Earth, the third planet from the sun and fifth largest, is the only planet who's name did not come from Greek or Roman mythology. The name 'earth' comes from old English or Germanic. Earth orbits 149,600,000 kilometers from the sun, or 1 Astronomical Unit (AU). Earth has a diameter of 12,756.3 kilometers, and a mass of 5.972e24 kilograms.

Until the sixteenth century, Earth was thought of as the center of the universe, and that all things revolved around it. During the time of Copernicus, people finally realized that Earth is just another planet.

The Earth is divided into several layers, the innermost being the core and the outermost being the crust. The inner core and crust are solid. The crust varies in thickness, being deeper under than continents than the ocean. The Earth is made up mostly of Iron, Oxygen, Silicon, and Magnesium. Most of the Earth's mass is in the core. The earth is the densest major body in the solar system. Most of the terrestrial (inner) planets probably have similar structures and compositions. The Earth, though, has plate tectonics.

Earth's surface is fairly young. Erosion and weathering destroy and recreate most of Earth's surface continually. Because of this, the very early history of Earth has been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are 4 million years old (and very, very rare).

71 percent of Earth's surface is covered with water. Earth is the only planet on which water exists (or can exist) in liquid form. Of course, liquid water is essential for life (as we know it, anyway....). Water is responsible for much erosion and is also critical in keeping the temperatures fairly stable.

Earth's atmosphere is made up of 77% nitrogen, and 21% oxygen. Traces of arbon, carbondioxide, and water account for about 2%. There was probably once more carbondioxide in the atmosphere, but it went into making carbon rocks. A balance of carbondioxide is important because carbondioxide contributes to the Greenhouse effect. The greenhouse effect raises temeratures and keeps them from droping to frigid temperatures like other planets. Scientists estimate that without the greehouse effect, Earth's temperatures would range from -21 degrees Celcius to 41 degrees Celcius.

Earth has one permanent satellite, the Moon. However, thousands of artificial satellites have been placed in orbit around the Earth by humans. Also, Asteroid 3793 has a complicated relationship with Earth, not really as a Moon but a "companion." The interaction of the Earth and the Moon slows Earth's rotation by 2 milliseconds per century. The Moon also creats tides.

Earth has a moderate magnetic field. The interaction of Solar Wind and the magnetic field create the Aurora Borealis. Earth's magnetic field and it's interaction with solar wind also produced the Van Allen Belts.