Biruni — the Persian who measured the Earth from a hill | PERSIA'S GREATEST MINDS Part 5

History Onion · Part 5 · 2:58

Around 1020, beside the fort of Nandana in the Salt Range of the Punjab (today in Pakistan), the Persian scholar Abu Rayhan Biruni (ابوریحان بیرونی) measured the size of the whole Earth from one hill. Two centuries earlier, the caliph al-Ma'mun's astronomers had measured a degree of the Earth by surveying a stretch of desert. Biruni found the hill's height from two sightings on the plain, climbed it, and measured how far the horizon lies below eye level — the dip, 34 minutes of arc, just over half a degree. Height and dip fix a right-angled triangle whose far corner is the centre of the Earth. He was born in 973 in Khwarezm, south of the Aral Sea; his mother tongue, Khwarezmian, was an eastern Iranian language. In his twenties he sent eighteen questions to the teenage Ibn Sina in Bukhara — ten of them challenging Aristotle — among them why a vessel of water breaks when it freezes and why ice floats. In 1017 Sultan Mahmud of Ghazna took Khwarezm and carried its scholars to his court; Biruni went too, and later wrote that he had never been free to come and go as he pleased. Around 1000, at the court of Gorgan, he had written The Chronology of Ancient Nations (al-Āthār al-bāqiya), which records the calendars of many peoples and how Iranians kept Nowruz, Mehregan, Sadeh and other festivals. In the lands Mahmud's armies took in India he learned Sanskrit and wrote his book on India (Taḥqīq mā li'l-Hind, finished just after Mahmud's death). In its preface, in his own Arabic: «وليس الكتاب كتاب حجاج وجدل … وإنّما هو كتاب حكاية فأورد كلام الهند على وجهه» — "This book is not a polemical one… I shall place before the reader the theories of the Hindus exactly as they are" (tr. Sachau). The lands beyond the ocean: the historian S. Frederick Starr ("So, Who Did Discover America?", History Today, 2013; The Genius of Their Age, 2023) reads Biruni's al-Qānūn al-Masʿūdī (c. 1037) this way — plotted on the globe, the known world from Africa's western tip to China covered only about two-fifths of the way around; Biruni rejected an all-water remainder and concluded that unknown lands lay in the ocean, and that people could live there. It is Starr's reading, and Starr himself says Biruni did not "discover" America in the literal sense: he never saw it. What we didn't have room for: he translated the Yoga Sutras into Arabic and, by his own account, Euclid and Ptolemy's Almagest into Sanskrit verse for Indian scholars; from river stones that shrink downstream he concluded that the plain of India "has once been a sea" filled by river silt; he measured the density of about eighteen metals and gems; and he wrote that a rotating Earth would not break astronomy, while calling the question "most difficult to solve". Writing after Mahmud's death, he also recorded that Mahmud had "utterly ruined the prosperity" of the country (India, ch. 1).