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

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, the goddess of 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 enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinary 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

650 – 2700: Lower mantle

2700 – 2890: D' ' Layer

2890 – 5150: Outer core

5150 – 6378: Inner core

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 belowx10^24 kilograms):

Atmosphere = 0.0000051

Oceans = 0.0014

Crust = 0.0026

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 olivine 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 other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for Earth.

Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates, which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the Sam Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at present) eight major plates:

• North America Plate – North America, western North Atlantic and Greenland
• South America Plate – South America and western South Atlantic
• Antarctic Plate – Antarctica and the "Southern Ocean"
• Eurasian Plate – eastern North Atlantic, Europe and Asia except for India
• African Plate – Africa, eastern South Atlantic and western Indian Ocean
• Indian-Australian Plate – India, Australia, New Zealand and most of Indian Ocean
• Nazca Plate – eastern Pacific Ocean adjacent to South America
• Pacific Plate – most of the Pacific Ocean (and the southern coast of California)

There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries (above).

The Earth's surface is very young. In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old. There is no record of the critical period when life was first getting started.

Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various "sinks" and back again. The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid –21 C to a comfortable =14 C); without it, the oceans would freeze and life as we know it would be impossible.

The interaction of the Earth and the Moon shows the Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year.

Earth has a modest magnetic field produced by electric currents in the core. The interaction of the solar wind, the Earth's magnetic field an the Earth's upper atmosphere causes the auroras. Irregularities in these factors cause the magnetic poles to move relative to the surface; the north magnetic pole is currently located in northern Canada.

The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. The outer belt stretches from 19,00 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.

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