How it works

	The Earth
Width at the equator	12,756 km
Distance from the Sun	150 million km on average: sunlight takes about 8 minutes to arrive
One turn	23.9 hours
One orbit	365.25 days
Tilt of the axis	23.4 degrees
Age	About 4.5 billion years
Sea	71 per cent of the surface, 3.6 km deep on average
Air near the ground	78 per cent nitrogen, 21 per cent oxygen, 1 per cent everything else
Moons	One

Inside the Earth. Nobody has seen any of this. It is worked out from the outside, above all from the way the waves from earthquakes travel through the planet.

Layer	What it is	Thickness
Inner core	Solid iron and nickel, at up to 5,400 °C	1,221 km from the centre to its edge
Outer core	The same metals, liquid	About 2,300 km
Mantle	Hot rock that flows very slowly	About 2,900 km
Crust	The solid rock at the surface	About 30 km under the land, about 5 km under the sea

The Earth sorted itself into these layers when it was young and molten. The heavy metals sank to the middle and the lighter rock floated to the top.

A surface that moves. The crust and the top of the mantle are broken into large plates, and the plates move. They go about as fast as a fingernail grows. Where two pull apart, new rock wells up between them, and the longest mountain range on the Earth is one of these seams, running along the floor of the oceans. Where two push together, mountains rise and volcanoes form. This slow movement is why the Earth has so few old craters. The surface is always being remade.

A football. The opening of this lesson says that on an Earth the size of a football the oceans would be a film of damp. Here is the sum, worked out for this lesson. A football is about 22 centimetres across and the Earth is 12,756 kilometres. On that scale the sea, at 3.6 kilometres, would be about six hundredths of a millimetre deep, which is about the thickness of a hair.

The magnet. The field comes from the liquid metal of the outer core, kept stirring by the planet’s heat and its spin. It is not fixed. Every so often, on average about every 300,000 years but very unevenly, north and south swap over.

The odd one out. Venus is almost the Earth’s twin in size, and Mars has a day nearly the same length. Neither has seas, moving plates or, as far as anyone knows, life. Why the Earth turned out so differently from its neighbours is one of the big questions about planets, and it matters for judging the planets being found round other stars.