c. 280 BCE

Sun-Centered Cosmos

Aristarchus (c. 310–230 BCE)

The geocentric model of Plato and Aristotle permeated ancient Greek thinking about the cosmos. And why not? Everyone could see that the Sun, Moon, and stars rotated around the Earth. Scholars added other supposedly irrefutable evidence: the Moon went through phases consistent with it orbiting our planet. If the Earth spun on its own axis, why was nothing flung off the surface? None of the stars showed any observed parallax, or shifting of position relative to other stars, that they would show if the Earth were moving in its own orbit. Case closed!

There were doubters and skeptics, however. The earliest one on record was the astronomer and mathematician Aristarchus, from the Greek island of Samos, who challenged the nearly 200-year-old common wisdom of his esteemed Greek colleagues by making detailed naked-eye observations of the Sun and Moon and trying to interpret them in a geocentric context. His methods were limited by the acuity of the human eye, but nonetheless he was able to deduce from geometrical calculations that the Sun was at least 20 times farther away than the Moon (the actual value is 400). He then deduced that, because the Sun and Moon have about the same apparent angular diameter in the sky, the Sun’s diameter must be at least 20 times larger than the Moon’s diameter and 7 times Earth’s diameter. Thus, according to his reasoning, the Sun’s volume was more than 300 times the volume of the Earth (the actual value is about a million). It must have seemed foolish to him, then, that such a giant Sun would be indentured to such a relatively tiny planet like the Earth, instead of the other way around. Naturally, he advanced the idea that the Earth and other planets orbit the Sun and that the stars are so far away that no parallax could be observed. Aristarchus’s universe was a much larger universe than anyone had described before.

Like most revolutionary ideas, Aristarchus’s idea of a Sun-centered cosmos was met with ridicule and disdain by most of his colleagues; 250 years later, the idea was effectively crushed by the geocentric teachings and writings of Ptolemy. Aristarchus had planted a critical seed of doubt, but it would not germinate until the sixteenth century.

Copy of a section of Aristarchus’s original third-century BCE calculations of the relative sizes of the Sun, Earth, and Moon, used to help support his then-radical notion of a heliocentric cosmos.