Earth

The third planet from the Sun is the Earth. Even from a distance, Earth appears to be unique in the Solar System. It is the only planet in the Solar System with a partially blue surface, sunlight glinting brightly off its oceans of water. Much of Earth is blanketed in an ever-shifting canopy of clouds. Glimpses of the land masses can be seen between the clouds.

Orbit

The Earth's orbit around the Sun is approximately circular (with an eccentricity of 0.017). The average Earth - Sun distance is 149 million km (93 million miles). This distance value defines the astronomical unit (or AU). Earth's elliptical orbit around the Sun defines a fixed plane in the sky, relative to the stars, which astronomers refer to as the ecliptic plane. The time it takes for the Earth to complete one orbit of the Sun with respect to the stars is called the sidereal year, which is 365.24 days long. The most readily noticed periodic phenomenon on Earth is the coming and going of the seasons. The seasons are caused by the Earth's rotational axis being tilted 23.5°. When traveling about the Sun, the Earth's rotational axis remains fixed toward the same point in the sky (toward the star Polaris). Therefore the average amount of sunlight falling on any one location changes during the year, which affects the average temperature.

Geology

Oceans of water cover more than two thirds of the Earth, making water one of the most abundant compounds. Most of the land area is concentrated in six massive continents. Geologically, the Earth is very active. The Earth has a strong magnetic field, generated by the solid metallic core and turbulent convection currents in the outer liquid core. The complex geology of the Earth is due to plate tectonics. The crust and mantle are divided into ten major continental plates that slowly move about and change. The plate boundaries are marked by long narrow regions where volcanoes and earthquakes are frequent. The force that drives the slow motion of the plates (a few centimeters per year) is the convective upward flow of molten lava that forces some adjacent plates apart. Once the lava has forced its way up it cools and accumulates, forming a range of high mountains deep in the ocean known as an oceanic ridge. The mid-Atlantic ridge is perhaps the best known of these submarine ridges. Since plates are pushed apart in some regions, it means that in other regions they are being pushed together. In some locations new mountains are being pushed up by the collision. A well known example is the Himalayan mountain chain. In other locations, one continental plate is forced to slide over the other. The lower plate is pushed downward where it remelts. This creates deep oceanic trenches, such as the one near the Mariana islands in the Pacific Ocean. Intense volcanism is often found in these regions as well (such as the Mariana islands, the Aleutians, and Japan).

Atmosphere

A particularily significant factor, as far as life is concerned, is the Earth's dense atmosphere. It is comprised of 78% nitrogen, 21% oxygen, 1% argon, and traces of other gases. Water vapor is also present in the air, the amount varies 3% to 0.1% depending upon surface location and season of the year. There is a constant exchange of water between the oceans and the atmosphere via a cycle of evaporation, condensation, and precipitation. Unlike the other planets of the Solar System, Earth's atmosphere has been largely formed and shaped by biological activity. Carbon dioxide is converted into sugar by photosynthetic organisms. Oxygen is a by product of this process. Other creatures use this oxygen for respiration, releasing carbon dioxide back into the air. Apparently the Earth is the only planet in the Solar System with an oxygen-rich atmosphere.

Moons

Moon (Luna)