People began to study the Moon a lot more centuries back. In the 17th century even the first lunar cards were made. The truth on them was represented only one side of the Moon. Studying the second, the back, became available to people only as a result of flights in space.
The moon makes a whole revolution around Earth in 29.53 days or in 29 days, 12 hours and 44 minutes. So much time passes between repetition of lunar phases. Besides, for the same interval of time of Long does a whole revolution round its pivot-center, as becomes the reason of constant invisibility of one of its parties for inhabitants of our planet. This phenomenon – at all not coincidence, but only a consequence of influence of the planet on the satellite. Better to understand how it occurs, make a small experiment. Take two balls of the different sizes, and then by means of a felt-tip pen or a marker draw on a small sphere the line so that it divided a ball into two hemispheres. Rotate a ball Moon myacha-Zemli around, watching at the same time that one of hemispheres of a small ball was always directed towards big. For the same interval of time the ball Moon will make a turn as around the second ball, and round its pivot-center. It is also necessary to notice that the statement as if we always see only one half of the Moon, i.e. exactly 50% of its surface, is incorrect. The fact is that though the Moon needs identical amount of time for commission of a whole revolution around Earth and round its pivot-center, the speed with which it orbits is not constant. At approach to Earth the movement of the Moon accelerates, and during removal slows down. It is connected with features of a gravitational attraction of celestial bodies: the closer the satellite is to the planet around which it rotates, or the planet – to the star, the is more rotation speed. Thanks to this phenomenon called a libration on longitude we can see the western and east edge of a reverse side of the Moon from time to time. Moreover, as the axis of rotation of the Moon is a little inclined in relation to Earth plane, we can also see south and northern region of a reverse side. The equator of the Moon is located at an angle to its orbit therefore, rotating around our planet, the satellite shows a part of south country, a part of northern. Considering all libration, we can see in total not 50% of a lunar surface, but 59%.