THE EARTH

The Earth is the third planet outward from the Sun. Its single most outstanding feature is that its near-surface environments are the only places in the universe known to harbour life.

Scientists have applied the full battery of modern instrumentation to studying the Earth in ways that have not yet been possible for the other planets; thus, much more is known about its structure and composition. It is convenient to consider separate parts of the planet in terms of roughly spherical regions extending from the interior outward: the core and mantle, the lithosphere (the rocky, near-surface crust of land), the hydrosphere (dominantly the oceans, which fill in low places in the crust), the atmosphere (itself divided into spherical zones such as the troposphere, where weather occurs), and the magnetosphere (which includes the interface with the upper atmospheric ionosphere, the radiation belts, and the bow shock). These parts of the planet are treated briefly, in turn, in this article, while they are treated in detail elsewhere.

Since the Copernican revolution of the 16th century, at which time the Polish astronomer Nicolaus Copernicus proposed a Sun-centred model of the universe, enlightened thinkers have regarded the Earth as a planet like the others of the solar system. Concurrent sea voyages provided practical proof that the Earth is a globe, just as Galileo's use of his newly invented telescope in the early 17th century soon showed various other planets to be globes as well. It was only after the dawn of the space age, however, when photographs from rockets and orbiting spacecraft first captured the dramatic curvature of the Earth's horizon that the conception of the Earth as a roughly spherical planet rather than as a flat entity was verified by direct human observation.

Humans for the first time saw the Earth as a complete globe in December 1968 when Apollo 8 carried astronauts around the Moon. In December 1990 the Galileo spacecraft, outfitted with an array of remote-sensing instruments, studied the Earth during the first of its two gravity-assisted flybys en route to the planet Jupiter. The information about the Earth gathered from Galileo was meagre compared with that obtained by the swarm of artificial satellites that have orbited the globe throughout the space age, but it provided some unique portraits of the Earth as a planet.



Viewed from another planet, the Earth would appear bright and bluish in colour. Most readily apparent would be its atmospheric features, chiefly the swirling white cloud patterns of mid-latitude and tropical storms, ranged in roughly latitudinal belts around the planet. The polar regions also would appear a brilliant white owing to the clouds above and the snow and ice below. Beneath the changing patterns of clouds are the much darker, blue-black oceans, interrupted by occasional tawny patches of desert lands. The green landscapes that harbour most human life would not be easily seen from space; not only do they constitute a modest fraction of the land area, which itself is a small fraction of the Earth's surface, but they are often obscured by clouds. Over the course of the seasons, some seasonal changes in the storm patterns and cloud belts on Earth would be observed. Also prominent would be the growth and recession of the winter snowcap across land areas of the Northern Hemisphere.


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