An Ancient Greek Philosopher Was Exiled for Claiming the Moon Was a Rock, Not a God

2,500 years ago, Anaxagoras correctly determined that the rocky moon reflects light from the sun, allowing him to explain lunar phases and eclipses. Anaxagoras the scholar did most of his work in Athens and had a knack for astronomy, an area of study that requires careful observational and calculation to unlock the mysteries of the universe. During his time in Athens, Anaxagoras made several fundamental discoveries about the moon. He reiterated and expended upon an idea: that the moon and sun were not gods, but rather objects. This seemingly innocuous belief would ultimately result in Anaxagoras’ arrest and exile.

Through persistent observation, Anaxagoras came to believe that the moon was a rock, not totally unlike the Earth, and he even described mountains on the lunar surface. The sun, he thought, was a burning rock. In fragment 18, Anaxagoras says, “It is the sun that puts brightness into the moon.” While Anaxagoras was not the first to realize that moonlight is reflected light from the sun, he was able to use this concept to correctly explain additional natural phenomena, such as eclipses and lunar phases.

“Anaxagoras turns the problem of lunar light into a problem of geometry,” Daniel Graham, a professor of philosophy at Brigham Young University says. He noted that when the moon is on the opposite side of the Earth than the sun, the full face is illuminated, “[producing] a model of the heavens that predicts not only phases of the moon, but how eclipses are possible.” The moon’s phases, Anaxagoras realized, were the result of different portions of the celestial object being illuminated by the sun from Earth’s perspective. The philosopher also realized that the occasional darkening of the moon must result from the moon, sun and Earth lining up such that the moon passes into the Earth’s shadow—a lunar eclipse. When the moon passes directly in front of the sun, the skies darken during the day, a phenomenon Anaxagoras also described and we now call a solar eclipse.

By describing the moon as a rock of terrestrial origin, and the sun as a burning rock, Anaxagoras moved beyond earlier thinkers, even those who realized the moon was a kind of reflector. This forward thinking got Anaxagoras labeled as a chief denier of the idea that the moon and sun were deities. Though his life was spared, the philosopher who questioned the divinity of the moon found himself in exile in Lampsacus at the edge of the Hellespont. But his ideas regarding eclipses and lunar phases would live on to this day, and for his recognition of the true nature of the moon, a lunar crater, visited by orbiting spacecraft some 2,400 years later, bears the name Anaxagoras.

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