First look
The idea, in plain words
On 7 December 1972 the crew of Apollo 17, on their way to the Moon, turned a camera back towards home and took this photograph. It shows the whole Earth, lit from edge to edge. For once you can look at our world as an astronomer looks at any other planet.
It is a ball of rock and metal 12,756 kilometres across, the biggest of the four rocky planets. Most of it is out of sight. At the middle is a ball of solid iron and nickel as hot as 5,400 degrees. Round that is a layer of the same metals, molten. Round that is a thick mantle of hot rock. The ground we live on is a thin skin on top.
Seas cover 71 per cent of the surface. No other planet in the Solar System has liquid water lying on its surface. Yet the sea is very shallow for the size of the world: on average 3.6 kilometres deep, on a planet more than 12,000 across. If the Earth were the size of a football, the oceans would be a film of damp on it.
The air is thin too. Nearly four fifths of it is nitrogen and one fifth is oxygen, and almost all of it lies within a few tens of kilometres of the ground. That thin layer keeps the planet warm, carries the weather, and stops most of the harmful light from the Sun.
The molten metal deep inside does one more thing. As the Earth turns, it makes the whole planet a magnet. That magnet reaches far out into space and turns aside most of the stream of particles that blows from the Sun. Some are steered down towards the poles, where they strike the air and make it glow. Those are the northern and southern lights.
And there is life. It is thought to have begun in the sea about 3.8 billion years ago, and it has since changed the air, the rocks and the colour of the land. So far, nobody has found it anywhere else.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Terrestrial planet
- A planet made mostly of rock and metal with a solid surface: Mercury, Venus, the Earth and Mars.
- Differentiation
- The sorting of a young, molten world into layers, with heavy metal sinking to the middle and lighter rock floating above it.
- Atmosphere
- The layer of gas held round a planet or moon by its gravity.
- Magnetic field
- The region round a magnet, or round moving electric charge, in which other magnets and charges feel a force.
- Magnetosphere
- The space round a planet ruled by its own magnetic field, which turns the solar wind aside.
- Aurora
- Light in the sky near a planet's magnetic poles, made when charged particles strike the upper air.
- Greenhouse effect
- The warming of a planet when gases in its air let sunlight in but hold back the heat trying to leave.
Where this comes from
- NASA, Earth facts
- NASA, Earth fact sheet
- NASA, View of the Earth seen by the Apollo 17 crew (AS17-148-22727)
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.