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A curious mind
The greatest of these scholars was Abu Raihan al-Biruni. Born in 973 near the Aral Sea in what is now Uzbekistan, while still a youth Biruni mastered mathematics, astronomy, mineralogy, geography, cartography, geometry and trigonometry. He spoke Persian, Arabic and Khwarazmian, the language of the Sunni dynasty that ruled Greater Iran between the 12th and 13th centuries. Later on he also studied Sanskrit.
While still a young man Biruni had calculated the latitude and longitude of his home town and had begun to collect similar co-ordinates for other places. Using ancient Greek sources he compiled data on hundreds of locales in the Mediterranean world and then began adding calculations on other locations from all points of the compass. From ancient writers like Claudius Ptolemy (c.150 BC), from more recent sources and from his own field observations he knew that the earth is round. By the time he was 30 Biruni was employing the most advanced systems of the day to calculate its precise circumference. In a pioneering effort not matched until the Renaissance he constructed a globe 16ft high showing the Earth’s terrestrial features.
Biruni followed in the footsteps of several other scientists from Central Asia. Among them was Ahmad al-Farghani, from what is now Uzbekistan, who in the ninth century had calculated the width of one degree of longitude at the equator, from which he deduced the earth’s circumference. His calculation, though less precise than Biruni’s, marked a significant improvement on those made by the ancient Greeks and assured a wide readership for his book on the subject, A Compendium of the Science of the Stars (c.833). Five centuries later Columbus came across a Latin translation of Farghani’s treatise. Besides welcoming confirmation that the Earth is round, Columbus used Farghani’s data to argue before sceptical potential sponsors that it was small enough for him potentially to circumnavigate. However, Columbus wrongly assumed that Farghani had presented his measurements in Roman miles rather than Arab miles. This caused him to understate the actual circumference of the earth by 25 per cent. His misreading caused (or, if it was deliberate, enabled) Columbus to place Cipango, or Japan, near the Virgin Islands. This convenient error proved crucial in Columbus obtaining funding for what he estimated would be a relatively short voyage to China.
Biruni had also delved into mineralogy, specifically the relative density and weight of minerals of all types and how the separate minerals interact in nature. In the process of this research he discovered the concept of specific gravity.
Just how Biruni acquired his passion for precise measurement is a mystery. It certainly owed something to his education, which included study of the classical Greek scientist Pythagoras, who had proclaimed that ‘things are numbers’. Biruni’s constant urge to quantify whatever he observed, combined with his enquiring mind, was to plunge him down a path that led to epochal insights, which in most respects put Columbus, Cabot and the Vikings in the shade.
By 1017 Biruni had become an honoured scientist at Gurganch, the intellectual capital of his home region of Khwarazm. But in that year a fierce and religiously fanatical Muslim ruler from Ghazni in Afghanistan, crushed Khwarazm and destroyed its capital. Mahmud of Ghazni, as he was known, was a brutal man but, like many rulers in the region, tried to surround himself with poets and learned scholars. He ordered Biruni to come to Ghazni and bring the results of his research with him.
Biruni, with no way out, not only complied but seized upon the move as an opportunity to learn more about India, which Mahmud had conquered over the previous decade. But Mahmud was as difficult as he was ruthless and Biruni quickly realised that he had to distance himself from his court. He removed to Lahore, now in Pakistan, where he penned the world’s first book on comparative religion, focusing on Hinduism and Islam. Gathering his notes and no equipment other than a simple astrolabe, he then withdrew to a heavily fortified hilltop castle at Nandana, not far from what is now Islamabad.
There Biruni returned to the old problem of measuring the earth’s circumference. To this end he devised a new technique, which involved careful observation, spherical trigonometry and the application of the law of sines. Besides being far simpler than using two distant points on flat land, this method produced a measure of the earth’s circumference that was a mere 10.44 miles less than the definitive modern measurement.
After Mahmud’s death in 1030, Biruni hauled his field notes and papers back to Ghazni in Afghanistan, where Mahmud’s son, Masud I (r.1031-40), welcomed him and helped him to settle into a quiet life of research and writing. Biruni wrote up his lifetime’s research on specific gravity and then turned to writing a vast tome, known as the Codex Masudicus, in which he summarised everything known at the time about astronomy and allied disciplines.
It was in the Codex Masudicus that Biruni considered the possibility that the sun is stationary and that the earth revolves around it. He stopped short of fully embracing a heliocentric view, noting instead that the notion of a heliocentric universe is no less logical than its alternative and called on mathematicians and astronomers either to refute it or accept it. It is no wonder that historians of science judge the Codex Masudicus to be the greatest work on astronomy from the period between late antiquity and the modern era. In his codex Biruni also hypothesised about the existence of North and South America.
Biruni began by presenting the research on the earth’s circumference that he had carried out at Nandana. He then set about fixing all known geographical locations onto his new, more accurate map of the globe. His list of longitudes and latitudes had grown substantially since his earliest collection and now included more than 70 sites in India alone, as well as hundreds of other locations stretching across the Eurasian land mass.
When Biruni transposed these data onto his map of the earth he noticed at once that the entire breadth of Eurasia, from the westernmost tip of Africa to the easternmost shore of China, spanned only about two fifths of the globe. This left three fifths of the Earth’s surface unaccounted for.
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