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.