Arabic Astrolabe: Definition, Navigation & History

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Imagine you had a device in your pocket that could tell you what time it was and where you were at any time of day or night. It might sound like a recent invention, but over a thousand years ago, people used a device like this, called an astrolabe, to make complex astronomical observations and navigate the world.

What is an Astrolabe?

When you want to know what time it is, what do you do? Look at a watch or your phone? Today, we have technology that can tell us exactly where we are and what time it is wherever we are. Imagine that you were living in the Middle East or Europe 900 years ago. Would you still be able to know where you were and what time it was? You certainly wouldn't have access to the kinds of technology that we now take for granted. However, it is likely that you might have a small handheld device called an astrolabe, with which you could calculate your exact location and the time of day.

An astrolabe from the 13th century
pictures of astrolabes

Astrolabes were amazing technological devices that allowed people to calculate not only the time of day (or night), and their precise location on Earth, but also to measure the positions of celestial objects like stars, determine the time of year, and determine what part of the sky would be visible at any given time. They were small and portable, and one of the most amazing inventions of the time.

How Does an Astrolabe Work?

Long before we had GPS and atomic clocks, people oriented themselves and kept time by looking upward at the stars and sun. Astrolabes took advantage of this, allowing people to make complex astronomical calculations easily. The basic idea behind an astrolabe is that you can line it up with objects above the horizon and find their altitude.

By lining up parts of the astrolabe with an object above the horizon, it is possible to calculate the altitude of the object.
Astrolabe diagram

Of course, people quickly developed this simple idea much further and added a variety of scales and measuring devices to the astrolabe, enabling it to be used for everything from timekeeping to navigation. A version of the astrolabe, called a mariner's astrolabe, was even developed for navigation on ships, where the motion of the waves made it difficult to use a traditional astrolabe.

Maritime astrolabes like this were used to navigate at sea for many years.
maritime astrolabe

Invention of the Astrolabe

Although we don't know exactly when the astrolabe was invented, we do know that a Greek man named Hipparchus, who lived in the 2nd century B.C., played a big role in its development. Hipparchus was the first to describe a method of using projection to solve astronomical problems, and this is the basis of how all astrolabes work.

In the 2nd century A.D. in Alexandria (modern-day Egypt), another scientist named Ptolemy wrote extensively about projection and the geometric relationships between the Earth and the sun. We don't know for sure that he had an actual astrolabe to help him make these calculations, but it is likely that he had one or something very much like it.

Although the early development of the astrolabe is still mysterious, we know for sure that true astrolabes were in use in the Middle East and North Africa by the 6th century, and probably had been for some time.

The Astrolabe in the Arabic World

The astrolabe was first invented in North Africa and it quickly spread throughout the Middle East. Astrolabes were first introduced to the Islamic world in the 8th century, and as Islam spread throughout the Middle East, North Africa, and Asia, the astrolabe went with it. Eventually, the astrolabe would reach Europe in the 1100s through Islamic settlements in southern Spain. Throughout the next few centuries, astrolabes continued to be refined and improved by Arabic scholars, and turned into true works of art. Many astrolabes were produced during this time period, and some Arabic astrolabes that were made in the 10th and 11th centuries are still around today!

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