Introductory Astronomy: The Planet Earth

Basic Facts:
Average distance from Sun: 1 AU (1.5 x 10^8 km)
Period of revolution about Sun: 1 year (365.25 days)
Period of rotation: 24 hours
Radius: 6378 km
Mass: 6 x 10^24 kg
Average Density: 5.5 g/cm^3
Magnetic Field: 3.05 x 10^-5 Teslas
Surface features: active surface, plate tectonics, erosion
Atmosphere: 78% N2, 21% O2, 0.9% argon, 0.03% CO2
Temperature: 62 degrees Fahrenheit (average)

The Earth is our home planet. It has one moon (see the Earth-Moon-Sun System page) and is the only planet known to support life (so far!). It has an iron core and an extensive, oxygen based atmosphere. Because we live here, it is important to us to understand the Earth and to know how it formed.

The Earth's development can be divided into four stages; each of the terrestrial planets went through similar stages in their development. The first stage is Differentiation. Differentiation means the separation of material according to density. During this stage, heavy elements sank to the center of the Earth, and lighter elements rose to the top to form the crust. Outgassing occurred as the lightest elements escaped the Earth's gravity.

The second stage is Cratering. During this stage, the surface of the Earth was solid enough for solar debris to leave dents (craters) upon impact. The Earth underwent heavy bombardment during the first 1/2 - 1 billion years of its formation.

Next came Flooding. During this stage, liquids such as lava and water dominated the evolution of the Earth. Liquids appeared as a result of the radioactive decay of elements in the Earth's interior. These decays released energy which heated the Earth, causing lava flows, the formation of oceans in basins, and (when the atmosphere cooled), rain.

Finally, the Earth moved into its current phase: Slow Surface Evolution. This stage comprises such changes as erosion, continental drift, and plate tectonics (the motion of large sections of the Earth's crust).

To learn more about the Earth, click here.