How can anyone tell the Earth is round without going to space?

The ship that vanishes from the bottom

Stand on a shore and watch a ship sail away. If the sea were flat it would simply get smaller while staying whole, until it was swallowed as a dot.

What really happens is different: the hull goes first, then the deck, and last of all the top of the mast. The bottom disappears first.

There is only one explanation — something bulges between your eye and the ship. Through binoculars you can still see the mast after the hull has gone, which shows this is not simply distance.

Two sticks, two shadows

More than two thousand years ago a man named Eratosthenes noticed that in one city, at noon on a particular day, an upright stick cast no shadow at all. On the same day at the same hour, in a city far to the north, an identical stick cast a clear one.

On flat ground, with a very distant Sun, two upright sticks would have to cast the same shadow. A difference between them is only possible if the ground itself curves.

Eratosthenes did not stop there. He measured the angle of the shadow, measured the distance between the cities, and calculated the circumference of the Earth from the two. His answer was close to what we know today — all from a stick and a shadow.

The Earth in numbers

The radius of the Earth is about 6,400 kilometres. Inside there is a thin crust, a hot rocky mantle beneath it, and a metal core at the centre.

Above everything is a surprisingly thin layer of air. Most of the atmosphere sits within the first hundred kilometres, which is about a sixtieth of the radius. If the Earth were the size of an apple, all the air would be thinner than its skin.

And here is the difference between a claim with evidence and one without. 'The Earth is round' can be checked from a beach, from a stick, and from an eclipse. A claim that cannot be checked any way at all is not a claim about the world but about whoever is making it.

Worked examples

1. A ship sails away and its hull disappears before its mast. What does that show?

   1. On a flat surface the whole ship would shrink together
   2. Here a lower part vanishes before an upper one
   3. Something is blocking the lower line of sight

   Answer: That the sea curves, and a bulge sits between eye and ship

2. At noon a stick in city A casts no shadow and in city B it does. What follows?

   1. The Sun is distant, so its rays arrive almost parallel
   2. On flat ground both sticks would cast the same shadow
   3. A difference means the sticks are not parallel to each other

   Answer: That the ground curves between the two cities

3. What is the ratio of the atmosphere's thickness to the Earth's radius?

   1. Most of the atmosphere is within about 100 km
   2. The radius is about 6,400 km
   3. 100 divided by 6,400

   Answer: About one sixtieth — a very thin shell

Common mistakes

Thinking a ship vanishes simply because it is far away

Distance shrinks everything equally. Here the bottom goes first, and distance alone cannot produce that order.

Assuming you need a spacecraft to know

The circumference was measured from a stick and a shadow more than two thousand years before the first flight. The photograph from space confirmed a measurement; it did not create one.

Picturing a thick atmosphere

It is thinner than a sixtieth of the radius. In photographs from space it appears as a narrow blue band, and that is its real size.

More in Earth and Space

• If the Earth spins eastward, why does the Sun rise in the east?
• Is summer hot because the Earth is closer to the Sun?
• Why does the Moon change shape — is it the Earth's shadow?
• If the Moon goes round us every month, why is there not an eclipse every month?
• What holds the planets in their orbits?