The form of the earth

Earth (IPA: /ɜ(ɹ)θ/) 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 a myriad of species including humans, it is also referred to as "the Earth", "Planet Earth", "Gaia", "Terra",[2] and "the World".

Earth is the only planet known to have liquid water on its surface[3][4] and 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[5] 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. This length of time is a sidereal year, which is equal to 365.26 solar days.[6] The Earth's axis of rotation is tilted 23.5°[7] away from the perpendicular to its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year.

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.

The Earth's shape is very close to an oblate spheroid—a rounded shape with a bulge around the equator—although the precise shape (the geoid) varies from this by up to 100 meters (327 ft).[21] The average diameter of the reference spheroid is about 12,742 km (7,913 mi). More approximately the distance is 40,000 km/π because the meter was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France.

Shape of the Earth

The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km (27 mi) larger than the pole to pole diameter.[23] The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m [29,028 ft] above local sea level) and the Mariana Trench (10,911 m [35,798 ft] below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in billiard balls. Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.