EARTH

DIAMETER: 7,926 miles
ROTATION: 23 hours, 56 minutes
ORBIT AROUND SUN: 365.24 days

Earth is the third planet from the Sun. Like Mercury and Venus it consists mostly of rock and metal. It is the fifth largest planet and the densest. Scientists believe that this high density is due to a tremendous loss of light weight material due to a major collision with something else, this may explain the existence of the moon, Earth's only natural satellite. Not only is Earth the densest but it is also the only planet to have water on the surface. More than 70% of the Earth's surface is covered by water. Although 97% of that water is the salty ocean waters, only 3% of the Earth's water is freshwater, the kind we drink. Earth's core is made up of iron and nickel. The very center of the core has solidified and spins freely within the molten metal of the outer core. The core is surrounded by a mantle of molten rock and a thin outer crust which is thicker beneath the continents than below the oceans.Earth's atmosphere consists of a mixture of nitrogen (78%), oxygen (21%), argon and other gases (1%). The atmosphere gets thinner the further away from the planet we get. Some of the oxygen in the atmosphere changed into the ozone layer over time. The ozone layer filters out the Sun's harmful ultraviolet rays. These ultraviolet rays heat the Earth and provide the light source which is vital to our existence. Too many of these rays can be harmful which is why the ozone layer is so important. Although too much ozone is also very harmful, trapping too much surface heat. This trapping of heat is called the greenhouse effect. Not only does the atmosphere protect us from the ultraviolet rays of the Sun, it also protects us from meteors by burning them up before they get to the surface.Not only does the Sun provide us with heat and light but it also gives rise to dramatic visual phenomenon in our atmosphere. When charged particles from the solar wind become trapped in Earth's magnetic field, they collide with air molecules above our planet's magnetic poles. These air molecules then begin to glow and are known as the auroras, or the Northern and Southern lights.Another aspect that makes Earth different from the other planets is that it is dynamic. It is in a constant state of change. The Earth's surface is separated into moving caps or plates. For example the North American continent continues to move west over the Pacific Ocean basin, roughly at a rate equal to the growth or our fingernails. We are made aware of this movement when it is interrupted by earthquakes. Earth quakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate. These movements are known as plate tectonics.