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Culture and history · The sky-measurers

Ulugh Beg and the Samarkand Observatory: observing with a giant arc

In 15th-century Samarkand, an arc about 40 metres in radius, cut into the ground, was used to measure how high the Sun and other celestial bodies stood. Try out the logic of meridian observation and finding latitude from the Pole Star, and see why a bigger instrument is more precise.

  • 18 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

On a hill just outside Samarkand, a giant curved channel runs down into the earth, lined with stone on both sides. At first glance it looks like a staircase or a ramp. Yet about 600 years ago this was the inside of one of the largest angle-measuring instruments in the world. Why would anyone build an instrument the size of a building to measure the sky? And what exactly was measured with this giant arc?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Protractor

The Pole Star at Samarkand

Turn the arm with the slider; the arrow keys move it 1 degree at a time. The horizontal line is the horizon, and the arm is your line of sight. When you're ready, press Check.

Prediction

Predict first

If you travel south from Istanbul (41° N) to Antalya (37° N), how does the Pole Star look?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Number line

How many millimetres is one arcminute?

Drag the marker or move it in 1 mm steps with the arrow keys, then press Check.

Prediction

Predict first

If you double the radius of a protractor, what happens to the length of a one-degree arc?

It doesn’t matter whether you’re right; the simulation opens once you choose.

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Frequently asked

5 questions

Why build an instrument the size of a building to measure the sky?

Because the length of an arc is proportional to its radius. On a 40-metre arc, 1° takes up about 70 cm, so even one arcminute is about 12 mm long and can be marked. Without telescopes, the way to gain precision was to make the instrument bigger.

Did the Samarkand Observatory have a telescope?

No. A telescope was first turned to the sky in 1609. In Samarkand and in every observatory before it, observations were made with the naked eye, using large angle-measuring instruments.

How do you find your latitude with the Pole Star?

The height of the celestial pole above the horizon equals your latitude. In Ulugh Beg's time the Pole Star was about 3.7° away from the pole and traced a small circle during the night. So astronomers measured its highest and lowest positions and took the average.

Why is Ulugh Beg's star catalogue considered important?

It is regarded as the most comprehensive star catalogue since Ptolemy to be based largely on new observations; it is usually described as listing 1018 stars. The same team measured the sidereal year to within about 1 minute of today's value.

Did Ulugh Beg do all this work on his own?

No, it was teamwork. Qadi Zada al-Rumi, born in Bursa, the master calculator Ghiyath al-Din Jamshid al-Kashi and the young Ali Qushji were part of the team. Ali Qushji later came to Istanbul and carried this knowledge to the Ottoman madrasas.