Planet Mug EARTH

EARTH. (Shown in true color) Earth, our home planet, is a world unlike any other. The third planet from the sun, Earth is the only place in the known universe confirmed to host life. With a radius of 3,959 miles, Earth is the fifth largest planet in our solar system, and it's the only one known for sure to have liquid water on its surface. Earth is also unique in terms of monikers. Every other solar system planet was named for a Greek or Roman deity, but for at least a thousand years, some cultures have described our world using the Germanic word “earth,” which means simply “the ground.” Earth orbits the sun once every 365.25 days. Since our calendar years have only 365 days, we add an extra leap day every four years to account for the difference. Though we can't feel it, Earth zooms through its orbit at an average velocity of 18.5 miles a second. During this circuit, our planet is an average of 93 million miles away from the sun, a distance that takes light about eight minutes to traverse. Astronomers define this distance as one astronomical unit (AU), a measure that serves as a handy cosmic yardstick. Earth rotates on its axis every 23.9 hours, defining day and night for surface dwellers. Earth's atmosphere is 78 percent nitrogen, 21 percent oxygen, and one percent other gases such as carbon dioxide, water vapor, and argon. Much like a greenhouse, this blanket of gases absorbs and retains heat. On average, Earth's surface temperature is about 57 degrees Fahrenheit. The atmosphere not only nourishes life on Earth, but it also protects it: It's thick enough that many meteorites burn up before impact from friction, and its gases—such as ozone—block DNA-damaging ultraviolet light from reaching the surface. But for all that our atmosphere does, it's surprisingly thin. Ninety percent of Earth's atmosphere lies within just 10 miles of the planet's surface.