Astronomy · a Light

Columbus sailed on a number he had converted wrongly

Al-Farghani wrote an introduction to astronomy at Baghdad in the 830s, thirty short chapters with almost no mathematics in them, which is exactly why it served as the standard textbook for seven hundred years. Translated twice into Latin in the twelfth century, it is where Dante took his figures for the size of the heavens. It also carried the caliph al-Ma'mun's measurement of the earth, given as fifty six and two thirds miles to one degree of latitude. Those were Arabic miles, longer than Roman ones. Columbus, working from that figure, used the shorter mile, and so calculated an earth about a quarter smaller than it is, with Asia correspondingly nearer. How much was error and how much was wishful thinking is still argued.

Al-Farghani · Baghdad, c. 833 CE