Half of the moon is always lit by the sun, unless the sun is eclipsed by the Earth. The portion of the illuminated moon viewed from Earth depends on the moon's orbit. As the moon orbits, different portions of shadow and illumination are seen from Earth, creating the moon's phases.
The moon goes through several phases during its orbit, either increasing or decreasing visibility of the portion of it that is illuminated. The entire half of the moon in shadows is visible during the new moon, with the entire half of the illuminated side visible during the full moon. During the first quarter and last quarter moons, one half of both the illuminated and shadowed side is visible. When a crescent moon is in the night sky, less than one half of the illuminated portion can be seen. If there is a gibbous moon, more than one half of the illuminated portion is visible.