Glossary of Astronomical Terms/Earth

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Earth is the third planet in the solar system.

Its days are 23 hours, 56 minutes, and four seconds long. Its years are approximately 365 days long.

Earth is the only planet on the solar system to possess life. It is also the only planet where water can be found in all three states of matter: liquid, solid, and gas.

Earth was probably formed about 4.5 billion years ago. It has one moon, named Luna, which probably formed from a collision with a large object early in Earth's history.

70% of Earth's surface is covered with water, and water can also be found frozen in the polar ice caps.