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