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Earth

Earth is the third planet from the sun and the fifth largest in the solar system. The diameter of Earth is 12,742 km (7,926 miles), just a few hundred kilometers larger than that of Venus. Earth is about 150 million kilometers (93 million miles) away from the Sun and has one satelite – the Moon. Seventy percent of the Earth surfice is covered in oceans and is only planet known to support life. The four seasons are a result of Earth axis of rotation being tilted 23.45 degrees with respect to the plane of its orbit around the sun. During part of the year, the northern hemisphere is tilted toward the sun and the southern hemisphere is tilted away, producing summer in the north and winter in the south. Six months later, the situation is reversed. When spring and fall begin, both hemispheres receive roughly equal amounts of solar illumination.

The atmosphere of Earth can be divided into five main layers - exosphere, thermosphere, mesosphere, stratosphere and troposphere.
The exosphere is the outermost layer of an altitude of about 700 km (440 miles) to about 10,000 km (6,200 miles) above sea level. The exosphere merges with the emptiness of outer space. This layer is mainly composed of extremely low densities of hydrogen and helium.
The thermosphere extends at an altitude of about 80 km (50 miles) up to an altitude range of 500–1000 km (310–620 mimiles) depending on the solar activity. The air is so rarefied that an individual molecule (oxygen, for example) travels an average of 1 kilometre (0.62 miles) between collisions with other molecules. This layer is completely cloudless and free of water vapor. However non-hydrometeorological phenomena such as the aurora borealis and aurora australis are occasionally seen in the thermosphere.
The mesosphere extends at an altitude of about 50 km (31 miles) to 80–85 km (50–53 miles) above sea level. Temperatures drop with increasing altitude to around −85 °C (−120 °F). The highest water vapor clouds in the atmosphere are situated just below the top of the mesosphere. The mesosphere is also the layer where most meteors are burned after their entrance in the atmosphere.
The stratosphere is the second-lowest layer of Earth atmosphere. This layer extends from about 12 km (7.5 miles) to about 50 to 55 km (31 to 34 miles) above Earth surface. The pressure at the top of the stratosphere is roughly 1000 times less than the pressure at sea level. It is the part of Earth atmosphere that contains the ozone layer. The absorption of ultraviolet (UV) radiation from the Sun by the ozone layer induces temperature rise with increasing altitude. Although the temperature below the stratosphere may be −60 °C (−76 °F), the top of the stratosphere is much warmer, and may be near 0 °C (32 °F). The stratosphere is almost completely free of clouds and other forms of water.
The troposphere is the lowest layer of Earth atmosphere. It extends from the surface to a height of about 12 km (7.5 miles). The temperature usually declines with the altitude because the troposphere is mostly heated through energy transfer from the surface. Therefore, the Earth surface is typically the warmest part of the troposphere. The troposphere presents roughly 80% of the mass of Earth atmosphere. The half of the total mass of the whole atmosphere is located in the close 5.6 km (3.8 miles) to the Earth surface. The stratosphere is mainly composed of nitrogen (78%) and oxygen (21%), with only small concentrations of other gases. Most of the Earth weather takes place here because almost all atmospheric water vapor is found in the troposphere.

The Moon is the only natural satellite of Earth. In the Solar System it is one of the largest natural satellites, and the largest one respective to the size of the orbiting planet with a diameter a quarter of that of the Earth. The Moon is possibly formed not long after Earth. The most recognized hypothesis is that the Moon emerged from the space debris left around the Earth after a giant impact with a planet called Theia, approximately the size of Mars. The Moon rotates about its axis almost the same time it takes to orbit Earth, so the Moon always keeps the same face turned towards Earth. The monthly changes of angle between the direction of illumination by the Sun and viewing from Earth result in different phases of the Moon. The foure main are New Moon, First Quarter, Fool Moon and Last Quarter. The Moon's gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day. The distance from the Moon to the Earth varies from 362,600 to 405,400 km (225,309 to 251,904 miles). The orbital period of the Moon is 27 days, 7 hours, 43 min and 6 seconds. Its mean radius is 1737 km(1079 miles), wich is 0.273 that of the Earth.

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