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Earth introduction

Earth basics

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.

The shape of the Earth
Exaggerated profile of the shape of the Earth

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:

Cross-section of the Earth
Internal structure of the Earth

  • the crust is the 22 km thick outer layer. This is made up of materials such as granite and basalt and is less dense than the core.

  • the mantle reaches down to a depth of nearly 3000 km. This consists of silicate rock which flows under pressure but shatters under impact.

  • the core is about 3500 km across and is made of iron and nickel under very high pressure. An outer molten layer surrounds a solid centre which has a temperature of 6400 K, about the same as the surface of the Sun. The core is responsible for Earth's global magnetic field.

The atmosphere

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:

  • an abundance of free oxygen
  • an abundance of liquid water
  • a moderate surface temperature

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.

The orbit of the Earth
The orbit of the Earth

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

1. Why is Earth unique in the solar system?


2. What problems does Earth's atmosphere create for astronomers? Give three features of The atmosphere that help human life thrive on Earth. Why are the changing levels of carbon dioxide and ozone a cause for concern?


3. Imagine you are an extraterrestrial being approaching Earth in a spacecraft after a long journey. What evidence would there be for intelligent life?


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