Like the measurements of Aristarchus and Eratosthenes, this calculation of Alhazen is simple enough in theory.
The most famous achievement of Eratosthenes was his measurement of the earth.
Hipparchus tries to invalidate this view of Eratosthenes, by sneering at the proofs on which it rests.
And Eratosthenes also says that he kept the poison in a hollow ring, which he wore about his arm.
Syene was one of the bases used by Eratosthenes in his calculations for the measurement of the earth.
His general conception of the earth was substantially that of Eratosthenes.
He justly objected to the arbitrary network of the map of Eratosthenes.
And here we must previously adduce the opinions of Eratosthenes.
Yet the method of Eratosthenes, like many another enigma, seems simple enough once it is explained.
But there is another method (says Eratosthenes) of proving this.