The Earth

This spectacular "blue marble" image of the Earth was produced using a collection of satellite-based observations. Scientists and visualisers stitched together months of observations of the land surface, oceans, sea ice, and clouds into a seamless, true-colour mosaic of every square kilometre of our planet.
Source: NASA Goddard Space Flight Centre.

Average Distance from the Sun: 149.6 million km (1 AU)

Size (Equatorial Diameter): 12 756 km

Mass: 5.976 x 1024 kg

Length of Day (Solar Rotation Period): 24 hours

Length of Year (Sidereal Orbital Period): 365.256 days

Temperature: 22°C

Gravity: 9.8 m/s²

The Earth and the Moon

The Earth and the Moon, as seen by the Galileo spacecraft on its way to Jupiter.
Source: Courtesy NASA/JPL-Caltech.

The Earth is the third, and largest, of the terrestrial planets of the inner Solar System. It is only in the last few hundred years, however, that we have come to think of the Earth as just another planet, like the ones we see in the sky.

Earth is very special because it is the only planet we know of that has the right concentrations and abundances of chemicals that support life.


Earth is composed mainly of rock but its surface is quite varied. The main features visible from space are the deep, blue oceans of salty water, surrounding the green and brown land masses. Water covers two thirds of the planet's surface. On the continents, some parts are covered with sandy soil and some are covered with vegetation. They also have networks of rivers and streams. At the polar regions the Earth has white ice caps whose size varies with the seasons. These ice caps contain nearly 90% of all the fresh water on Earth.

The surface of the Earth is geologically very active. It is broken up into large crustal plates, which move slowly and push against each other, forming mountains and causing earthquakes. The Earth remains volcanically active. Rock is formed at plate boundaries, on the ocean floor or by volcanoes.

The Earth's surface temperature varies, generally being warmer where the Sun is higher in the sky. The poles can be at temperatures below freezing, while at the equator temperatures can be as high as 40°C. The temperature also changes through the year, since the Earth's rotation is tilted by 23.5° from its orbit around the Sun. This gives us our cycle of four seasons: summer, autumn, winter and spring. When the north pole leans towards the Sun most of the Sun's light falls on the Northern Hemisphere, so it is summer in the north and winter in the south. Six months later the south pole leans towards the Sun and it is the other way around. Also see Craters.


The atmosphere of Earth is made mostly of nitrogen (77%) and oxygen (21%). There are also very small amounts of carbon dioxide and other gases. Water vapour in the air collects in clouds and the clouds are blown along by winds, which can sometimes be very strong, and water may fall as rain onto Earth's surface.

The presence of free oxygen in the atmosphere is actually quite unusual, since oxygen reacts easily with other elements. It is only because there is life on Earth that produces oxygen that it can still be found in our air.


Earth has one moon: The Moon (diameter: 3475.6 km). In addition to this, the Earth has thousands of artificial satellites, used for scientific and commercial applications.



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