The sun hung at its highest point over Athens, and one man was about to solve a problem that had tormented civilizations for millennia.

The Day Meton Counted the Moon's Return in Athens

One summer solstice, an astronomer cracked the calendar code that still governs Easter

In 432 BCE, Meton discovered the 19-year lunar cycle that still determines Easter's date today.

The sun hung at its highest point over Athens, casting almost no shadow at noon. It was the summer solstice of 432 BCE, and on the Pnyx hill, where citizens gathered to debate democracy's business, a different kind of revolution was unfolding. Meton of Athens had positioned his gnomon—a simple vertical rod—and waited for the moment when shadow and light would reveal the heavens' deepest secret.

For years, the Greek calendar had been chaos. Each city-state ran its own lunar calendar, inserting extra months whenever priests or politicians saw fit. Festivals drifted. Debts came due at unpredictable times. Wars were declared on days that meant different things to different armies. The moon's cycle of roughly 29.5 days refused to align neatly with the sun's 365-day journey. It was, as one Athenian playwright quipped, as if the gods themselves couldn't agree on the time.

Meton had been watching. For decades, he and his colleague Euctemon had tracked the risings and settings of stars, the lengths of seasons, the subtle dance between solar and lunar rhythms. And now, on this solstice morning, he was ready to announce what his observations had proven: nineteen solar years contained almost…

💡 The Jewish calendar adopted Meton's cycle around the 4th century CE, meaning both Easter and Passover calculations ultimately trace back to the same Athenian astronomer.