10 Things You Should Know About Earth

By Nick Greene, About.com


Earth, our home planet, is the only planet whose name is not derived from Greek/Roman mythology. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil, while the Greek goddes was Gaia, terra mater - Mother Earth. The name we use today, Earth, comes from Old English and Germanic.

1. Earth is the third planet from the Sun
"Earth Globe"NASA Glenn Research Center (NASA-GRC)
The Earth's average distance from the Sun is 149,597,890 km (92,955,820 miles) or one astronomical unit (AU). Located between Venus and Mars, some people have called it the "third rock from the sun."
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2. Earth is the fifth largest planet
"Full Earth "NASA
The diameter of the earth at the equator is about 7926 miles, but that's not the whole story. Because the earth is not a perfect sphere but is slightly flattened at the poles, the diameter of the earth measured around the North Pole and the South Pole is about 7899 miles.

3. Earth is the only planet known to harbor life
"SeaWinds Global Coverage with Detail of Hurricane Floyd"NASA/JPLQuikScat
All of the things we need to survive are provided under a thin layer of atmosphere that separates us from the uninhabitable void of space. Earth is made up of complex, interactive systems that are often unpredictable. Air, water, land, and life - including humans - combine forces to create a constantly changing world that we are striving to understand.

4. Earth is mostly covered in water
"Earth - Pacific Ocean"NASA/JPL
While the word earth is often used synonymously with dirt, seventy-one percent of the its surface is covered with water. It is the only planet where it exists in its liquid form on the surface. This is probably part of the reason that the Earth is the only planet known to contain life.

5. Early philosophy had the Earth as the center of the universe
"World Globes, Shaded Relief and Colored Height"NASA/JPL/NIMA
Although Aristarchus of Samos, in the 3rd Century B.C., figured out how to measure the distances to and sizes of the Sun and the Moon, and concluded that the Earth orbited the Sun, this view didn't attract followers until Nicolaus Copernicus, a Polish astronomer, published "On the Revolutions of the Celestial Spheres" in 1543.

6. Earth has four distinct seasons
"Global Images of Earth"NASA/JPL/Johns Hopkins University
This is a result of a result of Earth's axis of rotation being tilted more than 23 degrees. Seasons changes as the tilt of Earth's axis changes during it's revolution around the Sun.

7. Earth has an atmosphere that sustains life
"Pacific Ocean Surface Winds from QuikScat"NASA/JPL/QuikScat
Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. This atmosphere affects Earth's long-term climate and short-term local weather; shields us from nearly all harmful radiation coming from the Sun; and protects us from meteors as well - most of which burn up before they can strike the surface.

8. Earth has one natural satellite
"Pictures of the Moon - Moon Color Composite"JPL
Earth's Moon (called Luna) orbits at a distance of 384,000km, with a radius of 1738KM and a mass of 7.32e22kg. However, there are thousands of small artificial satellites which have been placed in orbit around the Earth. Also, asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they're not really moons, the term "companion" is being used.

Because of its size and rocky composition, the moon has also been called a terrestrial planet along with Mercury, Venus, Earth, and Mars. It has no atmosphere, but there is water ice in some deep craters. The moon is the only extra-planetary body that a human has visited.

9. Earth has a magnetic field
"South Polar Projection of Earth"NASA/JPL
Our planet's rapid spin and molten nickel-iron core give rise to a magnetic field, which the solar wind distorts into a teardrop shape. The magnetic field does not fade off into space, but has definite boundaries. Just like the field around a magnet, ours is also polarized. 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 aurorae, or the Northern and Southern Lights.

10. Our close proximity prevents us from seeing Earth in its entirety
"Earthrise - Apollo 8"Manned Spacecraft Center
To completely view our own planet, we must leave its surface and journey into space. From the vantage point of space we are able to observe our planet globally, as we do other planets, using similar sensitive instruments to understand the delicate balance among its oceans, air, land, and life. Viewing Earth from the unique perspective of space provides the opportunity to see Earth as a whole. Scientists around the world have discovered many things about our planet by working together and sharing their findings.