Mirzo Ulugbek (1394–1449) was the grandson of Amir Timur, ruler of Samarkand and one of the foremost astronomers of his age. Under him the city became a centre of learning: Ulugbek built a madrasah on the Registan and a famous observatory with a giant sextant. The star catalogue produced by his school, the Zij-i Sultani, charted the positions of over a thousand stars with an accuracy Europe would not surpass for two centuries.