Earth

**Earth** is a planet in the solar system that revolves around its star — the Sun — in the third innermost orbit. It is the densest and the fifth largest of the eight planets in the solar system. It is also the largest of the four terrestrial or rocky.

Earth was formed approximately 4.55 billion years ago and life emerged about a billion years later.18 It is home to millions of species, including humans, and is currently the only astronomical body where life is known to exist.19 The atmosphere and other abiotic conditions have been significantly altered by the planet's biosphere, favoring the proliferation of aerobic organisms, as well as the formation of an ozone layer that, together with the Earth's magnetic field, blocks harmful solar radiation, thus allowing life. on Earth.20 Earth's physical properties, geologic history, and orbit have allowed life to continue to exist. It is estimated that the planet will continue to be capable of supporting life for another 5000 million years, since according to current forecasts, after that time the increasing luminosity of the Sun will end up causing the extinction of the biosphere. 21 22 23

The earth's surface or crust is divided into several tectonic plates that slide over magma over periods of several million years. The surface is covered by continents and islands; These have several lakes, rivers and other sources of water, which together with the salt water oceans that represent about 71% of the surface constitute the hydrosphere. No other planet is known with this liquid water balance, note 6 which is indispensable for any known life. The Earth's poles are mostly covered with solid ice (Antarctic indlandsis) or ice floes (Arctic ice cap). The interior of the planet is geologically active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid inner core composed of approximately 88% iron.25

The Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. Currently, the Earth completes one orbit around the Sun each time it makes 366.26 turns on its axis, which is equivalent to 365.26 solar days or one sidereal year. Note 7 The Earth's axis of rotation is It is inclined 23.4 ° with respect to the perpendicular to its orbital plane, which produces seasonal variations in the planet's surface with a period of one tropical year (365.24 solar days) .26 The Earth has a single satellite natural, the Moon, which began to orbit the Earth 4.53 billion years ago; This produces the tides, stabilizes the inclination of the Earth's axis and gradually reduces the rotation speed of the planet. About 3.8 to 4.1 billion years ago, during the so-called late heavy bombardment, numerous asteroids struck Earth, causing significant changes to most of its surface.

Both the planet's minerals and the products of the biosphere provide resources that are used to sustain the world's human population. Its inhabitants are grouped into some 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. Human cultures have developed many ideas about the planet, including the personification of a deity, the belief in a flat Earth or Earth as the center of the universe, and a modern perspective of the world as an integrated environment that requires management.