(973–1052?). Al-Biruni was the most original scholar of the medieval Islamic world. He wrote some 150 books, about half of them on astronomy or mathematics. The others explored a wide range of subjects, from physics to gems to world cultures.

Al-Biruni was born on September 4, 973, in Khwarezm, in what is now Uzbekistan. Little is known of his early life. He was educated by a prince who was also a mathematician and astronomer. Al-Biruni lived during a period of political turmoil in the eastern Islamic world. Civil wars among local dynasties forced him to move frequently, and he served more than six different princes. During his stay in the city of Gurgan (now in northern Iran), he met the famous philosopher-scientist Avicenna (Ibn Sina). The two wrote letters to each other in which they shared scientific ideas.

In 998 the leader Mahmud seized the throne of the kingdom of Ghazna, which included the land that is now Afghanistan and northeastern Iran. He demanded that both al-Biruni and Avicenna join his court at Ghazna (now the city of Ghazni, in eastern Afghanistan). Avicenna managed to escape, but al-Biruni did not, and he worked in Ghazna until the end of his life. He traveled with Mahmud on some of the sultan’s many military expeditions to India. During these trips al-Biruni recorded observations about Indian life, language, and religion. He published these in what would become his most famous book. He died in Ghazna about 1052.

Al-Biruni was a master in applying mathematical theories to astronomy. He worked with the results of his own observations as well as those of other astronomers. He used principles of geometry to determine latitudes and longitudes, the distances between cities, and the direction from one city to another. The lunar eclipse of July 5, 1004, provides an example of his methods. Al-Biruni used timings of the eclipse at Ghazna and a city in what is now Turkmenistan to estimate the longitude difference between the two cities as 10.2°. His estimate was quite accurate—the correct figure is 9.3°. This technique was later widely adopted in Europe. Al-Biruni also made astronomical instruments and wrote a book about the astrolabe.

In physics, al-Biruni made important contributions to mechanics, the study of the motion of objects under the action of forces. He made studies of matter in motion and of matter at rest. Al-Biruni experimented with using balances to measure the weight and density of materials. He created a type of hydrostatic balance. This is used to weigh an object by submerging it in water and then measuring the liquid it displaces. He used this instrument to determine the density of gems. Along with his many other accomplishments, al-Biruni is remembered as one of the first scientists to apply experimental methods to the study of mechanics.