Earth is the third planet in order of distance from the Sun. It is part, together with Mercury, Venus and Mars, of the so-called solid planets, and is the only one to house, as far as we know today, evolved forms of life.

The Earth revolves around the Sun in an eccentric orbit approaching it up to 152.1 million km. Its average distance is 149.6 million km. and is used by astronomers as a unit of measurement of distances in the solar system, under the definition of Astronomical Unit (UA).

The plane of this orbit is taken as a reference to measure the inclinations of the orbital planes of the other planets which, with the exception of Pluto, separate into a few degrees or fractions with respect to it. The Earth uses 365,256 days to make a complete turn around the Sun (this period is called the sidereal period, that is, referred to the fixed stars), moving at a speed of 29.79 km / sec. along its orbit.

Our planet is also animated by a movement of revolution around its own axis that, measured with respect to the Sun, takes place in 24 hours (solar day). The inclination of the earth's axis with respect to the plane of the orbit is 66 33 '(with respect to the axis of the orbit is 23 degrees 27 minutes).

The mass of the Earth is 5.976 x 10 raised to 24 kg. and its average density of 5.52 g./cm3, that is to say about five times greater than that of water. It is the highest density with respect to the other inner planets. In terms of dimensions, the Earth has an average radius of 6,371 km., It is the largest of the solid planets, but has a volume 1,316 times smaller than that of the giant Jupiter. Its shape is not perfectly spherical, its radius being slightly larger in Ecuador (6,378 km.) Than at the poles (where it reaches 6,356 km.).

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