Letter: Ancient calculations

Sir: In his review of Colin Thubron's new book on Asia, William Dalrymple remarks ('Cracked domes on the silk route', 1 October) that al-Biruni was the first to compute the radius of the Earth.

Eratosthenes of Cyrene calculated the radius of the Earth in about 240BC by comparing the elevation of the Sun at noon in Alexandria and in Syene in Egypt on the same day of the year. That was more than 1,000 years before al-Biruni's time.

About a century and a half after Eratosthenes, Posidonius of Apamea repeated his experiments but came up with a less accurate figure. Eratosthenes's result was only re-established after Magellan's circumnavigation of the Earth in 1523.

Yours sincerely,

G. C. WRAITH

Kingston by Lewes

East Sussex