An alloy for the ages
In the 1420s, the Timurid ruler and astronomer Ulugh Beg constructed a scientific institution in Samarkand that had no equal. The centerpiece was a three-story circular building, 46 meters in diameter, housing a massive meridian arc. This instrument, often called the Fakhri sextant, had a radius of 40.04 meters. Its primary function was to measure the precise positions of celestial objects. A major challenge for any such large instrument is thermal expansion—the tendency for materials to change size with temperature, which introduces errors into observations. Samarkand's desert climate produces significant temperature swings, making this problem acute.
The solution lay in the metal used for the observatory's portable instruments, like astrolabes and quadrants. While the massive sextant was made of marble and brick, analysis of smaller instrument fragments reveals a sophisticated bronze alloy. Typically, 15th-century Islamic brass was an alloy of copper and zinc. The Samarkand instruments, however, were made from a quaternary bronze containing specific proportions of copper, lead, tin, and zinc. This particular mix of metals created a material with a very low coefficient of thermal expansion. The alloy remained stable across a wide temperature range, ensuring that measurements taken on a cold night were directly comparable to those taken on a warm day. This metallurgical innovation was a factor in the observatory's success.
Precision before the telescope
The stability of the instruments enabled Ulugh Beg and his team of around 60 scientists, including the brilliant Jamshid al-Kashi and Ali Qushji, to compile the Zij-i Sultani star catalog. Published around 1437, it was the most comprehensive and accurate catalog of its time. It listed the positions of 1,018 stars, with many redetermined from new observations rather than relying on older data from Ptolemy. The accuracy of these new observations surpassed anything that had come before.
The data produced at Samarkand was so precise that Ulugh Beg calculated the length of the sidereal year to be 365 days, 6 hours, 10 minutes, and 8 seconds—a value off by only 58 seconds from the modern calculation. A later calculation from the observatory was even more accurate, with an error of just 25 seconds. This was more precise than Nicolaus Copernicus's determination made 88 years later. He also determined the Earth's axial tilt to be 23.5047 degrees, a remarkably accurate figure. These achievements were made possible by the immense size of the main sextant, but by the advanced material science that kept the working instruments true. The observatory was destroyed in 1449 after Ulugh Beg's assassination, and its ruins were rediscovered by archaeologist V.L. Vyatkin in 1908.