Al-Biruni measured the Earth from Nandana hill and wrote Kitab al-Hind

Travelling from Jhelum towards Pind Dadan Khan, one comes upon a deserted hill fort on the eastern edge of the Salt Range: Nandana. Today it lies in ruins. Yet nearly 1,000 years ago, a scholar standing on the summit of this same hill worked out the radius of the whole Earth with the help of mathematics and observation. He was Abu Raihan Muhammad bin Ahmad al-Biruni, and his journey was not made by ship or caravan. It was a journey to understand a new civilisation.

From Khwarazm to Ghazni

The German orientalist Edward Sachau, who translated al-Biruni’s "Kitab al-Hind" into English, writes in his preface that al-Biruni was born in 973 in Khwarazm, the region of present-day Khiva in Uzbekistan. He made his name in science and letters while still young and served as an adviser to the Mamunid rulers of Khwarazm.

In the spring and summer of 1017, Sultan Mahmud of Ghazni annexed Khwarazm to his empire and brought its princes and leading men to Ghazni. According to Sachau, al-Biruni was among them. Sachau is of the view that for the next 13 years, from 1017 to 1030, al-Biruni enjoyed no particular patronage at court. He lived in various parts of India, probably as an honoured political guest or hostage, and spent his spare time studying India.

A foreigner’s scholarly journey

Al-Biruni learnt Sanskrit, talked with pandits and bought books. According to Sachau, hints in the book show that he did this at places such as Multan and Peshawar, as well as Ghazni. He himself writes that few people can have had such opportunities for studying the subject as he had, although he was never fully free in his movements.

The result of this labour is "Kitab al-Hind", completed around 1030. The title page of Sachau’s translation lists its subjects: religion, philosophy, literature, geography, chronology, astronomy, customs, laws and astrology. At the start of the book al-Biruni explains his method: "This book is not a polemical one". Rather than refute opponents’ arguments, he will present the Hindus’ theories "exactly as they are", and will also cite similar Greek theories so that the connection between the two becomes clear. This attempt, 1,000 years ago, to understand another civilisation through its own language and books is still regarded as exemplary.

He speaks openly about the difficulty of language too. By his account, he would sometimes write down a word he had heard from a Hindu with great care, and when he read it back to him, the man could not recognise it.

A map of our region, 1,000 years ago

"Kitab al-Hind" is of special interest to readers in Pakistan, because many of its pages concern the very lands where we live today.

On the Makran coast, al-Biruni writes that the coast of India begins at Tiz, the capital of Makran, and stretches from there south-east to the region of Daibal, a distance of 40 farsakhs. His "coast of India" thus begins in Makran and runs towards Sindh. He also notes that those going to Sindh leave by the road through Sistan, while the road to the rest of India runs through Kabul. In the mountains on India’s western frontier, he says, Afghan tribes live up to the vicinity of the Indus valley.

Multan appears repeatedly in his book. He quotes the method of calculation of Durlabha, an astronomer of Multan, and on his authority writes that the astronomers of Multan began the year with the month of Chaitra. In the chapter on rivers, he writes that the rivers of Punjab join below Multan to form a single river.

The most interesting item is a short list. Al-Biruni writes that he measured the latitudes of some cities himself, and then names Ghazni, Kabul, Peshawar (Purshawar), Waihind, Jhelum, Sialkot and Multan, along with "the fort of Nandana". In other words, this traveller was stopping at place after place in what is now Khyber Pakhtunkhwa and Punjab to observe the sky.

The summit of Nandana: how the Earth was measured

In the time of the Abbasid caliph Mamun, astronomers had travelled across open country and measured the length of one degree of the Earth. Al-Biruni wished to test this himself before relying on it. He thought of an easier way that did not require a long journey across open country: to see how far the horizon appears to dip when viewed from the top of a high mountain.

In 1951 the Iran Society of Calcutta published the "Al-Biruni Commemoration Volume". In it, Syed Hasan Barani translated this sentence from one of al-Biruni’s books: "When I was staying in the fort of Nandana in the land of India, and saw a high mountain to its west and a plain to its south, it occurred to me to try this method here."

The method was this. First, find the height of the mountain. Then, standing on the summit, measure the angle at which the horizon of the plain appears to meet the sky. From the height and this angle, the radius of the Earth can be worked out by trigonometry. Nandana suited the task because it had a high mountain on one side and a level plain stretching far away on the other. In Barani’s estimate, the measurement was made at some time between 407 and 416 AH (roughly 1016 to 1025). According to another of al-Biruni’s writings, he first determined the height of the Nandana hill, and then measured the dip of the horizon from its summit.

According to Barani, al-Biruni gives the radius of the Earth in his major work, "Al-Qanun al-Masudi", as 12,851,370 cubits, which by Barani’s calculation comes to about 3,939 miles. The modern mean radius of the Earth is about 6,371 kilometres, or roughly 3,959 miles. On Barani’s figures, al-Biruni’s result is thus only about half a per cent below the modern value. Barani also reminds readers that the bending of light in the air (refraction) made some error in this method unavoidable.

Nandana today

According to a 2021 report in Dawn, the Nandana fort stands on the eastern edge of the Salt Range in Pind Dadan Khan tehsil of Jhelum district. The report said the fort had long been in a dilapidated state and that a plan to conserve it was under consideration.

Al-Biruni’s journey reminds us that exploration is not only a matter of crossing new seas. Learning a foreign language, reading others’ books through their own eyes, and measuring the curvature of the Earth from a mountain top are also great expeditions. And one of them was accomplished on a hill in our own Jhelum.