Period of rotation (equatorial): 23 hr. 56 min. 4 sec.
Mean orbital velocity: 18.5 miles (29.8 km) per second
Inclination of axis: 23.44 degrees
Orbital eccentricity: 0.017
Orbital inclination: 0 degrees
Diameter at equator: 7,928 miles (12,756 km)
Oblateness: 0.003
Mass: 5.97 x 10^24 kg
Volume: 1.084 x 10^12 km^3
Density: 5.52 g/cm^3
Surface gravity: 9.78 m/s^2
Escape velocity: 6.94 miles (11.2 km) per second
Mean surface temperature: 72F / 22 C / 295 K
Albedo: 0.39
Number of known satellites: 1
Mean diameter of Sun as seen from the planet: 32' 01"
Satellite statistics
Moon
Mean distance from center of planet: 238,900 miles (384,400 km)
Inclination of Axis: 6.7 degrees
Rotation period: 27.321 days
Orbital period: 27.321 days
Orbital eccentricity: 0.055
Orbital inclination to planet's equator: 18.3 degrees to 28.6 degrees
Diameter: 2,160.1 miles (3,475.6 km)
Mass: 7.35 x 10^22 kg
Density: 3.34 g/cm^3
Escape velocity: 1.48 miles (2.38 km) per second
Maximum apparent diameter as seen from the planet: 31' 5"
Planet composition
The core of the Earth, composed of metallic nickel iron and iron oxides, is about 6800 km (4230 miles) in diameter. The outer 2100 km of the core is molten, and the rest is solid, with a specific gravity of about 16. Surrounding the core is a mantle composed of magnesium and iron silicates. On top of the mantle is a thin crust, approximately 33 km thick, which is divided into 15 plates. The composition of the crust varies: under the continents it consists of a substrata of granite rock, rich in silicon and aluminum, and under the oceans it is primarily basalt rocks, rich in silicon and magnesium.
Inner core: radius of 1300 km
Liquid outer core: 2100 km thick
Mantle: 2945 km thick
Crust: 33 km thick
Topography
The Earth is unlike the other terrestrial planets. Aside from being host to the only known life forms in the solar system, Earth has very few craters, many volcanic plains, and 70% of the surface is covered by water. Our planet also has folded mountain chains and platform deposits, which occur nowhere else in our solar system.
Erosion, weather, sea-floor spreading, and subduction are continually changing the outer face of the planet. The highest point on the Earth's surface is the summit of Mount Everest (29,028 ft.). The largest feature on the face of the planet is the Pacific Ocean (64,186,000 sq. mi. or 166,883,600 sq. km).
Atmosphere
The Earth's atmosphere today is thought to be the result of volcanic activity and the presence of living organisms, which produce oxygen through photosynthesis. Between sea level and an altitude of about 10 km lies the troposphere, which consists mainly of nitrogen (78%) and oxygen (21%), with traces of water vapor, argon, and carbon dioxide. This region contains 80% of the Earth's atmosphere (by mass) and is where virtually all recognizable weather takes place. Above the troposphere lies the stratosphere, where a layer of ozone absorbs ultraviolet radiation from the Sun. In the thermosphere, which begins about 80 km from the surface, the molecules are heated by radiation from the Sun, and the temperature increases with increasing altitude. Particles that are hot enough escape from the outermost layer, the exosphere, into interplanetary space.
The mean temperature on Earth is 60.53 F / 33.5 C / 306.5 K. Temperature extremes have been measured at el Azizia, Libya (136.4 F/ 75.6 C / 348.6 K) and Vostok, Antarctica (-126.9 F / -70.7 C / 202 K ).
Besides affecting weather and the shape of our atmosphere, solar activity gives rise to a dramatic visual phenomenon in our atmosphere. When charged particles from the solar wind become trapped in Earth's magnetic field, they collide with air molecules above the magnetic poles and begin to glow. These glowing molecules are known as the auroras, or the northern and southern lights.
Magnetic fields and gravity
The interaction of the Earth's magnetic field with the solar wind creates a bow shock and magnetosphere. The magnetic field of the Earth and other planets is similar to a dipole field--the type of field created by bar magnets. Due to the pressure of the solar wind, the magnetic field is compressed on the side of Earth facing the Sun. The magneto tail, on the other side of the Earth, streams outward for many thousands of kilometers. Within the magnetosphere are regions of charged particles which are trapped by the Earth's magnetic field. These regions are known as the Van Allen radiation belts.
Earth is the densest planet in the solar system, having a density of 5.52 g/cm^3 (water has a density of 1 g/cm^3). The surface gravity is 9.78 m/s^2.
Orbit and rotation
The Earth has an elliptical orbit with an eccentricity of 0.0167. Its orbital speed is 29.79 km/s.
Moons and/or rings
Earth has one satellite: The Moon. From rocks and materials gathered on the Apollo expeditions, researchers have been able to conclude the Moon is between 4.0 and 4.3 billion years old. Its origins are a mystery. Four theories attempt an explanation: The moon formed near Earth as a separate body; it was torn from Earth; it formed somewhere else and was captured by our planet's gravity, or it was the result of a collision between Earth and an asteroid about the size of Mars. The first few million years of the Moon's existence were so violent that few traces of this period remain. As a molten outer layer gradually cooled and solidified into different layers of rock, the Moon was bombarded by huge asteroids and smaller objects. Some of the asteroids were as large as Rhode Island or Delaware and their collisions with the Moon created basins hundreds of kilometers across. The bombardment tapered off approximately four billion years ago, leaving the lunar highlands covered with huge, overlapping craters and a deep layer of shattered and broken rock. Heat produced by the decay of radioactive elements began to melt the interior of the Moon at depths of about 200 km (125 miles) below the surface. Then, for the next 700 million years (from about 3.8 to 3.1 billion years ago) lava rose from inside the Moon and gradually spread out over the surface, flooding the large impact basins to form the dark areas that Galileo Galilei called maria (meaning seas). As far as researchers can tell, there has been no significant volcanic activity on the Moon since then. The surface of the planet has been altered by atomic particles from the sun and stars, by the rare impacts of large meteorites, and by spacecraft and astronauts.
Mythology
For many ages, the Earth was considered the center of the universe. It was also considered to be a flat disk or a square plate floating on the oceans. The Earth and atmosphere were composed of four elements: water, air, earth, and fire. Some believed in a fifth element: ether or space. The sky was believed to be a shell formed of solid crystalline material, studded with lights, and resting at the edges of the Earth. It was also believed that our world was sustained by animals, folk heroes, and other means. For example, Greeks placed the world on the shoulders of Atlas, the Vedic priests placed it on in pillars, the Hindus on the backs of animals, and the Norsemen in the branches of the world tree.
Discovery and exploration
First American satellite Explorer was launched from Cape Canaveral in Florida on Jan. 31, 1958 and brought to researchers' attention the intense radiation zone surrounding Earth, known as the Van Allen Belts. Between 1964 and 1968, the automated Ranger, Surveyor, and Lunar Orbiter spacecraft were dispatched by NASA to study the Moon . The first human footsteps on another planet were made by the Apollo expedition crews from 1969 to 1972.
