A circa 1560 Astrolabe - Attributed to Giovanni Battista Giusti - Italy

The planispheric astrolabe is a two-dimensional model of the celestial sphere in relation to the earth, based on the assumption that the earth is in the centre of the universe. It is a multifunctional instrument and can be used to tell the time, to determine the length of day and night, to simulate the movements of the heavenly bodies, for surveying and for astrological purposes.

The origins of the astrolabe are shrouded in mystery, but the underlying theory for its construction, the stereographic projection, may have been familiar to Hipparchus in the 2nd century B.C. and its seems certain that the instrument was well known in the 1st century A.D. An Islamic tradition attributes the invention of the instrument to the renowned astronomer Ptolemy (2nd century A.D.) who, when riding on a donkey and pondering on his celestial globe, dropped the globe. The beast trod on it and the result was the celestial sphere in two dimensions.

The name 'astrolabe' has a Greek origin and means essentially 'star holder'. Via the Arabic form the name came back to medieval Europe.

The earliest surviving astrolabe treatise was compiled by Theon Alexandrinus in the 4th century A.D., followed by a few more Greek texts on the same subject. With the introduction of Greek science to the Islamic civilisations through translations starting in the 9th century, Islamic treatises discussed in great detail and variation the construction and design of the instrument. One of the most influential of these, compiled by M?'sh?'a allah (Messehalla) in the 9th century, greatly influenced Chaucer in the late 14th century when he wrote a treatise compiled for his son Lewis. Through the Islamic conquest of parts of Europe and the translation of Arabic treatises into Latin the instrument was reintroduced to the Latin West and widely used until the 17th century.

The earliest astrolabe to survive is an Islamic instrument dated 927-28 A.D.

The basic design has not changed throughout the centuries, although many features were added for different purposes.

Essentially the planispheric astrolabe consists of the celestial part (the 'rete'), the terrestrial parts (the 'plates'), a thick brass plate with a rim (the 'mater'), an index for the front (the 'rule') and another one for the back with additional sights (the 'alidade'). All these parts have a central hole so that they can be assembled by means of a pin and a wedge.

Use of the Astrolabe

The astrolabe is a multifunctional instrument and can be used for different purposes of which the most prominent ones are time-telling, simulation of the movements of the stars, casting horoscopes, and surveying.

In use for time-telling, the sighting vanes are used to observe either a fixed star or the sun. In the case of the sun one would not look directly into it, but hold the astrolabe and orient the alidade so that the sun's rays can pass directly through both sighting vanes. The point where the alidade intersects with the altitude scale on the back determines the altitude of the sun at this particular time of day. From the calendrical scale also on the back one reads off the corresponding degree of the sign the sun is in on that date.

Turning to the front one opens the instrument and takes out first the rete, then the plates. Then one chooses the plate with the appropriate latitude and puts this on top of the other plates and fixes the rete on top of it. With the reading taken from the back one aligns the corresponding part of the ecliptic with the appropriate line of altitude (almucantar) on the plate. As soon as this intersection is found one lets the rule intersect with this point. The rule then points to one of the hour markings on the rim and this is the time in equal hours. To find the time in unequal hours one finds the point where the other arm of the ruler intersects with the opposite part of the ecliptic. Where this point intersects with the unequal hour line on the plate underneath one reads off the time in unequal hours. All the parts of the rete (i.e. the star pointers) now above the horizon line on the plate underneath can be seen in the sky at this particular time, all those below are out of sight.

For use in surveying the shadow square is used and the calculations are based on the principle of similar triangles. To measure for example the height of a tower one determines the distance one is standing away from the tower (for example 24 feet). Then one holds the instrument up and orientates it until one can see the top of the building through both sights of the alidade. The point on the shadow square where the alidade intersects with the 'Umbra recta' scale is read off (for example 6).

If the distance from the tower is O to B and the height of the tower A to B (plus the size of the observer who is holding the instrument at eye's level), while the full length of the 'Umbra versa' scale is a to b and the point of intersection on the 'Umbra recta' scale o to b, one gets the following equation: OB : AB = ob : ab. Since OB, ob and ab are known to be 24, 6 and 12 one can then determine AB, in this case 48 feet. To obtain the total height of the tower one only has to add the height of the observer's eye (for example 6 feet). The total height is 54 feet.

While originally only usable at particular latitudes, changes to the design in 16th and 17th-century Europe led to the development of the much more convenient universal astrolabe, for which different projections were used. These instruments, such as the 'astrolabum catholicum', can be universally used and differ from the above described ones as follows:

The universal projection is normally to be found on the back of the astrolabe (the features otherwise on the back are now on a plate). The vernal equinox is in the centre of the instrument and the equator is represented by the horizontal diameter. Depending on the projection used the meridians appear vertically as arcs of circles or semi-ellipses, while the declinations appear horizontally as arcs or straight lines. Attached to the back is a 'regula' with a 'brachiolus' and a 'cursor', essentially two graduated bars and a movable index arm.

H. Michel, Trait? de l'Astrolabe (Paris, 1947); J. North,"The Astrolabe", Scientific American, 230 (1974), pp. 96-106; [National Maritime Museum, London], The Planispheric Astrolabe (London, 1976); H. N. Saunders, All the Astrolabes (Oxford, 1984); A. J. Turner, The Time Museum Catalogues: Vol. 1, Time Measuring Instruments, Part 1, Astrolabes, Astrolabe Related Instruments (Rockford, 1985).

(The above borrows heavily from the EPACT Glossary - Epact is an electronic catalogue of medieval and renaissance scientific instruments from four European museums: the Museum of the History of Science, Oxford, the Istituto e Museo di Storia della Scienza, Florence, the British Museum, London, and the Museum Boerhaave, Leiden. Together, these museums house the finest collections of early scientific instruments in the world.)