The lunar phases are caused by the changing angles of the sun, the moon and Earth, as the moon revolves around Earth. Different amounts of the illuminated part of the moon are visible from Earth. The lunar phases include the new moon, first quarter, full moon, third quarter, crescent and gibbous.
A new moon takes place when the moon is between Earth and the sun. The full moon occurs when Earth is between the sun and the moon. The third and first quarter, sometimes referred to as the half-moon, takes place when the moon is at a 90 degree angle with respect to the sun and Earth.