Blue Planet

by Cesare Petitti
teacher of technical education
at the lower middle school and instructor for the CEMEA
(Centres of Practice in Methods of Active Education)

The astrolabe
An instrument for getting to know the stars

This instrument, whose invention is attributed to the Greek philosopher Hipparch (3rd century B.C.), makes it possible to recognize the stars present in the night sky and work out their relative direction and height. Its name derives from the Greek and literally means "star catcher".

L'astrolabe at the Astronomical Observatory of Arcetri in Florence, Italy

It is thought that Hipparch managed to list as many as 1500 stars, establishing their distance from the Pole Star (declination) and movement in time (ascension line).The first representation the celestial vault was a sphere with the stars drawn on it; this was followed later by the "armillary spheres" (a sphere created with rings). The celestial maps on the flat (celestial planispheres), that led to the construction of astrolabes, did not appear until some time later.

The astrolabe was introduced to the Western world by the Arabs and reached its maximum splendour during the Renaissance. Some of the finest examples can be found in the Museum of Science in Florence. This instrument has moreover been fundamental for navigation and was an essential element in the great geographical discoveries. Even today, however, this instrument is extremely useful for learning the names of the stars and constellations.