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It is known that the history of the Earth spans a period of more than 4 billion years, although how the Earth formed is still a matter of speculation. Some experts suggest that it originated from condensing gas and dust. Others regard it as having developed from a cloud of meteorites and meteoric dust revolving around the Sun. Gravitational forces caused the particles to come together, or accrete, with heavier particles gravitating to the center and lighter ones collecting outside. This contraction of the mass generated heat, which caused melting and the gradual development of three layers: core, mantle, and crust. Geologic history began when the crust first formed, about 4.6 billion years ago, as determined by radiometric age-dating of the oldest rocks and of meteorites. This event must have been preceded by a considerably longer period of astronomical history during which the solar system and, before that, the universe were created. Geologic history is divided into four main eras. The first, the Precambrian Era, began 4.6 billion years ago and includes the first 4 billion years of geologic history, about 85 percent of all geologic time. The second, the Paleozoic Era, lasted from 600 to 225 million years ago, roughly 10 percent of geologic time. The third, the Mesozoic Era, lasted from 225 to 65 million years ago, about 4 percent of geologic time. The fourth, the Cenozoic Era, embraces the last 65 million years, only about 1.5 percent of geologic time. Being the most recent, the Cenozoic is best known; the landscapes and life forms of modern times reached their present expression during this era. The Earth is a roughly spherical body that is somewhat flattened at its poles. Its equatorial circumference and diameter are, respectively, 40,075 km (24,902 mi.) and 12,756 km (7,926 mi.). Its polar circumference and diameter are 40,008 km (24,860 mi.) and 12,714 km (7,900 mi.). The shape of the Earth was considered to be a sphere by ancient Greeks such as Pythagoras and Aristotle. The first accurate measurement of the Earth's size was made in the 3d century BC by Eratosthenes of Cyrene. He knew that at the summer Solstice, the first day of summer, the noon Sun was reflected in a well dug at Syene (modern Aswan). This fact indicated that Syene was approximately on a direct line between the Sun and the Earth's center. Simultaneously, Eratosthenes determined that the Sun as observed at Alexandria (which he assumed to be on the same meridian as Syene) was south of the vertical by about 1/50 of a full circle. Because the rays of the distant Sun striking Syene and Alexandria can be assumed parallel, the angle of shadow at Alexandria is equal to the angle between there and Syene, as measured from the center of the Earth. The Earth's circumference would thus be 50 times the north-south distance between the sites. No way existed then to determine this distance accurately, but Eratosthenes' value was correct probably to within 15 percent. Later determinations of size were based on observations of stars to give angular differences in latitude. No substantial improvement was made, however, until 1615, when the Dutch mathematician and physicist Willebrord van Roijen Snell used triangulation to measure the linear distance between two stations. His value of the circumference was correct to within 5 percent. In 1669 a French astronomer, Jean Picard, used the telescope for both the astronomical and surveying operations. Sir Isaac Newton used Picard's determination of the Earth's size to confirm his law of gravitation (1687). These events opened a new phase of geodesy, the science that deals with the size and shape of the Earth. Newton found theoretically that under combined gravitational and centrifugal forces, the Earth's shape should be an oblate spheroid, formed by an ellipse of revolution. The polar axis would thus be shorter than the equatorial axis by about 1 part in 200. Measurements by Jean-Dominique CASSINI and his son Jacques, however, gave the opposite result--a prolate Earth, with the polar axis longer than the equatorial axis. To resolve the ensuing controversy, the French Academy of Sciences sent expeditions to Peru (1735) and Lapland (1736). The findings of these expeditions confirmed that the shape was an oblate spheroid. The difference in axes is about 1 part in 298. When the French created the Metric System, they defined the meter to be 1/10,000,000 of the quadrant of the meridian from North Pole to equator. Modern geodetic results make the quadrant equal to 10,002,000 m (32,814,960 ft); the meter, however, has since been redefined in terms of the speed of light. The planet Earth exhibits a number of different motions. The most familiar of these are its rotation, or spinning on an axis, and its revolution, or passage around the Sun. The time the Earth takes to turn once on its axis is called one DAY, and the time it takes to complete one orbit of the Sun is called one year. The Earth's orbit, in a plane called the ecliptic, is a nearly circular ellipse, with the Sun at one focus. The Earth is nearest the Sun (perihelion) about January 3 and farthest (aphelion) about July 4. The orbital speed averages 30 km/sec (18.6 mi./sec). It is greatest at perihelion and least at aphelion. The spin axis intersects the Earth's surface at the North Pole and South Pole. The great circle 90 degrees from the poles is the Equator. It intersects the ecliptic at an angle of about 23.5 degrees, at points called the vernal and autumnal Equinoxes. The Sun apparently passes through these points within a day or two of March 20 and September 23, respectively. The Sun is most northerly about June 21 and most southerly about December 22; in the Northern Hemisphere, these events are the summer and winter solstices, respectively. Because of its rotation, the Earth has an equatorial bulge, upon which the gravitational attractions of the Sun and Moon act. The Earth, as a result, moves like a top. The spin axis keeps a nearly constant inclination to the ecliptic, but describes a cone in space in a period of about 25,800 years. This motion is known as the precession of the equinoxes. Superimposed on this motion are small periodic effects (caused by the Moon's attraction) called nutation. The principal term has amplitude 9'' (0.0025 deg.) and a period of 18.6 years. Precession and nutation are motions of the spin axis in space. The Earth itself moves (aside from pure rotation) with respect to the spin axis because of unequal mass distribution about the axis. Consequently, the geographic poles move around the Earth's shortest axis, the axis of figure. This motion, called polar motion, has two components with periods of 12 and 14.2 months; these come in and out of phase, and therefore the pole spirals in and out. The maximum distance from the mean position, called the mean pole, is about 12 m (40 ft). The mean pole has also been observed to move about 8 m (26 ft) since 1900, towards about 70 deg W. If this drift continues for long intervals, it would change latitudes considerably and affect climates in different parts of the world. The astronomical data provide no basis, however, for assuming that large changes have occurred or will occur. The Earth, together with the Sun and the rest of the solar system, moves about 19 km/sec (12 mi/sec) with respect to neighboring stars. It also partakes of the Sun's motion as a member of our galaxy. The Sun revolves about the galactic center at a speed of about 250 km/sec (155 mi./sec). Earth tides, also known as body tides, are similar to TIDES in the ocean. Responding ever so slightly to the gravitational attraction of the Moon, the solid, almost perfectly rigid Earth bulges about 7 to 15 cm (3 to 6 in) as the planet rotates. That is, the Moon attracts the solid Earth as well as the oceans and the atmosphere, creating a high tide on its Moonward side, where the gravitational attraction is greatest, as well as on the opposite side of the planet, where the attraction is least. View a Voyager 1 image of Earth and it's moon.
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