Geodesy · a Light

One hill in the Punjab was enough to measure the earth

Around 1018, at the fort of Nandana in what is now Pakistan, al-Biruni measured the height of a hill by sighting its summit from two places on the plain below. He then climbed it and measured how far the horizon had dropped below the level line. From those two numbers and some trigonometry he obtained the radius of the earth. His answer works out at about 6,340 kilometres, within a fraction of a per cent of the modern figure. He liked the method mainly because it needed one person and one instrument, where the caliph al-Ma'mun's astronomers had needed survey parties pacing across a plain. Historians note that his instrument could not really support that much accuracy, so part of the agreement is luck.

Al-Biruni · Nandana fort, c. 1018 CE