Earth

Earth, the third planet from the sun (roughly 92 million miles distant), is the fifth-largest planet in the Milky Way galaxy and the largest of the four terrestrial planets—that is, the planets whose surfaces are rocky rather than gaseous. (The other three are Mars, Mercury, and Venus.) Earth is the only known planet with an oxygen atmosphere, which is produced by its plants and trees (oxygen makes up roughly one-fifth of its atmosphere). That, coupled with an abundant water supply (Earth is known as the Blue Planet), has allowed for the development of millions of species of life over more than 4.5 billion years. Many species, of course, are now extinct, from catastrophic changes in atmosphere or climate conditions across millions of years or from predatory and/or recreational hunting; and zoologists believe that of the living species today, only about one-third have actually been cataloged.

Earth’s diameter is just over 8,000 miles. Until late in the sixteenth century, Earth was believed to be flat like a dinner plate. Although it appears round in popular representations, it is in fact an oblate spheroid, which means it has a kind of flattened top at the two poles (that is, the ice-encrusted top and bottom of the planet) and bulges across its middle (the imaginary line known as the equator) due to its relatively high velocity of rotational spin. More than 70 percent of Earth’s surface is composed of water, and the rest is land masses, continents, or islands, themselves networked by water systems of lakes and rivers.

Structurally, Earth is made up of a layered lithosphere, the top of which is the hard solid crust fragmented into floating tectonic plates, which form the planet’s easily recognized land masses with their wide range of topographical features. The ocean bottom is thought to be volcanic and mountainous (only about one-quarter of it has been explored and charted) and made up largely of basalt. Beneath that outer crust is a cold, rigid layer roughly 1,800 miles thick called the mantle, made up of iron and magnesium. Beneath the mantle is a liquid outer core and then a massive (more than 4,800 miles across) solid inner core made up of iron and nickel. The inner core is responsible for Earth’s magnetic field, which shields the planet’s surface from much of the damaging radiation from the sun. Geologists believe temperatures at Earth’s core can reach more than 12,000 degrees Fahrenheit.

Earth rotates on an imaginary line that runs from pole to pole called its axis. It takes 23.9 hours (or roughly a single day) to complete a rotation. In turn, Earth itself follows an elliptical orbital path around the sun during which it rotates 365.26 times, or roughly one year. Because Earth is tilted 23.4 degrees toward or away from the sun, that rotational duration influences surface temperatures, creating the seasons. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere.

Atmosphere and Climate

Earth’s atmosphere is made up predominantly of nitrogen (78 percent) and oxygen (21 percent) along with trace amounts of gases including argon and carbon dioxide, a fortuitous combination that permits the florescence of carbon-based life on the planet. Earth is in turn wrapped by insulating layers that both shield and redirect the sun’s damaging ultraviolet (UV) rays. Immediately above the planet’s surface is the fluid and very active troposphere—it absorbs, deflects, and distributes much of the sun’s UV light and creates the familiar weather systems that are dominated and controlled by three principal energy fields: the trade winds, the westerlies, and ocean currents. About thirty miles above Earth’s surface is the stratosphere, known as the ozone layer, a thin layer of insulation responsible for deflecting most of the most harmful UV rays that constantly bombard Earth. The water vapor in the stratosphere traps heat from the sun and maintains a surface temperature that permits life.

Earth’s climate is divided generally into four broad types: polar, temperate (where more than two thirds of the world’s seven billion people live), subtropical, and tropical (or equatorial). The steadily escalating emission of carbon dioxide gases from the surface—largely from industrial plants, energy facilities, and transportation systems—has begun to erode the ozone, thus admitting more UV rays and incrementally raising the planet’s surface temperature. This process creates the potential for enormous climate changes and potentially devastating consequences for the delicate ecosystem that maintains life.

Planetary Rings and Moons

The moon is Earth’s only natural satellite. It is by far the brightest object in the night sky—although the surface itself, of course, is dark, it reflects sunlight, different portions illuminated at different times as the moon makes its way around the sun, thus creating the familiar phases of the moon. Its composition is very similar to Earth’s outer crust, which has led scientists to believe that the moon was actually broken off from Earth by the impact of a large object (perhaps the size of a small planet) shortly (about a billion and a half years) after Earth’s formation. The surface (as recorded by dozens of cameras from space missions) is forbidding and rocky, pocked by volcanic craters and ridges that, from Earth’s perspective, create the illusion of the face of the man in the moon.

Although not the largest moon in the solar system, Earth’s moon (2,159 miles wide) is the largest moon in relation to its planet—it is more than one-quarter the size of Earth. Because the moon is in an elliptical orbit, its distance from Earth varies; generally, it is 238,800 miles from Earth. That relative nearness gives the moon tremendous gravitational pull that is registered on Earth in the ocean tides. The moon’s rotational period is nearly equivalent to Earth’s—and when viewed from above, both rotate counterclockwise. Because of this synchronous orbit, the moon always shows the same side to Earth (hence the considerable folklore about the dark side of the moon). Earth’s moon remains the only natural satellite in the solar system visited by astronauts.

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