Close to the north pole of the moon lies the crater Anaxagoras, named for a Greek philosopher who lived in the fifth century B.C. The eponym is fitting, as Anaxagoras the man was one of the first people in history to suggest the moon was a rocky body, not all too dissimilar from Earth. Streaks of material thrown out during the impact that formed the crater extend 560 miles southward to the rim of another crater, this one named for Plato.
Like Plato, Anaxagoras the scholar did most of his work in Athens, but the similarities between the two men stop there. Influenced strongly by the Pythagoreans, Plato posited a mystical universe based on sacred geometric forms, including perfectly circular orbits. Plato eschewed observation and experimentation, preferring to pursue a pure knowledge he believed was innate in all humans. But Anaxagoras, who died around the time Plato was born, 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 likely emerged among his predecessors but was not widely accepted in antiquity: that the moon and sun were not gods, but rather objects. This seemingly innocuous belief would ultimately result in Anaxagoras’ arrest and exile.
Piecing together the lives of early philosophers such as Anaxagoras, who is thought to have written just one book, lost to us today, can be a major challenge for historians. Modern scholars have only “fragments” to describe the life of Anaxagoras—brief quotes from his teachings and short summaries of his ideas, cited within the works of scholars from later generations, such as Plato and Aristotle.
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.
Hailing from Clazomenae in the Ionian lands east of the Greek mainland, Anaxagoras grew up during the Ionian Enlightenment, an intellectual revolution that began around 600 B.C. As a young man, he saw Athens and Sparta align to drive the Persian Empire out of Ionia. When he relocated to Athens, Anaxagoras and his contemporaries brought philosophy to the budding Athenian democracy. Although many Greek philosophers of the sixth and fifth centuries B.C. believed in one or a few fundamental elements—such as water, air, fire and earth—Anaxagoras thought there must be an infinite number of elements. This idea was his way of resolving an intellectual dispute concerning the nature of existence that had emerged between the naturalistic-minded philosophers of Ionia to the east and the mystical-minded philosophers to the west, in Greek-colonized Italy, such as Pythagoras and his followers.
Daniel Graham, a professor of philosophy at Brigham Young University and one of the few Anaxagoras experts in the world, says that of the Italian-based philosophers, Parmenides in particular influenced Anaxagoras and his ideas about astronomy.
“Anaxagoras turns the problem of lunar light into a problem of geometry,” Graham 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.
Anaxagoras also wrestled with the origins and formation of the moon, a mystery that still challenges scientists today. The philosopher proposed that the moon was a big rock which the early Earth had flung into space. This concept anticipated a scenario for the moon’s origin that physicist George Darwin, son of Charles Darwin, would propose 23 centuries later. Known as the fission hypothesis, Darwin’s idea was that the moon began as a chunk of Earth and was hurled into space by the Earth’s rapid rotation, leaving behind the Pacific basin. (Today, many astronomers believe that a Mars-sized body slammed into the early Earth, expelling material that then coalesced into the moon, though other theories exist for the origin of our natural satellite.)
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.
Such an idea should have been welcome in democratic Athens, but Anaxagoras was a teacher and friend of the influential statesman Pericles, and political factions would soon conspire against him. In power for over 30 years, Pericles would lead Athens into the Peloponnesian wars against Sparta. While the exact causes of these conflicts are a matter of debate, Pericles’ political opponents in the years leading to the wars blamed him for excessive aggression and arrogance. Unable to hurt the Athenian leader directly, Pericles’ enemies went after his friends. Anaxagoras was arrested, tried and sentenced to death, ostensibly for breaking impiety laws while promoting his ideas about the moon and sun.
“In the Athenian democracy, with its ‘democratic’ trials before large juries on criminal charges being brought by private citizens—there was no district attorney—all trials were basically political trials,” Graham says. “They were often disguised as being about religion or morality, but they aimed at embarrassing some public figure by going after him directly if he was vulnerable, or a member of his circle if he was not. If you wanted to attack Pericles, but he was too popular to attack directly, you found the weakest link in his group. As a foreigner and intellectual with unorthodox new ideas, Pericles’ friend and ‘science advisor’ Anaxagoras was an obvious target.”
Still holding some political sway, Pericles was able to free Anaxagoras and prevent his execution. 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.