Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar Systems four terrestrial planets. It is sometimes referred to as theworld, the Blue Planet,or by its Latin name, Terra
Shape and Size of the EarthEratosthenes became head librarian at the RoyalLibrary around 235 BC. There, he made the firstaccurate measurement of the earthscircumference. His value (based on the altitude ofthe noontime sun as seen from Alexandria andSyrene on the first day of summer) wasapproximately 25,000 miles. The actual equatorialcircumference is 24,902 miles.
What is the diameter of the earth?The diameter ofthe earth at theequator is7,926.41 miles(12,756.32kilometers).
SHAPE OF EARTHNewton computed that the Earthsshape should be an oblatespheroid, a solid formed when anellipse is rotated about its axisExpeditions to Peru in 1735 and toLapland in 1736 confirmed thistheory.
RotationThe Earth spins on its axis fromWest to East (counter-clockwise). It takes the Earth 23hours, 56 minutes, and 4.09seconds to complete one fullturn. Day and night areproduced by the rotation of theEarth. The speed of rotation atany point upon the equator is atthe rate of approximately 1,038miles per hour, decreasing tozero
RevolutionWhile the Earth is spinning on its axis,it is revolving around the Sun in acounter-clockwise direction. It takesthe Earth one full year to complete onefull revolution around the Sun. Thispath is known as the Earths orbit. It isvery near a circle. The mean distanceof the Earth from the Sun is about 93milling miles and the distance variesby 3 million miles, forming a slightlyoval path.
Effects of Earths MotionSolsticesThe solstices are days when theSun reaches its farthestnorthern and southerndeclinations. The winter solsticeoccurs on December 21 or 22and marks the beginning ofwinter (this is the shortest dayof the year). The summersolstice occurs on June 21 andmarks the beginning of summer(this is the longest day of theyear).
EquinoxesEquinoxes are days in which day and night are ofequal duration. The two yearly equinoxes occurwhen the Sun crosses the celestial equator.The vernal equinox occurs in late March (this is thebeginning of spring in the Northern Hemisphereand the beginning of fall in the SouthernHemisphere); the autumnal equinox occurs in lateSeptember (this is the beginning of fall in theNorthern Hemisphere and the beginning of springin the Southern Hemisphere).
INTERNAL SRUCTURE OF EARTHThe Crust Within the crust, intricate patterns arecreated when rocks are redistributed crustanda deposited in layers through thegeologic processes of eruption andintrusion of lava, erosion, andconsolidation of rock particles, andsolidification and recrystallization ofporous rock.
The Mantle Our knowledge of the upper mantle, including the tectonic plates, is derived from analyses of earthquake waves (see figure for paths); heat flow, magnetic, and gravity studies; and laboratory experiments on rocks and minerals. Between 100 and 200 kilometers below the Earths surface, the temperature of the rock is near the melting point; molten rock erupted by some volcanoes originates in this region of the mantle
The Core (Outer Core)The outer core of the Earth isa liquid layer about 2,266kilometers thick composed ofiron and nickel which liesabove the Earths solid innercore and below its mantle. Itsouter boundary lies 2,890 km(1,800 mi) beneath the Earthssurface. The transitionbetween the inner core andouter core is locatedapproximately 5,150 kmbeneath the Earths surface.
Inner core The inner core of the Earth, its innermost hottest part as detected by seismological studies, is a primarily solid ball about 1,220 km (760 mi) in radius, or about 70% that of the Moon. It is believed to consist of an iron–nickel alloy, and may have a temperature similar to the Suns surface, approximately 5778 K (5505 °C).
Earths AtmosphereThe atmosphere of Earth is alayer of gases surrounding theplanet Earth that is retained byEarths gravity. The atmosphereprotects life on Earth byabsorbing ultraviolet solarradiation, warming the surfacethrough heat retention(greenhouse effect), andreducing temperature extremesbetween day and night (thediurnal temperature variation).