Earth

Rotating model of Earth
Earth - Rotating Model
Credit: Almond/NASA

Earth is the third planet from the Sun. It is the largest of four terrestrial planets and the only known place in the universe where life has evolved.

The Earth's atmosphere is composed mainly of Nitrogen (78%) and oxygen (21%), with traces of water vapor, carbon dioxide and other gases. About 70% of Earth's surface is covered with liquid water, the rest of surface consists of continents and islands with the polar regions being mostly covered with ice.

Earth's interior is divided into layers, it has a solid iron inner core, a liquid outer core, a mantle and a crust (see the diagram on the left).

As the Earth orbits the Sun it turns on its axis, taking 24 hours (1 day) to rotate right around. It takes 365.25 days to complete one orbit around the Sun.

Cut through the Earth
Cutaway showing Earth's internal layers
Credit: Wikimedia

The Earth has seasons because its axis is tilted by about 23°. When the northern half of the Earth (northern hemisphere) is pointing towards the Sun it has Summer. At this time the southern hemisphere is pointing away from the Sun and has winter.

Earth has one large natural rocky satellite, called the Moon, which orbits once every 27.3 days. The Moon is the fifth largest natural satellite (or moon) in the Solar System.