The astrolabe is a very ancient astronomical computer for solving problems relating to time and the position of the Sun and stars in the sky. Several types of astrolabes have been made. By far the most popular type is the planispheric astrolabe, on which the celestial sphere is projected onto the plane of the equator.

A typical old astrolabe was made of brass and was about 6 inches (15 cm) in diameter, although much larger and smaller ones were made.

Astrolabes are used to show how the sky looks at a specific place at a given time. This is done by drawing the sky on the face of the astrolabe and marking it so positions in the sky are easy to find.

To use an astrolabe, you adjust the moveable components to a specific date and time. Once set, the entire sky, both visible and invisible, is represented on the face of the instrument. This allows a great many astronomical problems to be solved in a very visual way. Typical uses of the astrolabe include finding the time during the day or night, finding the time of a celestial event such as sunrise or sunset and as a handy reference of celestial positions. Astrolabes were also one of the basic astronomy education tools in the late Middle Ages. Old instruments were also used for astrological purposes.

The typical astrolabe was not a navigational instrument although an instrument called the mariner’s astrolabe was widely used. The mariner’s astrolabe is simply a ring marked in degrees for measuring celestial altitudes. The history of the astrolabe begins more than two thousand years ago.

The principles of the astrolabe projection were known before 150 BC, and true astrolabes were made before AD 400.

The astrolabe was highly developed in the Islamic world by 800 and was introduced to Europe from Islamic Spain (Andalusia) in the early 12th century. It was the most popular astronomical instrument until about 1650, when it was replaced by more specialized and accurate instruments. Astrolabes are still appreciated for their unique capabilites and their value for astronomy education.

Origins of Astrolabe Theory

The origins of the astrolabe were in classical Greece. Apollonius (ca. 225 BC), the great codifier of conic sections, probably studied the astrolabe projection. The most influential individual on the theory of the astrolabe projection was Hipparchus who was born in Nicaea in Asia Minor (now Iznik in Turkey) about 180 BC but studied and worked on the island of Rhodes.

Hipparchus, who also discovered the precession of the equinoxes and was influential in the development of trigonometry, redefined and formalized the projection as a method for solving complex astronomical problems without spherical trigonometry and probably proved its main characteristics.

Hipparchus did not invent the astrolabe but he did refine the projection theory.

The earliest evidence of use of the stereographic projection in a machine is in the writing of the Roman author and architect, Vitruvius (ca. 88 - ca. 26 BC), who in De architectura describes a clock (probably a clepsydra or water clock) made by Ctesibius in Alexandria. Apparently, Ctesibius’ clock had a rotating field of stars behind a wire frame indicating the hours of the day.

The wire framework (the spider) was possibly constructed using the stereographic projection with the eye point at the north celestial pole.

Similar constructions dated from the first to third century and have been found in Salzburg and northeastern France, so such mechanisms were apparently fairly widespread among Romans.

The first major writer on the projection was the famous Claudius Ptolemy (ca. AD 150) who wrote extensively on it in. his work known as the Planisphaerium.

There are tantalizing hints in Ptolemy’s writing that he may have had an instrument that could justifiably be called an astrolabe. Ptolemy also refined the fundamental geometry of the Earth-Sun system that is used to design astrolabes.

Early Astrolabes

No one knows exactly when the stereographic projection was actually turned into the instrument we know today as the astrolabe. Theon of Alexandria (ca. 390) wrote a treatise on the astrolabe that was the basis for much that was written on the subject in the Middle Ages. Synesius of Cyrene (378-430) apparently had an instrument constructed that was arguably a form of astrolabe.

This is plausible since Synesius was a student of Hypatia, Theon’s daughter. The earliest descriptions of actual instruments were written by John Philoponos of Alexandria (aka. Joannes Grammaticus) in the sixth century and a century later by Severus Sebokht, Bishop of Kenneserin, Syria, although it is likely that Sebokht’s work was derivative of Theon. It is certain that true astrolabes existed by the seventh century.

The Astrolabe in Islam

The earliest surviving Arabic astrolabe treatises are from the seventh and eighth centuries and are often translations of earlier Greek or Syriac texts. Eighth century literary references from Baghdad and Damascus indicate that by this time the use of the astrolabe was widespread throughout the Arab world. Land under Arab control stretched from North Africa and Spain to India, enabling a wide range of astronomical influences to be combined. The early ninth-century tables of al-Farghani list the radii of the circles on the plate of the astrolabe for each degree of latitude.

These simplified the process of astrolabe construction by removing the need for mathematical calculation of these values, indicating that astrolabes were being manufactured in substantial numbers since the effort involved in producing the tables would have been considerable. The earliest surviving Islamic astrolabes date from the ninth century, and these are of such quality and craftsmanship that they represent a continuing tradition rather than a new activity.

The astrolabe was inherently valuable in Islam because of its ability to determine the time of day and, therefore, prayer times and as an aid in finding the direction to Makkah.

By the eleventh and twelfth centuries there are many surviving texts and astrolabes, the instruments varying in style and artistry but retaining many fundamental similarities in functionality and design.

Persian astrolabes became quite complex, and some were genuine works of art.

There are a number of interesting stylistic differences between astrolabes from the eastern Islamic areas (the Mashriq), Northern Africa (the Maghrib) and Moorish Spain (Andalusia).

The astrolabe was also used in Muslim India in a simplified and less artistic form.