Astrolabe

The Astrolabe was an ancient multipurpose astronomical instrument that provided early astronomers with a two-dimensional model of the celestial sphere with which to measure the position of celestial objects, measure the time of night or the time of year, and determine the altitude of any celestial object above the horizon. Historically, the astrolabe was an elaborately inscribed brass disk that functioned as the equivalent of a mechanical astronomical computer.

The earliest known astrolabes appeared around 300 to 200 B.C. and were possibly used by the famous Greek astronomer Hipparchus. The most important part of the traditional astrolabe was a circular brass plate typically about 15 centimeters or so in diameter with a convenient ring at the top by which the instrument could be suspended and hang perfectly vertical. Engraved upon the backside of the brass disk were several circles hosting convenient gradations, such as 360 degrees, 365 days, 12 months, and so forth. In time these backside engravings could also accommodate trigonometric calculations. The front of the brass disk also had several engraved circles. The outer circle usually had 24 divisions, corresponding to the hours in a day. Another circle was divided to serve as a calendar, using the constellations of the zodiac as a convenient reference. The celestial pole (northern or southern) was engraved in the central part of the brass front, as were lines representing the tropics and the equator.

Astronomers would place another disk (called the sky disk) on the front of the astrolabe in such a manner that they could rotate this removable sky disk. The thin brass sky disk had many opening cut into it. These cuts were precisely arranged so an astronomer could see the engraved front of the brass disk that made up the body of the astrolabe. By carefully adjusting each sky disk, early astronomers could determine the visible parts of the sky and the altitude of different celestial bodies for various times of the year and for different latitudes. The astrolabe also had a pivotal pointer or ruler attached to the center of the backside of the central brass plate. Early astronomers would suspend the astrolabe by its ring. Once the device was hanging in a proper vertical position, they would align the pointer with some target celestial object and then read its altitude from an appropriate measurement scale engraved on the backside of the instrument.

Questions to Ponder:

• List out a few developments in Astronomy owing to the Astrolabe.
• What are the uses of the Astrolabe?