Earth is the third planet from the Sun and the only astronomical object known to support life. It formed about 4.54 billion years ago. It is the largest of the four rocky planets in the Solar System and the fifth-largest planet overall. It is also the densest planet, with a mean density of about 5,513 kg/m³. Liquid water covers roughly 71% of its surface. A nitrogen–oxygen atmosphere, a global magnetic field and a single large natural satellite, the Moon, make Earth's surface conditions stable enough for life. The English name comes from a Germanic word meaning "the ground." Earth is the only planet in the Solar System not named after a Greek or Roman god.
Size, shape and motion
Earth is not a perfect sphere. It is an oblate spheroid, flattened slightly at the poles by its rotation. Its equatorial radius is about 6,378 km and its polar radius about 6,357 km, giving a mean radius of 6,371 km. The circumference is about 40,075 km around the equator and about 40,008 km through the poles. Earth's mass is about 5.97 × 10²⁴ kg. Mean surface gravity is close to 9.8 m/s², and the escape velocity is about 11.2 km/s.
Earth orbits the Sun at an average distance of about 149.6 million km, a distance that defines the astronomical unit. One orbit takes about 365.26 days, and the average orbital speed is close to 30 km/s. The orbit is nearly circular. Earth is closest to the Sun (perihelion) in early January and farthest (aphelion) in early July. The planet turns on its axis about once every 24 hours relative to the Sun. Its rotational axis is tilted about 23.4° from the plane of its orbit, and this tilt causes the seasons. The Moon's pull on Earth raises the ocean tides and slowly lengthens the day over geological time.
Internal structure
Earth's interior is divided into layers by chemical makeup and by physical behavior. The thin outer crust comes in two kinds. Continental crust is thicker and older. Oceanic crust is thinner and denser. By mass, the crust is made mostly of oxygen and silicon, followed by aluminium, iron and calcium. Below it lies the mantle, a thick layer of hot silicate rock that flows very slowly over long timescales. At the center is a core about 3,485 km in radius, made mainly of iron and nickel. The core has a liquid outer layer and a solid inner core. Convection in the liquid outer core is thought to generate Earth's magnetic field. The field shields the surface from much of the solar wind and cosmic radiation.
The rigid outer shell, made of the crust and the uppermost mantle, is broken into moving pieces. This process, called plate tectonics, drives earthquakes, volcanism and mountain-building. It also causes the slow drift of the continents, and over hundreds of millions of years the plates have repeatedly joined into supercontinents and split apart again. Plate tectonics also recycles carbon between the surface and the interior, which helps regulate climate over long periods.
Hydrosphere and atmosphere
Most of Earth's surface water is in the ocean. Smaller amounts are stored in ice sheets, groundwater, lakes and rivers. Water moves between these reservoirs and the air through evaporation, precipitation and runoff. This cycle shapes landscapes and carries heat around the planet.
The atmosphere is about 78% nitrogen and 21% oxygen by volume. The rest is argon, carbon dioxide, water vapor and other trace gases. Greenhouse gases such as water vapor and carbon dioxide trap heat. Without them, Earth's average surface temperature would be well below freezing, near its black-body temperature of about 254 K. The stratospheric ozone layer absorbs most of the Sun's ultraviolet radiation. Atmospheric and ocean circulation spread solar energy from the tropics toward the poles and create the planet's weather and climate zones. Since the Industrial Revolution, burning fossil fuels has measurably raised atmospheric carbon dioxide. This rise is the main driver of current climate change.
Geological and biological history
Earth formed from the disk of gas and dust that surrounded the young Sun. The leading explanation for the Moon is that it formed soon afterward from debris thrown out when a Mars-sized body collided with the early Earth. Over time the surface cooled, a solid crust formed, and oceans built up from water released by the interior and delivered by impacts.
Evidence of life goes back at least 3.5 billion years, to single-celled prokaryotes. Later, photosynthesis by cyanobacteria released oxygen into the atmosphere. Around 2.4 billion years ago this led to the Great Oxidation Event, which permanently changed the chemistry of the oceans and air. Complex eukaryotic cells appeared later, and multicellular animals spread widely during the Cambrian period, about 540 million years ago. Through evolution, life has diversified into the biodiversity seen today, though several mass extinctions interrupted this process. Living things in turn reshape the planet's air, water and rocks, linking every ecosystem to Earth's physical systems. Anatomically modern humans appeared roughly 300,000 years ago. The human population now exceeds eight billion.
History of human understanding
Scholars in ancient Greece concluded that Earth is round. Aristotle pointed to the curved shadow Earth casts on the Moon during lunar eclipses. In the 3rd century BCE, Eratosthenes used geometry and the angle of the Sun's rays at two Egyptian cities to estimate Earth's circumference, and his result was close to the modern value. For centuries, the dominant model placed a stationary Earth at the center of the universe. In the 16th century, Nicolaus Copernicus proposed that Earth orbits the Sun. Over the following century, the work of Johannes Kepler, Galileo Galilei and Isaac Newton established this view. Newton also predicted that Earth should bulge at the equator, and later geodetic surveys confirmed it. In 1798, Henry Cavendish's torsion-balance experiment allowed the first good estimate of Earth's density.
The modern geosciences began in the 18th and 19th centuries, when geology came to recognize Earth's great age. In the 20th century, radiometric dating of rocks and meteorites gave Earth's age as about 4.5 billion years. Plate tectonics became the accepted framework of geology in the 1960s. Since then, satellites have mapped Earth's shape, gravity field, surface and climate in great detail. Photographs taken from spacecraft, such as Apollo 17's 1972 image of the whole Earth, showed the planet as a single, finite world.
References
- Earth Fact Sheetnssdc.gsfc.nasa.gov
- Earthen.wikipedia.org
- How big is Earth?space.com
- The Earth - Imagine the Universe! - NASAimagine.gsfc.nasa.gov
- Planet Earth facts and informationnationalgeographic.com
- Quick Geography Facts About Planet Earth - Geography Realmgeographyrealm.com