# Advice 1: Why do we see one side of the moon

People started studying the moon many centuries ago. In the XVII century was drawn up the first lunar maps. However they were shown only one side of the moon. The study of the second, reverse sides, became available to people only as a result of space flight.
The moon makes a complete revolution around the Earth in 29.53 days or 29 days, 12 hours and 44 minutes. That's how much time passes between the repetition of the lunar phases. In addition, during this same period of time the Moon makes a complete revolution around its axis, which becomes the cause of permanent invisibility is one of its sides for the inhabitants of our planet. This phenomenon is not a coincidence, but rather a consequence of the influence of the planet on the satellite.To better understand how this occurs, try a little experiment. Take two balls of different sizes, and then use a felt tip pen or marker draw a small ball on a line so that it divided the ball into two hemispheres. Spin the ball is the moon ball around the Earth, while keeping an eye to one of the little hemispheres of the ball was always directed in the direction of big. For the same period of time the ball is the Moon will turn around the second ball and around its axis.It should also be noted that the assertion that we always see only one half of the moon, i.e. exactly 50% of its surface, is incorrect. The fact that although the moon takes about the same amount of time to commit a full revolution around the Earth and around its axis, the speed at which it revolves in its orbit, is not constant. When approaching the Earth the movement of the moon is accelerated, and removing slows. This is due to the gravitational attraction of celestial bodies: the closer a satellite is to the planet around which it revolves, or planet is to its star the greater the velocity of rotation. Due to this phenomenon that is called libration in longitude, we may from time to time to see Western and Eastern edge of the back side of the moon. Moreover, since the axis of rotation of the moon is slightly tilted relative to the plane of the Earth, we can also see southern and Northern edge of the reverse side. The equator of the moon is at an angle to its orbit, so it rotates around our planet, the satellite shows are part of the southern region, the Northern part. Given all of libration, we can see a total of 50% of the lunar surface, and 59%.

# Advice 2: Why day the moon is visible

The phenomenon of visibility of the moon is actually observed on the new moon. This is due to some cause. Side of the moon that is illuminated by the Sun, every time refers to the inhabitants of the Earth from a new angle, with the result that there is the changing of the moon phases. This process is not affected by the Earth's shadow, except for those moments when the Moon is eclipsed during a full moon. This phenomenon occurs twice a year.

During the new moon, the moon and the Sun interact as follows: the moon aligned with the Sun, resulting in the consecrated part of the moon becomes invisible. At the end of the day it can be observed in the form of a narrow Crescent, which is gradually increasing. This period is usually called the growing Moon.

During the movement of the earth's satellite along its orbit in the first quarter of the moon cycle begins to develop the apparent distance of the moon from the Sun. A week after the onset of the new moon, the distance from the moon to the Sun becomes exactly the same as the distance from the Sun to the Earth. At this moment it becomes visible a quarter of the lunar disk. Further, the distance between the sun and the satellite continues to grow, which is called second quarter of the lunar cycle. At this moment the Moon is at the farthest point of orbit from the Sun. Her phase at this moment will be called a full moon.

In the third quarter of the lunar cycle, the satellite begins its reverse movement relative to the Sun, coming closer to him. The waning Moon again reduced to the size of a quarter disk. The lunar cycle is completed by the fact that the satellite returns to its original position between the Sun and Earth. At this point, the consecrated part of the moon entirely ceases to be visible to the inhabitants of the planet.

In the first part of its cycle the Moon occurs in the morning over the horizon, with the rising Sun is at the Zenith by noon and in the visible area throughout the day to the setting sun. This pattern is usually observed in tropical and subtropical zones.

Thus, each appearance of the lunar disk depends on the phase in which there is a heavenly body at one time or another. In this regard, there are such thing as growing or waning moon, and blue moon.

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