’s more like a very primitive version of a camera obscura, the basic concept behind the pinhole camera. All you need to create a camera obscura is a small hole in a shade or a wall, so that light can pass from a sunlit garden, for instance, into a darkened room, projecting an inverted image on the wall opposite the pinhole.

Dolmen da Orca, a typical structure in Western Iberia, part of the megalithic cluster of Carregal do Sal.


The key difference with the megalithic clusters is that the narrow entrance creates an aperture of about 10 degrees, sufficiently restricting the view by naked eye to make it easier to observe the sky—specifically, certain stars—during pre-dawn and twilight hours. Simcox and his colleagues are exploring the precise conditions under which this might work in the laboratory. Is it just a question of eye adaptation, or do other factors, like the size of the aperture or background color of the sky, play a role?



“In a sense, the long passage of the passage grave would act as a long tube that would not only focus the attention on a narrow band of the horizon, but also block out most of the light, as well as the atmospheric scattering that brightens the sky around the point where the sun is about to rise,” Fabio Silva of the University of Wales told Gizmodo. “By spending the night inside the passage grave, the eyes will be dark-adapted and, therefore, might be able to see the faint star rising, whereas someone outside the megalithic structure, already with light-adapted eyes due to the pre-dawn scattering, wouldn’t.”

(Left) The “window of visibility” while standing in the dolmens’ chamber. (Right) Orca de Santo Tisco, a dolmen with a much smaller passage.

It’s always a tricky proposition to speculate about how such ancient structures may have been used by prehistoric peoples. But Simcox et al.’s work is based on an earlier pilot project focusing on passage graves in Carregal do Sal in Portugal. That yielded a new way of identifying alignments between how the passages and entrances of the prehistoric are oriented, and celestial objects. Based on this, Silva said that the mostly likely target would be the star Aldebaran.


“From the wider archaeological record connected to the societies that built the passage graves, we know that these were pastoral communities, that still practiced hunting and gathering,” he said. Such communities would pass the colder winter months in river valleys (where the dolmens are located) and move to higher ground in the spring—most likely to Serra da Estrela (“mountain range of the star”).

“When the youth would hit a certain age, they would be shown this ‘special’ way of viewing a star before it would usually be visible.”


Six thousand years ago, people would have been able to see Aldebaran’s first rise at dawn in late April or early May. Silva thinks this suggests those communities may have relied on such observations to tell them when it was time to move to higher pastures.

However, it could also be the case that such sites were used in initiation rituals, similar to the ritual use of caves in Mediterranean regions during the Neolithic era. “When the youth would hit a certain age, they would be shown this ‘special’ way of viewing a star before it would usually be visible,” Simcox said.


Along with forensic astronomy, the field of cultural astronomy is booming of late, although some form of so-called “archaeoastronomy” has been around since the late 1800s, according to Silva. “Because the sky is often ignored by the wider archaeological and anthropological communities, [such interdisciplinary work can yield] many insights,” he said. These include the use of celestial cycles as primitive calendars. how those cycles become encoded in myth and folklore, and illuminating deeper connections between the night sky, rock art, and architectural structures.

[Royal Astronomical Society]