Geodesy · a Light

Two teams walked in opposite directions to measure the earth

Around 830 the caliph al-Ma'mun sent surveyors to the flat plain of Sinjar, north of the Tigris, with a task: find the length of one degree of latitude. Two parties started from the same point, one walking north and one south, measuring the ground as they went, until each found the pole star had shifted by one degree. Their answer was about 56 and two thirds Arabic miles per degree. Converted into a circumference for the whole earth it comes close to the modern figure, though how close depends on which mile they were using, and that is still argued. What is not in doubt is the method: not a calculation from books, but people counting paces across a plain.

Plain of Sinjar · c. 830 CE