### The Earth-Moon system and its consequences

Earth, the third planet from the sun, has a single satellite: the Moon. According to the relationship between the size of the planets and their satellites, the moon is relatively large in the solar system, because its diameter is 3470 kilometers, a little over a quarter of the Earth's equatorial diameter, measuring 12,756 kilometers.

The two bodies interact through movements of rotation and translation, so that we can speak of an Earth-Moon system. The origin of the Moon is not exactly known, but given its density, similar to that of the earth's crust, it is possible that after our planet formed, a large meteorite crashed into him and a critical mass becomes detached. This mass had been hovering around the mother planet. The Earth's average density is 5.5 g/cm3, but its bark is of 3.36 g/cm3, much like the average density of the Moon: 3.34 g/cm3.
's Moon spins around same and also around the Earth in an orbit that is slightly elliptical. This satellite takes 27 days, 7 hours, 43 minutes and 12 seconds to complete one orbit around our planet, while matching its rotation period. As a result of this coincidence, the Moon always presents the same face when viewed from Earth.

The Moon's orbital plane forms an angle of 5.1 ° to the ecliptic, the name given to the plane of Earth's orbit around the Sun The points where the Moon's orbit intersects the ecliptic are called nodes. There is an ascending node and a downstream node, and this fact has a close relationship with the eclipses, both Sun and Moon. To produce a lunar eclipse, ie that the Earth's shadow falls on the moon and the tape, it must be located on a node, aligned with the Sun

As we have seen, to match their periods of rotation and translation, the Moon always shows the same face. This situation is not unique to the Earth and Moon. All Solar System satellites always show the same face to their respective planets. The reason for this lies in the gravitational gradient called a gravitational effect satellites makes restrain its rotation to synchronize with its movement around the planet.
tidal force due to the Moon is unimportant over the planet's solid, but causes the tides in the oceans. However, the tidal force exerted by the Earth to the Moon managed to retain after the formation of this star.

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