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.

'Rete' is Latin and means net or web, a description which quite nicely describes its appearance: it is a skeletal brass disc representing the ecliptic and some prominent stars. The ecliptic is marked with the names of the zodiacal signs, and sometimes additionally with the symbols, and a corresponding degree scale. The star pointers can have different shapes (flames, daggers, but also zoomorphic designs like dogs' heads or even court jesters). Normally the names of some selected stars (obviously particularly prominent or bright ones) are given on, or close to, the star pointer. They are either the Latin names or latinized forms of the original Arabic names. Sometimes, particularly on Flemish and Flemish-influenced instruments, the magnitudes of the stars and the corresponding planetary temperaments are also given. The shape of the rete can be very distinctive: for example, the inclusion of a Y-shaped solstitial bar as described by Chaucer, or a tulip design as found in the Flemish school. The rete rotates over the plates.

The latitude plates are brass discs with a small tab at one point for preventing them from moving once in place. They are laid out for different latitudes normally indicated on the plates. This is due to the fact that the apparent celestial movements actually depend on the latitude of the place from where they are being observed.

Plates usually have several circles and lines engraved on them. These can include: circles for the tropics and the equator; almucantars (circles of equal altitude parallel to the horizon) and the azimuths (lines of direction); a twilight curve engraved as a line 18? below the horizon, marking the beginning of the morning twilight and the end of evening twilight; unequal hour lines normally drawn below the horizon representing hours in a system where the time between sunrise and sunset and the time between sunset and sunrise is divided into 12 'hours' each (with the result that depending on the time of year the length of these 'hours' changes, only being of the same length at the equinoxes);
astrological houses dividing the sky into twelve equal parts for astrological purposes (overlapping and sometimes confused with the hour lines), marked according to one of several systems, the most usual one being of Islamic origin, but commonly associated with the 15th-century astronomer Regiomontanus, in which the boundary lines begin at the north point of the observer's horizon.

The almucantars and azimuths are local co-ordinates defining the location of a heavenly body in relation to the observer's horizon. The latitude for which they are constructed is usually marked on each plate, sometimes with the names of corresponding cities. They are analogous to latitude and longitude on earth, defining the location of a star in relation to the celestial equator instead of the location of a place in relation to the terrestrial equator.
The lowest of the almucantars represents the observer's horizon, and on Flemish and Flemish-influenced instruments is sometimes called the 'horizon obliquus'.

The plates and the rete are contained within the mater. The mater consists of a plate with a thick rim riveted or soldered to it. To the mater a suspension apparatus is fixed to a protruding part called the throne. The throne can have different shapes which can be rather distinctive for particular makers. The rim has usually both a 360 degree scale (either 0? to 360? or four quadrants 0? to 90?) and a 24 hour scale, numbered clockwise 1 to 12, 1 to 12. The inside edge of the rim has a small aperture to accommodate the tongues of the plates. The mater, i.e. the inside of the container, can be either left blank, be inscribed for another plate or be engraved with a 'quadratum nauticum'. The latter is a graduated square frequently to be found on Flemish and Flemish-influenced instruments which allows the seaman to find, without calculation, the course between two points of which the latitude and longitude are known. Some astrolabes have the names of the winds inscribed on the edge of the rim.

The back of the astrolabe contains several scales needed for the use of the instrument, but also serves as space for further information which could also be deduced from tables.
The usual main features are: a 360 degree scale; a zodiacal calendar made up of a calendrical scale (either for the Julian or the Gregorian calendar) with the names of the months and day divisions, together with a zodiacal scale with the names of the signs and sometimes their symbols with a degree scale; a shadow square whose scales are normally divided into 12 units, the vertical scale is called 'Umbra versa' (for shadows cast by a horizontal gnomon), the horizontal scale is called 'Umbra recta' (for shadows cast by a vertical gnomon); and an horary quadrant for unequal hours. The combination of the calendrical and the zodiacal scales indicates the date of the equinoxes which move back further and further in the Julian calendar (a reason for the introduction of the Gregorian calendar). Although this cannot be used to reliably date the instrument, it reveals something of the underlying table.

Additionally on the back of an astrolabe there can be further calendrical information such as the solar cycle, tables for the determination of the date of Easter, saints' names together with the calendrical scales (the choice of the saints' names sometimes allow the determination of the provenance of an instrument), astrological tables and so on.

Further components of an astrolabe often include a ruler, an alidade and the fixing mechanism.

The alidade is a piece of metal usually of the length of the diameter of the instrument. It normally has sights for observation. The alidade can have scales, in rare cases even hour markings when the alidade is used as a sundial with one of the sights acting as a gnomon (shadow caster). The alidade normally rotates over the back of the instrument.

The rule can have either the length of the radius or of the diameter of the astrolabe. It rotates over the front and acts as an index, sometimes with a declination scale.

The fixing normally consists of a pin which goes through the central holes in the mater, the plates and the rete. The pin is held in place by a small metal piece which often had the shape of a stylized horse's head and is therefore called the 'horse'.

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).

Silke Ackermann
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