The orientation of Sun and Moon determines the phase of the Moon. At new moon, the angular distance of the sun and moon is small, less than a few degrees. At first quarter, when the moon is half full, the Moon lies 90 degrees east of the Sun. As an example, if you were to point to the setting sun with one arm and to the moon with the other, the angle between your arms would be 90 degrees. At full, the moon is 180 degrees from the sun and at last quarter, it is 90 degrees west of the sun.

The lighted part of the Moon always points the way to the Sun. This means that a waxing crescent moon in the western sky at sunset has the lighted part on the right which is the direction of the Sun. The sequence of the lunar phases always proceeds with the lighted part of the Moon growing from right to left until the moon reaches full. After full moon, the light recedes from right to left until new moon.

The line boundary between dark and light on the Moon is called the terminator. It is rarely an even line because the surface of the Moon is not smooth. The terminator is closest to a straight line at first and last quarter. Librations affect exactly what portion of the face of the Moon is facing the Earth, during first and last quarter phase, the terminator's exact location may vary. This is illustrated when the terminator does not line up with the Moon's meridian at first and last quarter. In other words, you'd think that during first and last quarter when the Moon is half full, that the meridian (the imaginary line which runs from north to south on the Moon,) would be right on the terminator, but it isn't so. In fact the terminator can be almost 8 degrees off to either side of the meridian.