How long is a day on the moon?

The answer to this question depends on how we measure a "day". If we think of a day as the amount of time it takes the moon to rotate 360oon its axis, then one lunar day is about 27.32 Earth days long. However, if we consider the perspective of an observer standing on the moon, then a day is the amount of time between two sunrises (or sunsets, or noons; from any point, around and back to the same point). In this case, a lunar day lasts about 29.5 Earth days. This is usually the definition that astronomers prefer.