Planet Earth, Explained - National Geographic

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Our dance around the sun

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.

(Pictures: New HD View of Earth at Night)

Earth rotates on its axis every 23.9 hours, defining day and night for surface dwellers. This axis of rotation is tilted 23.4 degrees away from the plane of Earth's orbit around the sun, giving us seasons. Whichever hemisphere is tilted closer to the sun experiences summer, while the hemisphere tilted away gets winter. In the spring and fall, each hemisphere receives similar amounts of light. On two specific dates each year—called the equinoxes—both hemispheres get illuminated equally.

Many layers, many features

About 4.5 billion years ago, gravity coaxed Earth to form from the gaseous, dusty disk that surrounded our young sun. Over time, Earth's interior—which is made mostly of silicate rocks and metals—differentiated into four layers.

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