The shape of the Earth
Picture telling a friend exactly where you are standing, so precisely that they could find you anywhere on the planet. Street names and landmarks only help nearby. To describe a position anywhere on the Earth, you first need to understand the shape you are describing.
The Earth is very nearly a sphere. Its spin on its axis makes it bulge slightly at the Equator and flatten slightly at the poles, so its radius at the Equator differs a little from its radius at the poles. For most purposes, though, the Earth is treated as a sphere.
A sphere that is slightly flattened at the poles and slightly bulging at the Equator. It is the closest description of the Earth's true shape.
The idea of a round Earth is old. Around 500 CE the Indian astronomer Aryabhata described the Earth as a globe standing in space, spherical in shape, with the creatures of land and water living all around it.
A curved surface causes a problem for anyone drawing the whole Earth on paper. A sphere cannot be laid flat without breaking or stretching.
Peel an orange in three or four big pieces and press them flat on a table. The edges split and tear, because a curved skin cannot become a flat sheet without breaking. A flat map of the whole Earth runs into the same trouble.
A globe avoids the problem. It is a sphere with a map drawn on it, usually made of metal, plastic or cardboard. Because it has the same shape as the Earth, it shows the Earth's geography more faithfully than a flat map can. Globes can also be made of the Moon, Mars or the stars.
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