10 Things You Should Know About Earth

Earth is our home planet and home to the only life we know about (so far) in the solar system. It's also 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 of our planet 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 world.

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

The Sun is at the center of the solar system, and the planets move around it in their orbits. The inner planets, also known as the terrestrial worlds, are Mercury, Venus, Earth, and Mars. Earth's average distance from the Sun is 149,597,890 km (92,955,820 miles) or one astronomical unit (AU). 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 12,742 km (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 slightly less because it's slightly "flatter". 

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Earth is the Only Planet Known to Harbor Life

earth and the life it carries
Views of Earth from space show evidence of life on our planet. This one reveals streams of phytoplankton along the California Coast. NASA

All the things we need to survive on this planet are provided by several factors: a  thin layer of atmosphere that separates us from the uninhabitable void of space, a plentiful supply of water, and warmth to nurture life. 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

Earth's atmosphere seen from the ISS
Earth's atmosphere looks very thin when compared to the rest of the planet. The green line is airglow high in the atmosphere, caused by cosmic rays striking the gases up there. This was shot by astronaut Terry Virts from the International Space Station. NASA

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 with 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 part of the reason that Earth is (so far) the only planet known to contain life. The oceans sustain an incredible diversity of life and also act as a "sink" for carbon dioxide in the atmosphere.

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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

Philosopher and mathematician Aristarchus of Samos, who lived in the 3rd Century B.C., figured out how to measure the distances to and sizes of the Sun and the Moon. Based on his work, he correctly concluded that Earth orbited the Sun. However, not everyone agreed with that, particularly the Catholic church, which couldn't stand the idea that Earth wasn't the supreme center of all (since it contained humans).  It wasn't until Nicolaus Copernicus, a Polish astronomer, published ​On the Revolutions of the Celestial Spheres in 1543, that the idea of a Sun-centered solar system was acceptable. However, that doesn't make it the center of the universe. There is NO known cosmic center. 

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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 23.5 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. Actually, most of the other planets have seasons, too, for the same reasons.

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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 (sometimes referred to as Luna) orbits at a distance of 384,000 km (around 238,000 miles). It is the only known natural satellite of our planet. However, there are thousands of small artificial satellites which have been placed in orbit around the Earth. Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with our planet. However, they're not really moons, and the term "companion" perfectly describes them.

Our moon has no atmosphere, but there is water ice in some deep craters. It's the only solar system body that humans have visited.

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Earth Has a Magnetic Field

Earth's magnetic field
Earth's magnetic field is not a visible feature, but is important for life on our planet. NASA/Peter Reid/The University of Edinburgh

Our planet's rapid spin and molten nickel-iron core create a magnetic field. It gets distorted by the solar wind into a tear-drop shape. This magnetic field does not fade off into space but has definite boundaries.

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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Space Travel Allows Us to See the Entire Planet

Earthrise - Apollo 8
Earthrise - Apollo 8. Manned Spacecraft Center

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.