10 Things You Should Know About Earth

What do you want to know about Earth?  It's our home planet and the only world with a name not derived from Greek/Roman mythology. To the Romans, the goddess of Earth was Tellus, meaning "the fertile soil", while the Greek goddess was Gaia, or Mother Earth. The name we use today, Earth, comes from Old English and Germanic. Here are ten things you should know about our planet.

Revised and updated by Carolyn Collins Petersen.

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Earth is the third planet from the Sun.

Distant Earth and Moon
Earth and the Moon as seen from a passing spacecraft. Everyone you know is on (or orbiting) this world. NASA

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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Earth is the fifth-largest planet.

Full Earth
Full Earth. NASA

The diameter of our planet at the equator is about 7,926 miles, but that's not the whole story. Earth is not a perfect sphere but is slightly flattened at the poles. That means Earth's diameter measured around the North Pole and the South Pole is about 7,899 miles.

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Earth is the only planet known to harbor life.

SeaWinds Global Coverage with Detail of Hurricane Floyd
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.
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Earth has an atmosphere that sustains life.

Pacific Ocean Surface Winds from QuikScat
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.

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Earth is mostly covered in water.

Earth - Pacific Ocean
Earth - Pacific Ocean. NASA/JPL

While the word "Earth" is often used as a synonym for "dirt", seventy-one percent of our planet's 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 Earth is (so far) the only planet known to contain life.

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Early philosophers thought Earth was the center of the universe.

World Globes, Shaded Relief and Colored Height
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 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.

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Earth has four distinct seasons.

Global Images of Earth
Global Images of Earth. NASA/JPL/Johns Hopkins University

The reasons for the seasons stem from the fact that Earth's axis of rotation is tilted more than 23 degrees. Seasons changes as the tilt of the planet's axis changes during its revolution around the Sun and different hemispheres get different amounts of sunlight. 

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Earth has one natural satellite

Pictures of the Moon - Moon Color Composite
Pictures of the Moon - Moon Color Composite. JPL

Earth's Moon (called Luna) orbits at a distance of 384,000 kilometers. 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 our planet. However, they're not really moons, the term "companion" is being used to describe them.

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 solar system body that humans have visited.

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Earth has a magnetic field

South Polar Projection of Earth
South Polar Projection of Earth. NASA/JPL

Our planet's rapid spin and molten nickel-iron core create its 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.

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Our close proximity prevents us from seeing Earth in its entirety

Earthrise - Apollo 8
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.