Earth is the third
planet from the
Sun and the only object in the
Universe known to harbor
life. According to
radiometric dating and other sources of evidence, Earth
formed over 4
billion years ago.
Earth's gravity interacts with other objects in space, especially the Sun and the
Moon, Earth's only
natural satellite. Earth
revolves around the Sun in 365.26 days, a period known as an Earth
year. During this time, Earth
rotates about its axis about 366.26 times.
axis of rotation is tilted, producing
seasonal variations on the planet's surface.
gravitational interaction between the Earth and Moon causes
ocean tides, stabilizes the Earth's orientation on its axis, and gradually slows its rotation.
 Earth is the densest planet in the
Solar System and the largest of the four
Earth's lithosphere is divided into several rigid
tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is
covered with water, mostly by
 The remaining 29% is
land consisting of
islands that together have many lakes, rivers and other sources of water that contribute to the
hydrosphere. The majority of
Earth's polar regions are covered in ice, including the
Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the
Earth's magnetic field, and a convecting
mantle that drives plate tectonics.
Within the first billion years of
Earth's history, life appeared in the oceans and began to affect the
Earth's atmosphere and surface, leading to the proliferation of
anaerobic organisms. Some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earth's distance from the Sun, physical properties, and
geological history have allowed life to evolve and thrive.
 In the history of the Earth,
biodiversity has gone through long periods of expansion, occasionally punctuated by
mass extinction events. Over 99% of all
 that ever lived on Earth are
 Estimates of the number of species on Earth today vary widely;
 most species have not been described.
Over 7.4 billion humans live on Earth and depend on its
natural resources for their survival. Humans have developed diverse
cultures; politically, the world has
about 200 sovereign states.