The common term for the phase of the moon that is half illuminated is the "first quarter" (or "last quarter" when it wanes). During the first quarter, half of the moon’s surface appears illuminated from Earth’s perspective, presenting a half-circle shape. This phase occurs about a week after the new moon when the moon has moved a quarter of the way around the Earth in its orbit. The last quarter moon occurs about a week later, when again half of the moon is lit, but the illuminated portion is now decreasing. Both phases are frequently used by astronomers and enthusiasts to observe the moon.