Al-Mamun, however, persisted.

On the shores of the Red Sea, in the plains of Shinar, by the aid of an astrolabe, the elevation of the pole above the horizon was determined at two stations on the same meridian, exactly one degree apart.

The distance between the two stations was then measured, and found to be two hundred thousand Hashemite cubits; this gave for the entire circumference of the earth about twenty-four thousand of our miles, a determination not far from the truth.

But, since the spherical form could not be positively asserted from one such measurement, the khalif caused another to be made near Cufa in Mesopotamia.

His astronomers divided themselves into two parties, and, starting from a given point, each party measured an arc of one degree, the one northward, the other southward.