Philosophy professor Alexander Mourelatos said that he was shocked and skeptical upon learning, in the fifth grade, that the planets stay in groups, so it’s not all that surprising that it took the Ancient Greeks hundreds of years to discover the same thing.

As part of a series of History and Philosophy of Science Friday Talks, Mourelatos gave a lecture on what the Ancient Greeks knew about space and when they knew it.

Daniel Munoz, philosophy and linguistics senior, said Mourelatos was able to answer some of his questions about Thales, an Ancient Greek philosopher. According to Munoz, the importance of heliophotism, the idea that the moon gets its light from the sun, was the most surprising part of the talk.

“I didn’t realize that the big mystery was, ‘Why is the moon luminous?’” Munoz said.

Mourelatos also explored what the Greeks knew before the discovery of heliophotism. He said he suspected that, into the fifth and sixth centuries, the Greeks did not understand that the stars remained in a fixed orbit around each other.

“They knew about the constellations, and they knew about the seasonal rising and setting of stars,” Mourelatos said. “But does that mean they necessarily knew that all of the fixed stars stayed together?”

Mourelatos said some the earliest cosmologists had theories about objects in space that were on the right path toward the truth.

“[To Ancient Greeks,] all of the celestial bodies were ‘meteora,’ suspended above, so, if they are suspended, they can’t be heavy. It must be that they are of some airy or vaporous or fiery constitution,” Mourelatos said.

Mourelatos said an ancient theorist named Xenophiles theorized that celestial bodies contained some dark material that fueled flares, allowing humans to see them.

“So, the phases of the moon are just cases in which the flare just gets blocked. Something happens where the fire flares get closed,” Mourelatos said. “In the case of the moon, this happens with some regularity. In the case of the sun, it happens less frequently. That’s when you get an eclipse.”

Steve Bratteng, director of the Center for Inquiry, which is a nonprofit that aims to “foster a secular society based on science, reason, freedom of inquiry and humanist values,” said he was amazed by how much the ancient Greeks got right, given their limitations in the ability to observe space. Bratteng said he had been coming to these types of talks frequently. 

“It has an interesting connection between history, philosophy and science,” Bratteng said. “It’s often interesting to see how we’ve developed ideas, and what effect they’ve had on the progress of society, or vice versa: how society affects the progress of science, and how it affects the way people think about things.”