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rare earths,in chemistry, oxides of the rare-earth metals. They are widely distributed in the earth's crust and are fairly abundant, although they were once thought to be very scarce...... A substance such as a compound can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved...... These substances were first called elements (stoicheia) by Plato. element,in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. Earth was one of the four "roots" of the Greek philosopher Empedocles, the other three being air, water, and fire.
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The earth is undoubtedly much denser near the center than it is at the surface, because the average density of rocks near the surface is c.2.8 g/cc, while the average density of the entire earth is c.5.5 g/cc.
The outer shell, or crust, varies from 5 to 25 mi (8 to 40 km) in thickness, and consists of the continentscontinent,largest unit of landmasses on the earth.
However, the earth is not an exact sphere, being slightly flattened at the poles and bulging at the equator.
The equatorial diameter is c.7,926 mi (12,760 km) and the polar diameter 7,900 mi (12,720 km); the circumference at the equator is c.24,830 mi (40,000 km).
The sial and sima together form the crust, beneath which lies a shell called the mantle.
The boundary between the crust and the mantle is marked by a sharp alteration in the velocity of earthquake waves passing through that region.
Knowledge of the earth's interior has been gathered by three methods: by the analysis of earthquake waves passing through the earth (see seismology), by analogy with the composition of meteorites, and by consideration of the earth's size, shape, and density.
Research by these methods indicates that the earth has a zoned interior, consisting of concentric shells differing from one another by size, chemical makeup, and density.
Halfway between, in spring and in autumn, there is a time (see equinoxequinox, either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. These temperature gradients, together with the Earth's rapid rotation, drive the circulation of the atmosphere.
The vernal equinox, also known as "the first point of Aries," is the point at which the sun appears to cross the celestial equator from south to north...... On average the Earth emits into space an amount of radiant energy equal to that absorbed by surface plus atmosphere. that needed to keep Earth in thermal equilibrium with space, is –18 °C; this is the Earth's temperature as measured from space.
) when all parts of the earth have equal day and night. Studies of the refraction and reflection of seismic waves propagating through the Earth show that it consists of three main internal layers: the crust, mantle, and core.