Thursday, 24 February 2011


Earth is the third planet from the Sun and the fifth largest: 

      orbit:            149,600,000 km (1.00 AU) from Sun
     diameter:    12,756.3 km
       mass:           5.972e24 kg

Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother Earth).

It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet.

Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful. 

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km): 

                    0-  40         Crust

                   40- 400      Upper mantle

                   400- 650   Transition region
   The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.

Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms):

  atmosphere                           = 0.0000051
  oceans                                    = 0.0014
  crust                                        = 0.026
  mantle                                     = 4.043
  outer core                              = 1.835
  inner core                              = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is:

  34.6%             Iron
  29.5%             Oxygen
  15.2%            Silicon
  12.7%            Magnesium
  2.4%              Nickel
  1.9%              Sulfur          
  0.05%           Titanium

The Earth is the densest major body in the solar system.

                                       Distance              Radius                   Mass
Satellite                        (000 km)                (km)                      (kg)
---------                            --------                     ------                        -------
Moon                            384                         1738                     7.35e22 

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