For thousands of years, human beings have noticed that the planets—from the Greek word planetes, meaning wanderers—move differently than the rest of the stars in the sky. Only the five naked-eye planets—Mercury, Venus, Mars, Jupiter, and Saturn—were known before the eighteenth century and, because of their unusual movement through the heavens, had all manner of mystical significance attributed to them.

Conjunctions of two planets in the sky—or massings, as they are called when more than two planets are involved—were important celestial events, and even today remain among the favorite targets of amateur astronomers. But what makes a planetary conjunction and how do we know when one will occur? It turns out the truth is far less mysterious than the ancients would have had us believe.