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The phenomenon of libration, discovered by Galileo Galilei in 1635, makes it possible to observe about 59% of the lunar surface. The fact is that the Moon rotates around the Earth with a variable angular velocity due to the eccentricity of the lunar orbit (it moves faster near the perigee, slower near the apogee), while the rotation of the satellite around its own axis is uniform. This makes it possible to see the western and eastern edges of the far side of the Moon from the Earth (optical libration in longitude). In addition, due to the inclination of the Moon's axis of rotation to the plane of its orbit, from the Earth one can see the northern and southern edges of the far side of the Moon (optical libration in latitude).

Although the Moon rotates around its axis, it always faces the Earth with the same side, that is, the Moon's revolution around the Earth and rotation around its own axis are synchronized. This synchronization is caused by the friction of the tides produced by the Earth in the shell of the Moon[40]. According to the laws of mechanics, the Moon is oriented in the Earth's gravitational field so that the semi-major axis of the lunar ellipsoid is directed towards the Earth.

Since the Moon does not glow itself, but only reflects sunlight, only the part of the lunar surface illuminated by the Sun is visible from the Earth (in the phases of the Moon close to the new moon, that is, at the beginning of the first quarter and at the end of the last quarter, with a very narrow crescent, you can observe " the ashen light of the moon" - weak illumination of it by the rays of the sun reflected from the earth). The Moon revolves in an orbit around the Earth, and thereby the angle between the Earth, the Moon and the Sun changes; we observe this phenomenon as a cycle of lunar phases. The period of time between successive new moons averages 29.5 days (709 hours) and is called the synodic month. The fact that the duration of the synodic month is longer than the sidereal one is explained by the movement of the Earth around the Sun: when the Moon makes a complete revolution around the Earth relative to the stars, the Earth by this time has already passed 1/13 of its orbit, and so that the Moon again finds itself between the Earth and the Sun, She needs an extra two days.