The phase of the Moon as seen from the Earth arises because of the constantly changing position of the Moon relative to the Sun and the Earth. As the Moon goes round the Earth, one half of its surface that faces the Sun is always illuminated. But from Earth, the entire illuminated side is visible only when it faces us, that is, only at full Moon. At other times, depending on the position of the Moon in the sky relative to the Sun, only a part of the illuminated face becomes visible, which grows and shrinks in size, giving rise to the familiar phases of the Moon.
