A sol is a solar day on Mars, which is the apparent interval between two successive returns of the Sun to the same meridian as seen by an observer on Mars. It is one of several units for timekeeping on Mars and is slightly longer than an Earth day, lasting approximately 24 hours, 39 minutes, and 35 seconds. A Martian year is approximately 668 sols, equivalent to approximately 687 Earth days or 1.88 Earth years. The sol was adopted in 1976 during the Viking Lander missions and is a measure of time mainly used by NASA when, for example, scheduling the use of a Mars rover. When a NASA spacecraft lander begins operations on Mars, the passing Martian days (sols) have been tracked using a simple numerical count. The spacecraft that are sent to Mars have a unique clock known as "The Mars Clock" that helps project and keep track of the solar local time. All of the space crew members that are part of any Mars mission are given these unique wristwatches that help keep track of the Martian local time.