As the Earth moves in its orbit, the positions of stars appear to shift back and forth just as the apparent direction of falling raindrops changes as we move back and forth during a rainstorm. This effect, the aberration of starlight, was the first direct evidence for the revolution of the Earth.
The rotation of the Earth causes the planet to have an oblate shape and have an equatorial bulge. The Coriolis effect, also caused by the Earth's rotation, causes apparent deflections of moving objects and currents of air and water. The Coriolis effect is responsible for spiral patterns in atmospheric circulation.
Minerals are solid chemical compounds. Rocks are assemblages of minerals. We classify rocks according to whether they were formed from cooling magma (igneous) or collected sediments (sedimentary), or were transformed by high pressure and temperature (metamorphic).
The Earth's surface is divided into ocean basins and continents. The continents contain very old rocks, but the ocean basins are relatively young.
Pressure increases as depth within the Earth increases. A typical value of pressure in the Earth is 1 million atmospheres. Such high pressures have an effect on the density, mineral structure, and molten state of rock.
The magnetic field of the Earth resembles that of a bar magnet, but it is actually produced by flows of electrically conductive material in the interior. The magnetic field direction reverses on a timescale of hundreds of thousands of years or less.
The study of seismic waves has led to knowledge of the variation of density with increasing depth in the Earth. Zones in which density changes rapidly as depth increases correspond to regions of changing mineral structure, molten state, or chemical composition.
The layers of the interior are the crust, which is made of lighter rock; the mantle, which is made of denser rock; and the core, which is composed mostly of metal. The crust and outer mantle make up the lithosphere, a rigid shell. Beneath the lithosphere is the aesthenosphere, a partially melted, plastic zone.
Plate tectonics describes the Earth's crust as divided into about a dozen moving plates. New crust is produced at oceanic ridges, where magma reaches the surface. The plates are driven apart by the spreading of the seafloor. When plates collide or move past each other, earthquakes, volcanoes, mountain building, and ocean trenches result.
Atmospheric layers are defined by changes in temperature with altitude. Warm layers are those in which solar radiation is absorbed. Transparent layers are cool. The lowest layer, the troposphere, is heated by the ground. Convection in the troposphere causes weather.
The greenhouse effect insulates the Earth by making it difficult for infrared radiation to escape into space. The gases mainly responsible for the absorption of infrared radiation are carbon dioxide and water vapor. Without the greenhouse effect, the Earth would be about 14 K (25° F) cooler.
The Earth's magnetic field dominates the motion of electrons and ions in the magnetosphere. Within the magnetosphere are regions of trapped electrons and ions known as the Van Allen belts. Although gas moving outward from the Sun can't usually penetrate the magnetosphere, it can do so near the magnetic poles. When these energetic particles strike the upper atmosphere, they cause the aurorae.
The Earth formed by the accumulation of infalling bodies, some of which were as large as the Moon or Mercury. Energy from the impacts of these bodies kept the Earth molten and permitted heavy materials to sink to the center to form the core. The oceans and atmosphere accumulated from gas that emerged from the interior in volcanic eruptions. There would be much more carbon dioxide in the present atmosphere if it had not dissolved in the oceans and formed carbonaceous rocks. The oxygen in the atmosphere is mostly the result of photosynthesis by plants.
