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Earth is the third planet from the Sun, orbiting at a mean distance of 150 million kilometers.
It is an oblate spheroid, which means that the equatorial diameter of 12,756 km is larger than the polar diameter of 12,714 km.
It has a mass of 6 x 1024 kg.
The altitude of the surface features range from 11 km below to 9 km above sea level.
70% of its surface is liquid water, which makes it unique in the solar system.
The internal structure of Earth
Earth is about 4.6 billion years old. It contracted from the dust and gas cloud from which the Sun also formed. Earthquakes allow scientists to probe the internal structure, which consists of three main layers:
Earth has an atmosphere reaching several hundred km into space, consisting of 78% nitrogen, 21% oxygen, 1% other gases including carbon dioxide and water vapour. Earth is the only planet in our solar system to have large quantities of free oxygen in its atmosphere.
The ozone layer, at an altitude of 12–50 km, shields Earth's surface from ultraviolet radiation from the Sun.
Carbon dioxide traps sunlight re-emitted from Earth's surface as infrared radiation. This greenhouse effect is responsible for raising the temperature of Earth by around 20 K.
Earth has certain properties which make it conducive to life, the most important of which are:
The orbit of the Earth
Earth orbits around the Sun in a path with the shape of a stretched circle - an ellipse. The plane of the orbit is called the ecliptic.
In early January it is closest to the Sun - perihelion - and in July it is furthest away from the Sun - aphelion. Seasons are not connected to Earth's distance from the Sun - our northern winter takes place when the Earth is actually closest to the Sun - but to the fact that the Earth's axis is tilted away perpendicular to the ecliptic.
The mean distance from Earth to the Sun defines the Astronomical Unit (AU) where:
1 AU is approximately 150 million km
Astronomers measure distances within the solar system in AU.
Questions to think about
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