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Earth
Description The Earth consists of three distinct layers. The core is the central part. It is made of molten nickel and iron; their movement generates Earth's magnetic field. The next layer is the mantle. It consists of solid, hot rock. The crust upon which we walk and live is the upper-most layer, consisting of soil, clay, and fine rock. The atmosphere has five layers. The layer closest to the surface is called the Troposphere. It is about 3.2 km (two miles) thick, and can contain low lying clouds. The next layer is the stratosphere. The stratosphere is about 21 km (13 miles) thick and contains the high cirrus and cirrostratus clouds. The highest altitude commercial planes fly at this level. The level above the stratosphere is the Mesosphere. Above the Mesosphere is the Thermosphere. Combined, the Thermosphere and the Mesosphere create the Ionosphere, which is 435 km (270 miles) thick. The last layer of our atmosphere is the Exosphere, which extends into space. Unique Characteristics The Earth is the only planet, that we currently know of, capable of sustaining life. Several factors led to the development of life: First, Earth had to be at a temperate distance from its host star. This means that the planet needs to be at a distance from the star to allow liquid water to exist, and for the temperature range to be not too extreme. Second, Earth has an atmosphere, and that atmosphere had (and has) a chemical makeup that allows the development of life. Third, Earth has water. Biologists believe that this is the most essential constituent for life to exist. The Seasons If you were to talk to the average passer-by, they would probably tell you that the seasons are caused by Earth not having a perfectly circular orbit about the sun, so when it's farther away it is winter and when it's closer it is summer. However, there is a simple reason why this is incorrect: That would mean the northern and southern hemispheres have winter at the same time and summer at the same time (which they, in actuality, don't). The explanation behind the seasons has to do with the Earth's tilt on its axis. As shown in the data table below under "Axial Tilt," the Earth's rotation axis is tilted approximately 23.5° from the plane in which it orbits (this is represented by the green arrows pointing "To North Star") in the above picture. This tilt results in parts of Earth receiving more direct sunlight than other parts. When they get more direct sunlight, it is warmer (Summer), and less direct is cooler (Winter). In the diagram above, the triangle and diamond represent locations on Earth. In the Earth on the left of the sun, the diamond is facing the sun more directly, and it is experiencing Summer. The diamond is at a less-direct angle and is experiencing Winter. In the Earth on the right, the triangle and diamond are in the same location as before, but now the diamond gets more direct sunlight and the triangle less direct, so their seasons are switched. It is because of this constant* axial tilt that different sections receive different amounts of light at different times of the year, causing the seasons. *The tilt of Earth's axis is relatively constant, but a process known as precession causes the "North" to move over the course of approximately 24,000 years. For example, when the Egyptian pyramids were built, the north star was not Polaris, but a star called Thuban. Mythology In Greek mythology, the Earth was called Gaea. Gaea married Uranus, and became mother of all living things. Data for the Planets
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