First Quarter Moon Picture

What is a first quarter moon? The first quarter moon, sometimes known as 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 90 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 increasing. This first quarter moon picture illustrates this spectacular event.