Earth

Orbital Characteristics

The Earth spins on an imaginary line called an axis that runs from the north pole to the south pole, while also orbiting the sun. It takes Earth 24 hours to complete a rotation on its axis, and roughly 365 days to complete an orbit around the sun.

The Earth's axis of rotation is tilted in relation to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. This means the northern and southern hemispheres will sometimes point toward or away from the sun depending on the time of year, varying the amount of light they receive and causing the seasons.

Earth's orbit is not a perfect circle, but is rather an oval-shaped ellipse, like that of the orbits of all the other planets. Earth is a bit closer to the sun in early January and farther away in July, although this variation has amuch smaller effect than the heating and cooling caused by the tilt of Earth's axis. Earth happens to lie within the so-called "Goldilocks zone" around its star, where temperatures are just right to maintain liquid water on its surface.

History: Earth's formation and evolution

Earth probably formed at roughly the same time as the sun and other planets some 4.6 billion years ago, when the solar system coalesced from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun. Other particles within the disk collided and stuck together to form ever-larger bodies, including the Earth. The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost worlds, rendering Earth and its siblings into small, rocky planets.

Scientists think Earth started off as a waterless mass of rock. Radioactive materials in the rock and increasing pressure deep within the Earth generated enough heat to melt Earth's interior, causing some chemicals to rise to the surface and form water, while others became the gases of the atmosphere. Recent evidence suggests that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape.

The history of Earth is divided into four eons — starting with the earliest, these are the Hadean, Archean, Proterozoic, and Phanerozoic. The first three eons, which together lasted nearly 4 billion years, are together known as the Precambrian. Evidence for life has bee found in the Archaean about 3.8 billion years ago, but life did not become abundant until the Phanerozoic.

The Phanerozoic is divided into three eras — starting with the earliest, these are the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic Era saw the development of many kinds of animals and plants in the seas and on land, the Mesozoic Era was the age of dinosaurs, and the Cenozoic Era we are in currently is the age of mammals.

Most of the fossils seen in Paleozoic rocks are invertebrate animals lacking backbones, such as corals, mollusks and trilobites. Fish are first found about 450 million years ago, while amphibians appear roughly 380 million years ago. By 300 million years ago, large forests and swamps covered the land, and the earliest fossils of reptiles appear during this period as well.

The Mesozoic saw the ascendence of dinosaurs, although mammals also appear in the fossil record about 200 million years ago. During this time, flowering plants became the dominant plant group and continue to be so today.

The Cenozoic began about 65 million years ago with the end of the age of dinosaurs, which many scientists think was caused by a cosmic impact. Mammals survived to become the dominant land animals of today.

Composition & Structure

• Atmosphere

The atmosphere is roughly 78 percent nitrogen, 21 percent oxygen, with trace amounts of water, argon, carbon dioxide and other gases. Nowhere else in the solar system can one find an atmosphere loaded with free oxygen, which ultimately proved vital to one of the other unique features of Earth — us.

Air surrounds Earth and becomes thinner farther from the surface. Roughly 100 miles (160 kilometers) above Earth, the air is so thin that satellites can zip through with little resistance. Still, traces of atmosphere can be found as high as 370 miles (600 kilometers) above the surface.

The lowest layer of the atmosphere is known as the troposphere, which is constantly in motion, causing the weather. Sunlight heats the Earth's surface, causing warm air to rise. This air ultimately expands and cools as air pressure decreases, and because this cool air is denser than its surroundings, it then sinks, only to get warmed by the Earth once again.

Above the troposphere, some 30 miles (48 kilometers) above the Earth's surface, is the stratosphere. The still air of the stratosphere contains the ozone layer, which was created when ultraviolet light caused trios of oxygen atoms to bind together into ozone molecules. Ozone prevents most of the sun's harmful ultraviolet radiation from reaching Earth's surface.

Water vapor, carbon dioxide and other gases in the atmosphere trap heat from the sun, warming Earth. Without this so-called "greenhouse effect," Earth would probably be too cold for life to exist, although a runaway greenhouse effect led to the hellish conditions now seen on Venus.

Earth-orbiting satellites have shown that the upper atmosphere actually expands during the day and contracts at night due to heating and cooling.

• Magnetic field

The Earth's magnetic field is generated by currents flowing in Earth's outer core. The magnetic poles are always on the move, with the magnetic north pole recently accelerating its northward motion to 24 miles (40 km) annually, likely exiting North America and reaching Siberia in a few decades.

Earth's magnetic field is changing in other ways, too — globally, the magnetic field has weakened 10 percent since the 19th century. These changes are mild compared to what Earth's magnetic field has done in the past — sometimes the field completely flips, with the north and the south poles swapping places.

When charged particles from the sun get trapped in Earth's magnetic field, they smash into air molecules above the magnetic poles, causing them to glow, a phenomenon known as the aurorae, the northern and southern lights.

• Chemical composition

Oxygen is the most abundant element in rocks in Earth's crust, composing roughly 47 percent of the weight of all rock. The second most abundant element is silicon at 27 percent, followed by aluminum at 8 percent, iron at 5 percent, calcium at 4 percent, and sodium, potassium, and magnesium at about 2 percent each.

The Earth's core consists mostly of iron and nickel and potentially smaller amounts of lighter elements such as sulfur and oxygen. The mantle is made of iron and magnesium-rich silicate rocks. (The combination of silicon and oxygen is known as silica, and minerals that contain silica are known as silicate minerals.)

• Internal structure

The Earth's core is about 4,400 miles (7,100 kilometers) wide, slightly larger than half the Earth's diameter and roughly the size of Mars. The outermost 1,400 miles (2,250 kilometers) of the core are liquid, while the inner core — about four-fifths as big as Earth's moon at some 1,600 miles (2,600 kilometers) in diameter — is solid.

Above the core is Earth's mantle, which is about 1,800 miles (2,900 kilometers) thick. The mantle is not completely stiff, but can flow slowly. Earth's crust floats on the mantle much as a wood floats on water, and the slow motion of rock in the mantle shuffles continents around and causes earthquakes, volcanoes, and the formation of mountain ranges.

Above the mantle, Earth has two kinds of crust. The dry land of the continents consists mostly of granite and other light silicate minerals, while the ocean floors are made up mostly of a dark, dense volcanic rock called basalt. Continental crust averages some 25 miles (40 kilometers) thick, although it can be thinner or thicker in some areas. Oceanic crust is usually only about 5 miles (8 kilometers) thick. Water fills in low areas of the basalt crust to form the world's oceans. Earth has more than enough water to completely fill the ocean basins, and the rest of it spreads onto edges of the continents, areas known as the continental shelf.

Earth gets warmer toward its core. At the bottom of the continental crust, temperatures reach about 1,800 degrees F (1,000 degrees C), increasing about 3 degrees F per mile (1 degrees C per kilometer) below the crust. Geologists think the temperature of Earth's outer core is about 6,700 to 7,800 degrees F (3,700 to 4,300 degrees C), and the inner core may reach 12,600 degrees F (7,000 degrees C), hotter than the surface of the sun. Only the enormous pressures found at the super-hot inner core keep it solid.

Orbit & Rotation

Average Distance from the Sun

English: 92,955,820 miles      

Metric: 149,597,890 km

Perihelion (closest)

English: 91,400,000 miles

Metric: 147,100,000 km

Aphelion (farthest)

English: 94,500,000 miles                  

Metric: 152,100,000 km

Average Length of Solar Day

24 hours

Length of Year

365.24 Earth days

Equatorial Inclination to Orbit

23.45 degrees

(Source: NASA.)

Moon

Earth's moon is 2,159 miles (3,474 kilometers) wide, about one-fourth of Earth's diameter. Earth has one moon, while Mercury and Venus have none and all the other planets in our solar system have two or more.

The leading explanation for how the moon formed was that a giant impact knocked off the raw ingredients for the moon off the primitive molten Earth and into orbit. Scientists have suggested the impactor was roughly 10 percent the mass of Earth, about the size of Mars.

Species Overview

Earth is the only planet in the universe known to possess life. There are several million known species of life, ranging from the bottom of the deepest ocean to a few miles into the atmosphere, and scientists think far more remain to be discovered. Scientists figure there are between 5 million an 100 million species on Earth, but science has only identified about 2 million of them.