The planet Earth

Earth is neither a very large nor a very small planet. But it is the home planet, the site of all life, and the place from which the entire universe is viewed.

Astronomers and geologists study other worlds to learn about Earth and its planetary family. They now believe that the planets condensed from spinning rings of debris left over from the Sun’s birth.

The shape of Earth

Earth is not in a perfect spherical shape. It is flattened at the poles. Its diameter is 7,900 miles (12,700 kilometers) from pole to pole. It is 7,920 miles (12,750 kilometers) around at equator.

Geodesy is the study of Earth’s shape. Modern geodetic scientists know Earth’s shape from photographs taken from space. But people had long-ago reached that conclusion. For example, people noticed that distant ships seemed to drop below the horizon. Early astronomers watched the movement of a lunar eclipse: Earth cast a curved shadow on the Moon. So Earth was round.

Mass and density

The mass of an object tells us how much matter is in it. In 1735, mathematician Pierre Bouguer estimated Earth’s mass. Today scientists know that Earth weighs 6.59 x 1021 tons (5.98 x 1021 metric tons). To figure its density, they divide Earth’s mass by its volume. The planet’s air, water, and rock would weigh about 5.5 times the same amount of water. This makes Earth the densest of all planets.

Gravity and magnetism

Gravity is the force that pulls everything toward Earth’s center. Earth also acts as a huge magnet. It has north and south poles. (The magnetic poles are not in the same place as the geographic poles.) A compass works because of Earth’s magnetism.

Three parts of Earth

Earth is made up of air, water, and solid ground. Scientists call these the atmosphere, hydrosphere, and lithosphere. These terms come from Greek roots. Atmos means vapor. Hydro means water. Lithos means stone.

The Atmosphere
Earth is surrounded by layers of gases called the atmosphere. This shields the planet from deadly radiation and meteors. The atmosphere also acts as a blanket to keep Earth’s temperature stable. This blanket is made of 21 percent oxygen, 78 percent nitrogen and 1 percent other gases. It also contains dust and water vapor.

The Hydropshere
When viewed from space, Earth might be called the blue planet. Three-quarters of its surface is covered with water. This is the hydrosphere. Earth is the only planet known to have liquid water. The oceans, lakes, and rivers are filled with plants and animals. The hydrosphere provides food, fertilizers, and industrial materials. The oceans produce salt and many other important chemicals.

The Lithosphere
The lithosphere is the solid part of Earth. It has a solid outer mantle about 1,740 miles (2,800 kilometers) thick. The crust is made up of rock and soil and constitutes the top 25 miles (40 kilometers) of the mantle.

Geophysicists use seismic waves to learn about the interior. Below the mantle lies the core. It is about 4,400 miles (7,100 kilometers) across. The core has a solid inner core and a molten outer core.

Age of Earth

Fossils convinced scientists that Earth was very old. Scientists now measure Earth’s age from radioactive elements. By knowing how long these take to decay, geologists can calculate the age of the rocks or minerals they are in. Several radioactive elements, or isotopes, are commonly used. The most famous of these is carbon-14.

Scientists have learned that the oldest known rocks were formed about 4.2 billion years ago. They believe Earth itself to be about 4.6 billion years old.

Earth’s rotation

People in ancient times believed that Earth stood still. They thought that the Sun, Moon, and stars moved around us. It is now known that Earth turns on its axis. The planet spins once every 23 hours, 56 minutes, 4.09 seconds. In 1851 the French scientist Jean-Bernard-Léon Foucault proved this with a pendulum. He attached a heavy, pointed weight to a long rope that was suspended from a tall ceiling over a layer of sand. As the pendulum swung back and forth, the point marked its motion in the sand on the floor. The markings followed different directions. The Foucault pendulum showed that Earth must be turning.

It is the earth’s rotation that causes day and night. It makes air currents turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The name of this phenomenon is Coriolis Effect.

Earth’s revolution

As Earth rotates, it also revolves around the Sun. The journey takes 365 days, 6 hours, 9 minutes, 10 seconds. This is called a sidereal (star) year. Earth orbits the Sun in a squashed circle called an ellipse. So Earth’s distance from the Sun changes slightly during the year. Its closest position is called perihelion, which occurs in early January. Its farthest position is called aphelion, which occurs in early July. At perihelion, Earth lies about 3 million miles (4.8 million kilometers) nearer to the Sun than at aphelion.

Earth’s axis is tilted about 23.5 degrees from its orbit. Each hemisphere tilts toward the Sun during part of its orbit. This is called summer. Winter occurs when the hemisphere is tilted away from the Sun. Earth speeds along its orbit at about 19 miles (30 kilometers) per second. It moves most rapidly at perihelion. Earth also follows the Sun’s motion. The Sun pulls the solar system on its own journey through the heavens. It revolves around the center of the Milky Way galaxy every 200 million years. This is called a cosmic year. The speed along this journey is about 143 miles (230 kilometers) per second.

Precession

Like a spinning top at a slant, Earth wobbles on its axis. This is known as precession. Precession is caused mostly by the pull of the Moon‘s gravity as it orbits Earth. It takes about 25,800 years to complete one wobble.

As Earth wobbles, its axis points toward different places in space. Today the axis aims toward Polaris, the North Star. This changes over many centuries. When the Egyptians were building pyramids, the star Thuban was the North Star. By A.D. 14,000, Vega will be the North Star. Polaris will return to that spot after one complete wobble.