The moon revolves in its orbit around the Earth at an average distance 384 392 km. This spacecraft is in two movements: a translation around the Earth and one rotation about its axis. The time period in which these two movements are performed is identical, why the moon always shows one face to our planet. But that face is not always visible in the same way. The moon has no light of its own and acts as a giant mirror that reflects the rays it receives from the Sun changes in form, such as perceived by an observer located at a given point on the planet, depends on the relative positions of G, Earth and the Moon. When Earth is between the Sun and the Moon, the observer will see it as a round disc and intense brightness: this is the stage known as the full moon. If the Moon is located between Earth and the Sun, the same observer will perceive a completely opaque disk called new moon. There are intermediate stages, like the waxing and waning.
The moon completes a translational motion of our planet environment within 27.3 days. However, as in this period the Earth also moves around the sun, the time lag between a full moon and the other reaches to 29.5 days.