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Earth (IPA: /ɝθ/) is the third planet from the Sun and is the largest of the terrestrial planets in the Solar System, in both diameter and mass. Home to the human species, it is also referred to as "the Earth", "Planet Earth", "Gaia", "Terra", and "the World".
The Earth is the first planet known to have liquid water on the surface and is the only place in the universe known to harbor life. Earth has a magnetic field that, together with a primarily nitrogen-oxygen atmosphere, protects the surface from radiation that is harmful to life. The atmosphere also serves as a shield that causes smaller meteors to burn up before they strike the surface.
The Earth formed around 4.57 billion years ago and its only known natural satellite, the Moon, began orbiting it around 4.53 billion years ago. At present, the Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis (which is equal to 365.26 solar days). The Earth's axis of rotation is tilted 23.5° (away from the perpendicular to its orbital plane), producing seasonal variations on the planet's surface.
Atmospheric conditions on Earth have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered with salt-water oceans. The remaining 29% consists of continents and islands. The planet's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid-iron inner core.
Earth interacts with outer space to a significant degree. Its relatively large moon provides ocean tides, stabilizes the axial tilt and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet played a role in the formation of the oceans. Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the orbit of the planet are believed to have caused the ice ages that have covered significant portions of the surface in glacial sheets.