A moonphase is a complication on a watch that displays the current phase of the moon as seen from Earth, often found on the dial of the watch.
Most moonphase complications use a 59-tooth disc that treats the lunar cycle as 29.5 days. The real cycle is about 44 minutes longer, so the display drifts by one day roughly every two years and eight months. Higher-precision mechanisms use finer gearing and stay accurate to within a day for 122 years or more. The record belongs to independent watchmaker Andreas Strehler, whose Sauterelle à lune perpétuelle 2M is accurate to one day in around two million years.
Yes, moonphase displays require manual adjustment whenever the watch stops or loses power. You'll need to advance the moon disc to match the current lunar cycle using the watch's pusher or crown. Most require adjustment once every few years if kept running.
Moonphase complications display the current lunar phase, historically useful for maritime navigation, agriculture, and tidal predictions. Today, they're primarily aesthetic and traditional features appreciated by collectors for their romantic connection to astronomy and classical watchmaking.