The Planet Earth

Earth is the third planet from the Sun and the fifth-largest of the nine major planets. The mean distance of the Earth from the Sun is 149,503,000 km (92,897,000 mi). It is the only planet known to support life, although some of the other planets have atmospheres and contain water. The Earth is not a perfect sphere but is slightly pear-shaped. Calculations based on perturbations in the orbits of artificial satellites reveal that the Earth is an imperfect sphere, because the equator bulges, or is distended, by 21 km (13 mi); the North Pole bulges by 10 m (33 ft); and the South Pole is depressed by about 31 m (100 ft). In common with the entire solar system, the Earth is moving through space at the rate of approximately 20.1 km/s or 72,360 km/h (approximately 12.5 mi/s or 45,000 mph) towards the constellation of Hercules. The Milky Way galaxy as a whole, however, is moving towards the constellation Leo at about 600 km/s (375 mi/s). The Earth and its satellite, the Moon, also move together in an elliptical orbit about the Sun. The eccentricity of the orbit is slight, so that the orbit is virtually circular. The approximate circumference of the Earth’s orbit is 938,900,000 km (583,400,000 mi), and the Earth travels along it at a velocity of about 106,000 km/h (66,000 mph). The Earth rotates on its axis once every 23 hr 56 min 4.1 sec. A point on the equator therefore rotates at a rate of a little more than 1,600 km/h (about 1,000 mph), and a point on the Earth at a latitude of 45° North, rotates at about 1,073 km/h (667 mph). An oxygen-rich protective atmosphere, moderate temperatures, abundant water, and a varied chemical composition allow earth to support life, the only planet known to do so. The planet is composed of rock and metal, solid in the outer parts but molten throughout most of the core.
The Earth may be regarded as consisting of five parts: the first, the atmosphere, is gaseous; the second, the hydrosphere, is liquid; the third, fourth, and fifth, the lithosphere, mantle, and core, are largely solid. The atmosphere is the gaseous envelope that surrounds the solid body of the planet. Although it has a thickness of more than 1,100 km (700 mi), about half its mass is concentrated in the lower 5.6 km (3.5 mi). The lithosphere, consisting mainly of the cold, rigid, rocky crust of the Earth, extends to depths of 100 km (60 mi). The hydrosphere is the layer of water that, in the form of the oceans, covers approximately 70.8 per cent of the surface of the Earth. The mantle and core are the heavy interior of the Earth, making up most of the Earth’s mass. Radiometric dating has enabled scientists to arrive at an estimate of 4.65 billion years for the age of the Earth. Although the oldest Earth rocks dated this way are not quite 4 billion years old, meteorites, some of which correlate geologically with the Earth’s core, give dates of about 4.5 billion years, and crystallization of the core and of the parent bodies of meteorites is considered to have occurred at the same time, some 150 million years after the Earth and solar system first formed.