Diameter: 7,926 miles (12,756 km).

Mass: 1.00 Earth masses

Rotation period: 24 Hour (solar) 23hr, 56 (sidereal).

Inclination of equator to orbit: 23.4 degrees.

Mean orbital speed: 18.5 Miles per second (29.8 km/s)

Mean distance from the sun: 93 million miles (150 million km)

THE STRUCTURE OF EARTH

Earth is the largest terrestrial planet and one of the most geologically active worlds in the solar system. It is the only planet temperatures allow surface water to exist in solid, liquid and gaseous forms, and as we all know it is (thus far) the only planet we know to be supportive of life.

Several factors combine to make Earth the only planet in the solar system hospitable to human life. It has vast oceans of liquid water and a protective atmosphere rich in oxygen. It orbits in a stable, nearly circular path, so that it is never too far from the suns warmth, nor too close. It has one moon, which is large by the standards of the solar system, that acts as a kind of gyroscopic stabilizer, preventing the tilt of the earths spin axis from shifting wildly. Most important of all it lies within a habitable zone in the solar system, not too near or too far from the sun...but just right.

On no other planet but earth does water exist as a liquid, and with salt water oceans covering 71% of the planets surface there is an abundance. The Pacific ocean reaches a depth of 35,800 feet, but the global average is 12,500. As well as supporting marine life forms of great variety and abundance, the ocean plays a vital role in sustaining ground based life. Heat from the sun evaporates seawater, freshwater moisture precipitates from this evaporation, cooling the oceans and bringing rain to the land, where it refreshes lakes, rivers and streams, and eventually flows back to the sea. All the water in the oceans goes through this cycle once in two million years.

Each of us is surrounded by an envelope of air that stretches upward for over 75 miles. Earths atmosphere is almost entirely made up of just two gases, nitrogen (78%) and life sustaining oxygen (21%), traces of carbon dioxide are enough to create a moderate greenhouse effect, a key factor in retaining the warmth of the sun and preventing extreme temperature swings from day to night.  The atmosphere gets thinner and colder the higher you go. The densest and warmest part, known as the troposphere, lies between the surface and an altitude of ten miles. In this zone air warmed by the sun rises from the service to be continually replaced with descending particles of cool air. This circulatory pattern generates most of earths weather. Above the troposphere lies the stratosphere. This region, which extends to approximately 37 miles above the surface, is cold and cloudless. If you go higher still, you would encounter the ozone layer, which absorbs most of the dangerous ultra violet radiation from the sun. At higher altitudes the air rarefies even more and temperatures take a dive. In the upper mesosphere, which lies between 37 and 50 miles above the surface, the temperature drops to –130F. This is the realm of meteors and auroras. Above this elevation the atmosphere is so intangible that orbiting satellites and space crafts undergo little or no atmospheric drag. In the exosphere, the outer most region of the atmosphere 310 miles above the surface, the air density is less than one million millionths that of sea level.

A slice through earth reveals a series of layers much like those of an onion. The crust, which is mostly basalt and granite, is only a few miles thick. Below this is a mantle, composed mainly of metals and igneous rock. This is the largest portion of the planet, extending from the base of the crust to a depth of 1800 miles. Below the mantle is a liquid outer core, of which the material is thought to be composed of iron and nickel as well as quantities of sulphur and other elements, and it is 10 times denser than water. Convective currents in the outer core are believed to produce electrical currents that generate earths magnetic field. Deeper and denser still is the inner core, where enormous pressure keeps the nickel iron solid, even though the temperature is more than 9400F. This planets inner core is 1500 miles in diameter.

Because of the earths abundant supply of internal heat , our world is very geologically active by planetary standards. Heat from the interior causes earthquakes and volcanic eruptions, builds mountains and pushes continents across the surface. According to the theory of plate tectonics the crust and part of the upper mantle form a zone that is broken up into several relatively rigid slabs called plates. Continental drift occurs when currents in the upper mantle cause these plates to float as distinct masses a couple of inches each year. here plates collide, great mountains rise, and where a plate moves over a plume of hot rock in the mantle, volcanoes form. Recycling of rock by plate tectonics, as well a s weathering by wind, rain and ice means that the surface of earth shows little signs of impacts by objects such as comets, meteorites and asteroids.  b

SATELLITE

DISTANCE (KM)

RADIUS (KM)

The Moon

384,400

3,467