The Antikythera mechanism also traced patterns that took much longer to repeat. For instance, ancient astronomers knew that occasionally, on the night of a full moon, a lunar eclipse happens. During a lunar eclipse, the fully illuminated face of the full moon goes dark for a time. However, they noticed that this didn’t happen every full moon—in fact, over a year would sometimes pass between their observations of lunar eclipses. Through careful record-keeping, the ancient astronomers realized that eclipses, although rare, happened in patterns. They kept track of the patterns and recorded that knowledge in the workings of the Antikythera mechanism. As a user turned the date dial of the Antikythera mechanism, the mechanism counted the days and displayed exactly when people in Greece could expect to observe a lunar eclipse.