The earth, the earth, is the third planet from the sun and the only known living object in the universe. It is the most dense planet in the solar system and the largest of the four terrestrial planets.

According to radiometric dating and other sources of evidence, the earth formed about 4.54 billion years ago. The gravity of the earth interacts with other objects in space, especially the sun and moon, the only natural satellites of the earth. In an orbit around the sun, the earth rotates about its axis 365 times; therefore, the earth is about 365.26 days a year. The earth's axis of rotation is tilted, resulting in seasonal changes on the earth's surface. The gravitational interaction between the earth and the moon will cause ocean tides, stabilize the direction of the earth on its axis, and gradually slow down its rotation speed.

The lithosphere of the earth is divided into several rigid tectonic plates, which move across the earth's surface in millions of years. About 71% of the earth's surface is covered by water, most of which is covered by the ocean. The remaining 29% is made up of continents and islands, which together have many lakes, rivers and other water sources that contribute to the hydrosphere. Most polar regions of the earth are covered with ice, including the Antarctic ice sheet and the sea ice of the Arctic ice bags. The earth's interior remains active, with a solid iron inner core, a liquid outer core that generates the earth's magnetic field, and a convective mantle that drives plate tectonics.