First quarter Moon. Image 7 of 27. Optical image of a waxing (increasing in apparent size) half Moon 7 days into its 28day cycle (first quarter). The lunar phases arise as the Moons orbit of the Earth shows the Earthfacing side moving into and out of the light of the Sun. This Earthfacing side of the Moon is locked in place by the gravitational influence of the Earth. This results in a lunar day that equals the length of the lunar Earth orbit. On a waxing Moon the daynight line (terminator) marks the lunar dawn. Taken in September 2001. A sequence of the monthly cycle of Moon phases is shown on images R340644670.