Ulugh Beg Observatory: 15th-century astronomy

Ulugh Beg's observatory sat on Kuhak hill northeast of Samarkand's Registan. Its centrepiece was the Fakhri sextant: a 40-metre-arc quadrant, half of it buried in a trench cut into the bedrock and half rising above ground on a marble arc.

Working with a team of astronomers (including Qadi-zada al-Rumi and later Ali Qushji), Ulugh Beg produced the Zij-i Sultani in 1437 — a star catalogue of 1,018 stars and planetary tables that were the most accurate in the world until Tycho Brahe's work 150 years later.

Their calculations: - Length of the tropical year: 365 d 5 h 49 min 15 s (modern value: 365 d 5 h 48 min 45 s) — accurate to ~25 seconds. - Axial tilt of Earth: 23°30'17" (modern: 23°26'20") — accurate to about 4 minutes of arc.

The observatory was demolished by religious conservatives after Ulugh Beg's assassination in 1449. It was rediscovered in 1908 by archaeologist Vasily Vyatkin, whose grave is next to the excavated sextant trench.

The site now includes a small museum with copies of the star catalogue and models of the observatory.