One of the oldest of all the altitude measuring devices, the astrolabe is an angle-measuring tool. Its name comes from the Greek, “to take a star.”
Likely invented by Hipparchus, an ancient mathematician, the astrolabe was originally used to tell the time based on the location of the sun, moon, and stars in the sky. However, the astrolabe comes in many sizes and designs and has many different uses, so many that Persian astronomer, Abd al-Rahmân, wrote 386 chapters on the 1,000 uses of an astrolabe.
The astrolabe is simply an angle-measuring tool. In the beginning of its existence, the astrolabe was used for astronomy, for the study of the stars, and for telling time – but not necessarily for navigation. The astrolabe was adopted for sea-going use when sailors learned that they could navigate by measuring the angle between Polaris (the North Star) and the horizon. This version of the astrolabe is called the mariner’s astrolabe. The first documented use of an astrolabe at sea is in 1481, on a voyage down the African coast by Portuguese explorers.
So how does it work? To correctly measure the angle of the sun or a star, the astrolabe must hang down so that it is perpendicular (⊥) to the ocean. To keep it straight, the user holds it with a finger through the ring at the top and lets the astrolabe dangle. Next, the navigator looks through two pinholes in a rotating arm and tries to line up both holes with the star they are measuring. The navigator then reads the angle measurement from the scale around the circumference of the astrolabe. This measurement would help the navigator find his location.
Other navigational instruments are impossible to use when the horizon is not visible. But one advantage of the astrolabe is that a clear horizon is not needed to use the instrument. This means that a navigator can take measurements at night, or on foggy days when it might be hard to see the horizon. However, the astrolabe is not a particularly accurate tool at sea because it is difficult to keep it steady on a rolling ship and in high winds. Using an astrolabe at sea could result in errors of as much as five degrees, or 300 miles. In order to avoid miscalculation at sea, Portuguese explorers would take their astrolabes ashore and preset them. Astrolabes set on steady ground were much more accurate, being less than one-half degree, or 30 miles off at the most. By setting their astrolabes before sailing, the Portuguese explorers were able to accurately map the coast of Africa in their early exploration.
Waldman, Carl, and Alan Wexler. Encyclopedia of Exploration, Vol. 1-2. New York: Facts On File, 2004.
The Astrolabe. Accessed January 19, 2016. http://www.astrolabes.org/index.htm.