Ulugh Beg Observatory
How precisely can you measure stars without a telescope?
That time, that place
A grandson of Timur and the ruler of Samarkand — whose real interest was not governing but the sky. He founded a madrasa in the city and gathered the finest mathematicians and astronomers of the age.
Why this question
Before the telescope there was only one way to observe more precisely: build the instrument larger. The wider the gap between two marks, the finer you can read what falls between them.
What was found
His sextant had a radius of some forty meters. Too large to stand above ground, it was sunk into the earth: a vast arc cut below the surface, along which the sighting apparatus slid.
With it he remeasured the positions of 1,018 stars from scratch — not a revision of Ptolemy's catalogue but a new one, the first since Hipparchus. His figure for the length of the year differs from today's by less than a minute.
It was close to the limit of what the unaided eye can reach.
Star tables were something you corrected in Ptolemy
The positions of 1,018 stars remeasured with a forty-meter sextant
In 1449 he was killed by an assassin sent by his son and the observatory was destroyed. Only the catalogue survived, published in seventeenth-century Europe