Shape of the Earth: a ball, slightly flat
The Earth is round like a ball, not flat like a plate. Some easy proofs: a ship goes down below the horizon with its hull first, then its top; the Earth's shadow on the Moon during a lunar eclipse is always round; and photos from space show a ball.
The Earth spins once a day. Spinning pushes the middle outwards a little, so the Earth bulges at the equator and is a bit flat at the poles. The shape is called an oblate spheroid (a squashed ball). The squash is tiny: the radius is 6,378 km at the equator and 6,357 km at the poles, a gap of only 21 km. On a globe of radius 30 cm the gap would be about 1 mm, so a globe is a very good model.
Size of the Earth
Mean (average) radius R ≈ 6,371 km. The distance round the equator is the circumference, 2πR ≈ 40,000 km. The surface area is about 510 million km², and about 71% of it is water.
Why does the ground look flat to us? Because we see only a very small patch. A person 1.7 m tall sees the horizon only about 5 km away, and 5 km is tiny compared with 40,000 km.
Inside the Earth: four layers
- Crust: the thin, solid, rocky skin we live on. About 5–10 km thick under oceans and 30–70 km under big mountains.
- Mantle: a very thick layer of hot rock, down to about 2,900 km. It is solid but flows very slowly, like thick toffee over millions of years.
- Outer core: from 2,900 km to 5,150 km. Melted iron and nickel, so it is liquid.
- Inner core: a ball of iron and nickel about 1,220 km in radius. It is hottest (about 5,500 °C) but solid, because the pressure on it is enormous.
The 3D shows the crust thicker than real so that you can see it.
How do we know what is inside?
The deepest hole ever drilled is about 12 km, which is only 0.2% of the way to the centre. So scientists use clues. The most important clue is earthquake waves. Some waves (S-waves) cannot pass through liquid. They stop at the outer core, so the outer core must be liquid. Other waves bend and speed up or slow down at layer edges, which tells us where the layers begin. Also, mines and drilling show that it gets hotter with depth, and volcanoes bring up hot rock from below.
Try it: build the Earth with fruit
Cut a peach or a boiled egg in half. Name the thin skin (crust), the thick middle (mantle) and the stone or yolk (core). Then draw the Earth with a ruler: radius 6.4 cm for 6,371 km. How thick is the crust on that drawing? About 0.03 cm, thinner than a pencil line! In the 3D, move the depth slider and say the layer name every time the colour changes.
Key formulas and definitions
- Circumference = 2 × π × R, R ≈ 6,371 km, so C ≈ 40,000 km
- Equatorial radius 6,378 km, polar radius 6,357 km, difference ≈ 21 km
- Layer depths: crust 0–35 km (average), mantle to 2,900 km, outer core to 5,150 km, inner core to 6,371 km
- Thickness of a layer = lower depth − upper depth
- Distance = speed × time
Worked examples
1. Use R = 6,371 km and π = 3.14 to find the circumference of the Earth.
C = 2 × π × R = 2 × 3.14 × 6,371 = 40,011.88 km. So about 40,000 km.
2. A plane flies at 800 km/h along the equator. How long to go once round the Earth (40,000 km)?
Time = distance ÷ speed = 40,000 ÷ 800 = 50 hours.
3. The outer core starts at 2,900 km depth and ends at 5,150 km depth. How thick is it?
Thickness = 5,150 − 2,900 = 2,250 km.
4. The inner core has radius about 1,220 km. What percent of the Earth's radius (6,371 km) is that?
1,220 ÷ 6,371 = 0.1915, so about 19%. The inner core is small compared with the whole Earth.
5. On a model globe of radius 25 cm, by how many mm would the polar radius be shorter than the equatorial one? (real difference is 21 km out of 6,378 km)
Fraction = 21 ÷ 6,378 = 0.0033. 25 cm = 250 mm. 250 × 0.0033 ≈ 0.8 mm. A hair is thicker, so a globe looks perfectly round.
6. The deepest drill hole is 12 km. What percent of the way to the centre (6,371 km) is that?
12 ÷ 6,371 = 0.0019, so about 0.19%. We have barely scratched the surface.
Common mistakes
- Thinking the Earth is a perfect sphere. It is slightly flat at the poles (gap 21 km).
- Thinking the whole inside is liquid lava. Only the outer core is liquid. The mantle is solid rock that flows slowly and the inner core is solid.
- Thinking the core is cold or the crust is thick. The crust is the thinnest layer and the core is the hottest part.
- Mixing up the order. From the surface down it is crust, mantle, outer core, inner core.