Earth (Planet)
Earth is the third planet from the Sun in the solar system. Earth's average distance from the Sun is 92,900,000 mi (149,476,000 km), and defines one astronomical unit. Earth is an oblate spheroid with a surface area of 196,938,800 square miles (510,485,000 square kilometers). The elliptical orbit of Earth around the Sun takes approximately 365.26 days with respect to the background stars, and defines one sidereal year. Radiometric dating has led to a widely accepted date for Earth's age of about 4.6 billion years.
The bulk of scientific knowledge about Earth has accumulated only within the past few centuries and has evolved into the disciplines of geophysics and geology. Geophysics is the study of Earth as a planet and the space surrounding it, and the interactions between Earth and extraterrestrial forces. Geophysics contains three major divisions of study: the solid-earth, the atmosphere and hydrosphere, and the magnetosphere.
Solid-earth studies are concerned with Earth's surface and interior. The surface of Earth is very young compared to its age. In a period of about 500 million years erosion and tectonic processes destroy and re-create most of the surface features. Earth's surface is unique from the other planets in the solar system because it has liquid water in large quantities.
The knowledge learned about the interior of the solid-earth is mostly based on evidence and deductions made from the propagation of seismic waves through Earth. Studies of seismic waves propagating through the planet show that the interior consists of three main layers: the crust, mantle, and core.
The "crust," immediately below the surface, is of variable thickness. In some areas the crust is only a few miles or kilometers thick--in other areas it may measure 25 mil (40 km) in thickness. It has an average density of about three times that of water. The crust consists primarily of quartz and other silicates in the form of sedimentary rocks resting on a base of igneous rocks. It is divided into several separate solid tectonic plates that float independently on top of the mantle.
The mantle generally extends from a depth of 25-1,802 mi (40-2,900 km), with a density reaching 5.5 times that of water. Its composition is thought to include a high proportion of silicon, magnesium, and oxygen, with some iron, calcium, and aluminum.
Earth's core extends from a depth of 1,802-3,964 mi (2,900-6,378 km), with an increase in density from 10 times that of water at its mantle boundary to 13 times that of water at the center. Its composition may be similar to nickel-iron meteorites although it is possible that lighter elements also may exist. The outer portion of Earth's core is known to be in a liquid state and it is the rotation of the ion filled outer core around the inner core that is thought responsible for Earth's magnetic field. Temperatures at the core of Earth may range as high as 7,500K (hotter than the Sun's surface).
The material that comprises Earth's interior has an average strength similar to steel, but maintains a fluidic consistency. As a result of Earth's consistency and angular momentum, materials tend to migrate to the equator causing the equatorial radius 3,964.2 mi (6,378.4 km) to be 13.2 mi (21.3 km) larger than the polar radius, and thereby give Earth its oblate spheroid shape.
The atmosphere of Earth is a mixture of gases, with dry air composed of 78% nitrogen, 21% oxygen, 1% argon, and trace percentages of carbon dioxide and other gases. The majority of these gases exist at the lowest layer of the atmosphere, the troposphere, which extends an average thickness of 4-8 mi (7-13 km). The stratosphere, mesosphere, ionosphere, and the outermost layer, the exosphere, lie in progressive layers above the troposphere.
Two-thirds of the planet Earth is covered by water (hydrosphere) at an average ocean depth of 12,795 ft (3,900 m). The water area is 1.394 x 108 mi2 (3.617 x 108 km2 ).
The magnetosphere of Earth extends from the upper atmosphere to thousands of miles into space. The magnetic field of Earth dominates this region, trapping various charged particles. Similar to a bar magnet, lines of force stretch between the magnetic poles of Earth, whose positions shift over time. The rotation of Earth causes the magnetic field to be aligned approximately with the rotation axis. In 2000 the position of the north magnetic pole was 77.3 degrees north latitude and longitude 101.8 degrees west, and located on the Bathurst Island in northern Canada. The overall magnetic field strength of Earth is fairly weak, averaging about one-half gauss at Earth's surface.
The sidereal rotation of Earth about its axis takes approximately 23 hours, 56 minutes, 4.099 seconds. The sidereal rotation is variable because it changes slightly as Earth's rate of spin varies.
Earth has one natural satellite, the Moon. Earth's average distance from the Moon is 238,857 mi (384,321 km).
The mass of Earth is approximately 5.977 x 1024 kg. Earth's volume is 260.3 billion mi3 (1,084.2 billion km3 ).
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