Was the Greek philosopher Anaxagoras of Clazomenae (ca. 500–428 BCE) the first person to propose the correct theory of both solar and lunar eclipses? Thales, more than a century earlier, has a claim. Anaxagoras’s contemporary Empedocles of Acragas has also been credited. Philosopher Daniel W. Graham and astronomer Eric Hintz team up to argue for giving the laurels to Anaxagoras, whom they call “perhaps the first empirical astronomer.”

“There are serious doubts about Thales’ ability either to predict or to explain an eclipse,” they explain, “and no sign that anyone after him had a correct understanding of eclipses until the generation of Anaxagoras and Empedocles. Whatever Thales’ insights may have been, explanations of eclipses were not founded on a scientific basis until the time of Anaxagoras.”

Anaxagoras’s theory of eclipses was “essentially correct,” Graham and Hintz write. The caveat is that Anaxagoras thought the Earth was a thin, flat disk at the center of the cosmos. The other heavenly bodies, held aloft by vortices, orbited the Earth. The Sun generated its own light, the Moon reflected the Sun’s light.

Anaxagoras conjectured that when the Earth was between the Sun and the Moon, there was a lunar eclipse: the shadow or umbra of the Earth darkened the surface of the Moon. When the Moon was between the Sun and Earth, there was a solar eclipse: the umbra of the Moon darkened the surface of the Earth. This is basic stuff now, but “in its time [Anaxagoras’s] theory marked a sudden advance to the first scientifically correct explanation of eclipse in the history of the world, as far as we know.”

But, continue Graham and Hintz, Anaxagoras didn’t stop there. He realized that a solar eclipse could be used as a scientific tool. The Moon’s shadow could be used to figure out the size of the Moon. And the Sun.

“The area in which the eclipse was visible would be limited to the area shadowed by the Moon,” they write, “and the Moon’s size would be roughly the same as its shadow on Earth. If the Sun were significantly farther from the Earth than the Moon, it could be significantly larger than the Moon.”

By Anaxagoras’s calculation, the Moon was as large as the Peloponnese, the peninsula at the southern tip of mainland Greece. The Sun, therefore, must be larger than the Peloponnese.

[Obviously, the Sun is much larger than the Peloponnese, which covers some 8,320 square miles/21,549 square kilometers. It would take 1.3 million Earths to fill the Sun’s volume. But the point is that Anaxagoras showed that the Sun was definitely larger than it looked.]

So which solar eclipse sparked this calculation on Anaxagoras’s part? Scholars in the 1970s proposed the eclipses of 557 BCE and 463 BCE. Graham and Hintz disagree: 557 BCE was half a century before Anaxagoras’s time; the 463 BCE eclipse was visible further north on the Greek mainland, not on the Peloponnese. The only other solar eclipse “noticeable in Greek lands during Anaxagoras’s maturity” was the eclipse of 478 BCE, when Anaxagoras was about twenty-two. In this eclipse, the Moon’s shadow covered almost the entirety of the Peloponnese, which, conveniently, is a roughly circular landmass. This eclipse, the authors argue, is the “only immediate basis for Anaxagoras’s measurement of the Sun’s size.”

There are solar eclipses at least twice—and as many as five times—a year. Most solar eclipses, partial or total, are unseen by the majority of the world’s population: about one-quarter the size of Earth, the Moon casts a narrowly focused shadow across the surface of its big sibling. This has led to the phenomena of eclipse-chasing, traveling to catch eclipses far from home, especially total eclipses like this year’s April 8, 2024, eclipse, which passes across a narrow swath of North America. Eclipse-chasers, also known as umbraphiles (shadow lovers), could well be called Anaxagorasians in honor of the symbolic first of their kind.

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