Al-Biruni and His Study of India: A Scholar in Ghazni

Al-Biruni was a scholar from Central Asia who came to Ghazni around 1017 in the train of the conquering sultan Mahmud and went on to write the most remarkable medieval study of India. He learned Sanskrit, studied Indian science and religion from their own texts, and completed his book on India around 1030. Along the way he measured the radius of the Earth with surprising accuracy.

Early life in Khwarazm

Abu Rayhan Muhammad ibn Ahmad al-Biruni was born on 4 September 973 near Kath, a capital of Khwarazm, the region around the delta of the Amu Darya river south of the Aral Sea. His name comes from the Persian word for “outside”, probably meaning he came from the outer district of the town.

He studied mathematics and astronomy young and corresponded with the philosopher and physician Ibn Sina, debating questions of physics and Aristotle’s ideas in a series of letters. Around 1000 he wrote his Chronology of Ancient Nations, a comparison of the calendars and festivals of many peoples, for the ruler of Gorgan. He then served the Ma’munid rulers of Khwarazm at Gurganj.

Coming to Ghazni

In 1017 Mahmud of Ghazni, the powerful Turkish ruler of what is now Afghanistan and eastern Iran, conquered Khwarazm. He took a number of its scholars back to Ghazni, al-Biruni among them. Whether al-Biruni went as a prisoner, a hostage or a reluctant courtier is unclear, but he spent most of his remaining life under Ghaznavid rule.

Mahmud led repeated raids into northern India, including the sack of Somnath in 1025. Al-Biruni travelled in the Punjab, then under Ghaznavid control, and met Indian scholars, from whom he learned Sanskrit. He could now read Indian works on astronomy, mathematics and philosophy directly.

Measuring the Earth

At the fort of Nandana, in the Salt Range of the Punjab, al-Biruni carried out his most famous measurement. He first found the height of a nearby mountain using an astrolabe and trigonometry. From the summit he then measured the dip of the horizon, the small angle by which the horizon lies below the true horizontal.

From these two values he calculated the Earth’s radius. His result corresponds to about 6,340 km, within about 1 percent of the modern mean radius of roughly 6,371 km, though modern scholars note that some of the agreement owes to luck, because the dip angle is very hard to measure precisely. His method, which needed only one observer at one place, was nonetheless elegant and original. Earlier measurements, such as the one ordered by the Abbasid caliph al-Ma’mun in the ninth century, had required teams to measure a long distance on the ground.

The book on India

Al-Biruni’s study of India was completed around 1030, soon after Mahmud’s death. Its full Arabic title can be translated as “Verifying all that the Indians recount, the reasonable and the unreasonable”. In some 80 chapters it covers Hindu beliefs about God and the soul, sacred texts, the caste system, marriage and law, festivals, weights and measures, geography, and above all Indian astronomy and calendars.

What makes the book exceptional is its method. Al-Biruni insisted on describing Indian beliefs from Indian sources, often quoting Sanskrit texts in translation, and he compared them with Greek philosophy to show their similarities. He was frank about what he considered errors, but tried to report fairly, and he criticised the hostility that Mahmud’s raids had created among Indians towards foreigners. He also translated Sanskrit works on yoga and Samkhya philosophy into Arabic.

Science and open questions

Al-Biruni was also a careful critic of his own field. He discussed whether the Earth might rotate on its axis, noting that the idea did not contradict astronomical observations, but concluded that the question belonged to natural philosophy rather than mathematics. He measured the specific gravity of many metals and gems with notable precision and compared Greek, Persian, Indian and Arabic calendars in great detail.

Later works and legacy

Al-Biruni dedicated his great astronomical encyclopedia, al-Qanun al-Mas’udi, to Mahmud’s son Mas’ud. He also wrote on precious stones and minerals and on medicinal drugs, the last of these in his old age. He died in Ghazni around 1050, still working.

His book on India was little known in Europe until the German scholar Edward Sachau published the Arabic text and an English translation, Alberuni’s India, in 1887–1888. Today he is honoured in Uzbekistan, Iran, Afghanistan and Pakistan, and a lunar crater bears his name.

Timeline

• 973, 4 September: Al-Biruni is born in Khwarazm.
• c. 1000: He writes the Chronology of Ancient Nations.
• 1017: Mahmud of Ghazni conquers Khwarazm; al-Biruni is taken to Ghazni.
• c. 1020s: He travels in the Punjab and measures the Earth’s radius at Nandana.
• 1030, April: Death of Mahmud of Ghazni.
• c. 1030: Al-Biruni completes his book on India.
• c. 1050: Al-Biruni dies in Ghazni.

Why it matters

Al-Biruni showed that a scholar could study a very different civilisation with curiosity, accuracy and respect for its own sources. His work on India is still read by historians, and his measurement of the Earth is a landmark of medieval science.