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The '''Earth''' is the only known [[planet]] in our '''Solar System''' which can support [[life]]. It contains [[water]], reasonable levels of [[oxygen]], and a stable [[temperature]] range. Geometrically speaking, Earth is the largest of the terrestrial (rocky) planets in the [[solar system]].
The '''Earth''' is the only known [[planet]] in our '''Solar System''' which can support [[life]]. It contains [[water]], reasonable levels of [[oxygen]], and a stable [[temperature]] range. Geometrically speaking, Earth is the largest of the terrestrial (rocky) planets in the [[solar system]].
==Physical Description==
Within the [[solar system]], Earth is the third planet from the [[Sun]]. It is approximately 8,000 miles (13,000 km) in diameter, which means that its equatorial circumference (measured around the equator) is 24,901 miles (40,075 kilometers), and the polar circumference is slightly less at 24,809 miles (40,008 km).<ref>http://geography.about.com/library/faq/blqzcircumference.htm</ref> The reason its metric circumference is so close to the "round number" of 40,000 is that the [[kilometer]] was defined (by the French) as 1/10,000th the distance from the equator to the [[North Pole]].
Its surface area is approximately 4·π (4000 mi)<sup>2</sup> = 200 million square miles (510 million km<sup>2</sup>). It has been said that Earth should really be called "water", as the larger part of it—about 70%—is covered by water. In fact, the [[Pacific Ocean|Pacific]] and [[Indian Ocean|Indian]] oceans alone cover about half of the Earth's surface.
The Earth orbits at an average distance of about 93 million miles (150 million km) from the Sun in an almost circular orbit. The plane of this orbit, or '''ecliptic''', is the common reference plane for the inclinations of the orbits of all [[planet]]s and [[dwarf planet]]s in the [[Solar System]], though in fact Earth's orbit is inclined 7.25° to the plane of the [[Sun]]'s equator. It takes light (and other forms of electromagnetic radiation) approximately 500 seconds to travel from the Sun to the Earth, i.e. the distance can also be stated as "500 light-seconds." The distance to nearby stars is made by measuring the stellar [[parallax]] between observations when the earth is at opposite ends of its orbit, so the earth's orbit itself is a measuring stick for astronomical distances, and is known as the ''astronomical unit'' (A. U.) It is worth noting that if the Earth were slightly closer to the Sun, it would be too hot for life while if Earth were slightly further away from the Sun, water would freeze and life would be impossible.
==Origin==
==Origin==