Last Quarter Moon Picture

What is a last quarter moon? The last quarter moon, sometimes known as the third quarter moon or a half moon, is defined to occur when the excess of the apparent ecliptic (celestial) longitude of the Moon over that of the Sun is 270 degrees. This means that one-half of the Moon appears to be illuminated by direct sunlight and the fraction of the Moon’s disk that is illuminated is decreasing. This last quarter moon picture illustrates this spectacular event.