Astrolabe History

Astrolabe History

An astrolabe is a two-dimensional model of the celestial sphere. The name has its origins from the Greek words astron and lambanien meaning "the one who catches the heavenly bodies. An astrolabe is an instrument that once was the most used, multipurpose astronomical instrument. Historically, astrolabes were elaborately inscribed brass discs. The portability and usefulness of an astrolabe made it something like the multipurpose "lap-top computer" of our predecessors. With an astrolabe, an astronomer could make quite accurate measurements of the following things:

position of celestial objects
measure the time of the night (or of the day, using it as a mobile sundial or, more accurately by measuring the altitude of the sun)
measure the time of the year,
compute what part of the sky is visible at any time,
determine the altitude of any object over the horizon,
determine the current latitude, and
determine (very accurately) the NPS orientation.

While the oldest known astrolabes were created a few centuries BC, possibly by Hipparchus. They were improved and more features were added to them until the Middle Ages, by which time they had become very complex instruments. The Arabian astronomers made extensive use of the astrolabe. One of the best descriptions of the astrolabe and its use was written in 1392 by Geoffrey Chaucer, in England; the TOPS astrolabe is inspired from that text.

The most important part of the traditional astrolabe is a circular plate of metal, usually about 6 inches in diameter, which could be suspended by a ring from which it would hang perfectly vertical. On one side of the disk (the "back") was engraved several circles divided by different kinds of gradations, such as 360 degrees, or 365 1/4 parts for the days, 12 for the months, etc. The engravings could be used for trigonometric calculations. The other side of the plate (which was called the front) was also engraved. The outer circle had 24 divisions for the hours (here numbered by letters). Another circle was divided as a calendar (using the zodiacal constellations). The tropics and equator were engraved in the central part, the celestial pole being at the center of the disk.

Another disk could be fixed on the front side of the astrolabe so that it could rotate. Many openings were cut into that disk in order to let the astronomer see through to the body of the astrolabe. These cuts were made in order to form a map of the sky: a broad annulus corresponding to the zodiac (divided by the constellations) and several "tongues" or "flames" pointing to important stars. Thin engraved disks or paper could also be put between the sky disk and the astrolabe body. By adjusting the "sky" disk, it was possible to determine the visible part of the sky, the altitude of celestial bodies, etc.
A ruler was also generally available, to be fixed on the back of the astrolabe. Suspending the instrument by its ring, one could measure the altitude of a celestial body by pointing at it with the ruler, and reading the measurement off one of the engraved circles.

To purchase astrolabes and other recreations of antique astronomical instruments visit the Saunders and Cooke website, the Classic Science website, or Norman Greene Astrolabes.

Last Updated on April 18, 2000

This page has been visited times since October 1999.
Karen Meech, Institute for Astronomy, University of Hawaii meech@.ifa.hawaii.edu