Earth

Earth is the third of nine planets in our solar system. It orbits the Sun at a distance of about 93,000,000 mi (150,000,000 km), taking 365.25 days to complete one revolution. Earth is small by planetary standards; with a diameter of 7,921 mi (12,756 km), it is only one-tenth the size of Jupiter. Its mass is 2.108 × 10²⁶ oz (about six trillion kg), and it must speed this huge bulk along at nearly 19 mi (30 km) per second to remain in a stable orbit. The mean density of our planet is 5.5 grams per cubic centimeter. Unlike the outer planets, which are composed mainly of light gases, Earth is made of heavy elements such as iron and nickel, and is therefore much more dense. These characteristics—small and dense—are typical of the inner four planets, or terrestrial planets.

It was not until 1957, when the first man-made satellite was launched, that humans could see Earth as a beautiful whole. It seemingly floats in empty space, a world distinguished first by its vast oceans and only secondarily by its landmasses, everywhere draped in white swirls of clouds. This is a planet in the most fragile ecological balance, yet resilient to repeated catastrophe. It is our home, and it bears close examination.