The shape and size of the Earth

In ancient times peoples imagined the Earth as flat or carried by animals. In the 4th century BC Aristotle proved that the Earth is a sphere: when a ship approaches the shore its mast appears first and then its hull; during a lunar eclipse the Earth’s shadow is round; and the horizon widens the higher you climb. A globe is a reduced model of the Earth; the first globe in Europe was made by Martin Behaim in 1492. The Earth rotates on its axis, and the two points where the axis meets the surface are the North and South Poles. The great circle at equal distance from both poles is the equator; it is about 40 000 km long (more exactly 40 075 km) and divides the Earth into the Northern and Southern Hemispheres. Circles parallel to the equator are parallels, and half-circles joining the poles are meridians; the meridian through the Greenwich observatory is the Prime Meridian. The Earth is not a perfect ball: the distance from the centre to the surface is 6378 km at the equator and 6357 km at the poles, so its shape is called a geoid; the surface area is about 510 million km².