The Astrolabe

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                T H E   A S T R O L A B E   A S   A R T - T H E   A S T R O L A B E   I N   A R T 






Many modern books on Islamic art use the image of the astrolabe rete (the star map on the front of an astrolabe) on the cover or as an example of the Islamic decorative arts. Some astrolabes are genuine works of art rather than simply astronomical calculating devices - elaborately engraved and beautifully made. The decorator of the astrolabe was sometimes a different person to the astrolabist who calculated the scales, indicating that the astrolabe's function as a beautiful object was of importance at the time it was made. Astrolabes seem to have been highly prized possessions, and were treated as objects with a dual purpose: functionality and beauty.
Some Islamic observatories were huge, with many scholars and instruments and generous funding. Others were small groups of scholars, often centred on the muwaqqit (a professional astronomer who made calculations for religious purposes) at a mosque. The astrolabe became symbolic of astronomy, astronomers and observatories in Islamic art. Both visual and textual use was made of the astrolabe as an astronomical object; the picture illustrates astronomers using instruments including the astrolabe:

The large observatories were seen as status symbols for the patron funding them, and were represented in writing or pictures about that patron. For example, an epic poem called the History of the King of Kings was written by 'Ala ad-Din Mansur-Shirazi in honour of Sultan Murad II. It describes the work of the astronomers in the observatory of Taqf ad-Din at Istanbul and illustrates this section with a picture of a group of people, some of whom are using astrolabes to take observations.


Recommended Reading

Sarah Schechener Genuth "Astrolabes: a Cross-Cultural and Social Perspective" introduction to Webster (ed) Western Astrolabes
David King Astronomy in the Service of Islam 1993

John North, The Fontana History of Astronomy and Cosmology, London 1994 

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An Islamic Astrolabe
 
The front (above) and back (below)                                                                                                          of an Islamic Astrolabe in the Whipple Museum.




 


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-Farghânî 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. 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.
This astrolabe is signed "Husain b. Ali" and dated 1309/10 AD. It is probably North African in origin, and is made of brass. It has four plates (for the front of the astrolabe, representing the projection of the celestial sphere and marked with lines for calculation), each for a specific latitude, and 21 stars marked on the rete (the star map, with pointers, fitting over the plate). I have chosen this astrolabe since it is right in the middle of the time frame for Islamic astrolabe use (ca. 600 to ca. 1800) and because it demonstrates many of the features common to Islamic astrolabes.

On the back is a shadow square for measuring the heights of inaccessible things and other similar calculations (shadow squares are quite common, but not on all astrolabes), and scales for calendrical calculations and calculation of the qibla (the direction to face during prayers).

A typical text on the astrolabe describes more than forty uses of the astrolabe, indicating its versatility as an astronomical calculating device. Some of its principal uses to the Islamic astronomers were to provide answers to astrological, calendrical, and meteorological questions. Although less accurate than direct mathematical calculations (the astrolabe is only as accurate as the positioning of the rete and so on) it provided an easy and quick way to calculate values.



Recommended Reading

Sarah Schechener Genuth "Astrolabes: a Cross-Cultural and Social Perspective"

David King Astronomy in the Service of Islam 1993

John North, The Fontana History of Astronomy and Cosmology, London 1994


http://www.hps.cam.ac.uk/starry/isaslabepoemsmed.jpg

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                                            A S T R O L A B E   H I S T O R Y







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.

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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 (a.k.a. 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.

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THE  ISTANBUL OBSERVATORY
The painting shows workers at the observatory of Taqi al-Din at Istanbul in 1577 (AH 985). Two observers are working with an astrolabe. A universal astrolabe of the saphea form is on the table in front of the man with the dividers and paper.

The painting is from Shahinshah-nama (History of the King of Kings), an epic poem by 'Ala ad-Din Mansur-Shirazi, written in honor of Sultan Murad III (reigned 1574-95 [AH 982-1003]).









The Astrolabe in Islam


 The astrolabe was introduced to the Islamic world in the eighth and ninth centuries through translations of Greek texts. The astrolabe was fully developed during the early centuries of Islam. Arab treatises on the astrolabe were published in the ninth century and indicate a long familiarity with the instrument (the oldest existing instruments are Arabic from the tenth century, and there are nearly 40 instruments from the 11th and 12th centuries). 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 Mecca. It must also be noted that astrology was a deeply imbedded element of early Islamic culture and that astrology was one of the principle uses of the astrolabe. The picture is from a larger painting of the observatory at Istanbul in the 16th century.

 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 Moslem India in a simplified and less artistic form.

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The Astrolabe in Europe


The astrolabe moved with Islam through North Africa into Spain (Andalusia) where it was introduced to European culture through Christian monasteries in northern Spain. It is likely that information about the astrolabe was available in Europe as early as the 11th century, but European usage was not widespread until the 13th and 14th centuries. The earliest astrolabes used in Europe were imported from Moslem Spain with Latin words engraved alongside the original Arabic. It is likely that European use of Arabic star names was influenced by these imported astrolabes. By the end of the 12th century there were at least a half dozen competent astrolabe treatises in Latin, and there were hundreds available only a century later. European makers extended the plate engravings to include astrological information and adapted the various timekeeping variations used in that era. Features related to Islamic ritual prayers were generally discarded in European instruments.

The clock in the picture is on the Prague, Czech Republic town hall and was originally constructed in about 1410. 

The astrolabe was widely used in Europe in the late Middle Ages and Renaissance, peaking in popularity in the 15th and 16th centuries, and was one of the basic astronomical education tools. A knowledge of astronomy was considered to be fundamental in education and skill in the use of the astrolabe was a sign of proper breeding and education. Their primary use was, however, astrological. Geoffrey Chaucer thought it was important for his son to understand how to use an astrolabe, and his 1391 treatise on the astrolabe demonstrates a high level of astronomical knowledge.

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Astrolabe manufacturing was centered in Augsburg and Nuremberg in Germany in the fifteenth century with some production in France. In the sixteenth century, the best instruments came from Louvain in Belgium. By the middle of the seventeenth century astrolabes were made all over Europe. It is likely that most early astrolabes were designed and built by a single individual. It is known that some particularly lovely examples were made by a team consisting of a designer, engraver and decorator. Later European instrument makers established workshops with several employees, but the style and level of workmanship was defined by the master and the workshops often closed when he retired or died. A particularly interesting workshop was founded by Georg Hartmann in Nuremberg in about 1525. It is clear that Hartmann used an early form of mass production to produce his high quality instruments. It is very likely that most workshops acquired parts of finished instruments from specialists or other shops were employed for services such as gilding. Brass astrolabes were quite expensive, and only the wealthy could afford a good one. Paper astrolabes became available as printing developed, and many were surely made, although few survive.

The picture above is a page from Elucidatio fabricae ususque astrolbii (1512), by Johannes Stöffler, who was professor of mathematics at the University of Tübingen. This treatise was one of the most popular astrolabe references and established something of a standard for European astrolabe design.

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Several interesting astrolabe variations known as "universal astrolabes" which make a single instrument usable in all latitudes were invented in the 15th and 16th centuries, but due to their high cost and complex operation, never gained the popularity of the planispheric type. These instruments projected the celestial sphere on the equinoctial colure and lacked the intuitive appeal of the planispheric type.






The Saphea Arzachelis


Many would argue that the highest level ever achieved by the astrolabe was an instrument style introduced by Gemma Frisius (1508-1555) in Louvain, Belgium in the middle of the 16th century. This instrument, called the "Astrolbum (sic) Catholicum" by Gemma, included innovations in the instrument itself and a standard of artistic execution that greatly influenced European instrument manufacturing. Hans Dorn (1430-1509) of Vienna produced an astrolabe in 1486 with virtually the same format as the Astrolabum Catholicum (the instrument survives and is in Poland). There is no way of knowing whether Gemma ever saw or heard of Dorn's astrolabe and simultaneous inventions in a time of rapid progress are not unusual.

Gemma's "Astrolabum Catholicum" was an attempt to make an astronomical instrument that could be used anywhere by a wide range of users. An ordinary astrolabe requires a separate plate for each latitude which makes it impractical to produce an instrument that can be used anywhere at reasonable cost and convenience of use. In addition, the ordinary astrolabe, as flexible as it is, is not well suited for certain types of problems, particularly those expressed in celestial latitude and longitude. In order to overcome these shortcomings, Gemma Frisius designed an instrument that had an ordinary astrolabe on one side and adopted a form of astrolabe that can be used at any latitude for the other side and included a magnetic compass in the throne. This form makes a lot of sense since different problems are better suited for one type of astrolabe or the other.

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The latitude independent astrolabe variation was originally described by Ibn az-Zarqellu of Toledo in the 11th century, probably in about 1048, and had been discussed in several treatises in the Middle Ages. It was called the Saphea Arzachelis by Profat Tibbon (Jacob ben Makir), better known under the name of Prophatius Judaeus, in a treatise dated 1263. The name, which derives from al-Safihat (the plate) of al-Zarqellu, has endured, although it is often shortened to simply saphea.

As Henri Michel put it, "The "Astrolabum Catholicum" was definitely not Gemma Frisius' invention and, with the typical lack of concern of the time, the learned cosmographer 'forgot' to say that this instrument had been contrived five centuries earlier". Regardless of the facts, Gemma Frisius is usually credited with the invention. There is certainly no argument that the instruments he inspired had a profound effect on instrument making.

The principle of the saphea is similar to the plane astrolabe. Both instruments use the stereographic projection. The projection origin is at the south celestial pole and the sky is projected onto the equator on the planispheric astrolabe. On the saphea, the projection is from one of the equinoxes and the celetstial sphere is projected onto the great circle of the celestial sphere defined by the poles and the solstices (the solsticial colure). That is, the projection is from the "side".

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At this point we note that the projection would have exactly the same arcs if the origin of the projection were at the autumnal equinox instead of the vernal equinox. Thus, we can divide the ecliptic for the entire range of solar longitudes and simply imagine that we are looking at the sphere from one side or the other depending on the time of year.

The really clever part of this application of the stereographic projection is, if the projection is rotated so the ecliptic is horizontal, the arcs above the ecliptic represent arcs of celestial latitude and the perpendicular arcs represent celestial longitude. That is, if the horizontal diameter through the center (the equinoctial line) is considered to the be ecliptic, the poles are the ecliptic poles and the arcs are celestial latitude and longitude.

Similarly, if the projection origin is moved to a point on the celestial sphere on the extension of the local horizon then the equinoctial line is the horizon, the poles represent the zenith and nadir of the place and the arcs represent angles of altitude and azimuth or hour angles.

Thus, the projection can represent the celestial coordinates of a point in space in three different coordinate systems at the same time. In fact, the simplest applications of the saphea involve converting between coordinate systems. For example, if the celestial latitude and longitude are known, it is very simple to find the declination and right ascension or the altitude and azimuth of the same point, and vice versa.

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The ecliptic is divided by the zodiac on most saphea instruments. These divisions represent the position of the Sun on the ecliptic and require a little imagination to visualize. Recall that the projection arcs are identical whether the projection origin is at the vernal equinox or the autumnal equinox. Therefore, the half of the ecliptic we are considering depends on the origin. We see the half of the ecliptic from Aries 0° to Gemini 30° and Capricorn 0° to Pisces 30° when the origin is the autumnal equinox. These signs are printed above the ecliptic line. We see the half of the ecliptic from Cancer 0° to Sagittarius 30° when the origin is the vernal equinox. These signs are printed upside down and below the ecliptic line. Thus, we can visualize the Sun moving back and forth along the ecliptic over the course of a year if we mentally shift our view of the projection origin. It is much more difficult to describe than it is to do.

The margin of the plate is divided into four 90° quadrants. The sequence of the numbering for a quadrant depends on the use. The numbering in QI proceeds from 0° at the equator to 90° at the north pole. This set of divisions is used when the arcs are interpreted as declinations or latitudes. QII can be divided in the same way or in the reverse order so 0° is at the north pole, increasing in the counterclockwise direction to 90° at the 9 o'clock position. This set of divisions is the polar distance and are used to orient the local horizon. Instrument makers were not particularly consistent in the labeling of the saphea limb. A mental subtraction is needed for certain problems if QII and QIV are not labeled with polar distance.

The meridians are generally not labeled along the equinoctial line for two reasons: the labels would have to be very small in order to be consistent and the polar arcs can have a variety of meanings. Among the meanings are right ascension, hour angles, equal hours, longitudes and azimuths. It would not be possible to label all of these uses in a coherent way. The hours (equal hours, hour angles, right ascensions) are labeled near the tropics since they are used rather often and would otherwise be difficult to locate. One hour is 15° on the celestial sphere. The labels along the 30° parallel identify the equal hour associated with each 15° and can be converted to right ascension.

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Two additional accessories are required to solve problems with the saphea plate. A rule that is free to rotate around the center of the instrument having one edge as a diameter is called the regula. "Regula" is Latin for "rule" and we use the term to differentiate the device from the rule used on the ordinary astrolabe. The regula may be divided by degrees. Connected to the regula is an articulated arm of two or three sections called the brachiolus (Latin for "little arm"). Some old instruments did not have a brachiolus but had an auxiliary rule oriented 90° to the regula that could slide up and down the regula and point to any location on the plate. Others had both, with the brachiolus attached to the cursor.

Some old instruments showed some stars on the saphea plate even though they were of limited use. Stars shown on the projection plate were located in the equatorial coordinate system by their declination and right ascension. Stars visible from the vernal equinox were sometimes shown as a solid star figure with stars visible from the autumnal equinox shown as a star outline.

The saphea is a very versatile device. Problems involving conversion of coordinate systems are very easy. Thus, if the user has an ephemeris that lists celestial positions by latitude and longitude, which are very awkward on a planispheric astrolabe, it is very easy to convert the positions to declination and right ascension, which are very convenient on a plane astrolabe.

The saphea can be used to find the time of sunrise and sunset, and thus the length of the day, for any latitude. It can also be used to find the latitude from the Sun's meridian altitude, although this is such a simple problem arithmetically that it is unlikely anyone would go to the trouble to use an astrolabe to solve it [Sun's noon altitude = (90° - latitude)+ declination].

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Time cannot found directly on the saphea but requires an iterative procedure. This shortcoming of the saphea is very likely the reason Gemma included a regular plane astrolabe on the instrument.

For the mathematically inclined, the saphea can be used to solve spherical triangles. It is unlikely that that the instrument saw much use in this area even though such problems could be solved by a user with little knowledge of spherical trigonometry. As John North points out, such instruments were not very popular since it is unlikely that people with no knowledge of spherical trigonometry would be interested in solving spherical trigonometry problems.

Gemma Frisius was a professor of medicine in Louvain who apparently became interested in astronomy and astronomical instruments through the astrological aspects of medicine as it was practiced at that time. He wanted to create an instrument that would be attractive to users ranging from astronomers and astrologers to surveyors and sailors and named it the "Astrolabum Catholicum" ("Latin for "Universal Astrolabe" ) to convey idea that he had succeeded. Whether he did, in fact, succeed in creating a truly universal astrolabe is arguable, but the quality, flexibility and ingenious design of the instrument assured success for the makers and fame for the designer.

The original instruments are identified with the workshop of Gemma's nephew, Gualterus (Walter) Arsenius. Gemma Frisius probably did not actually make any instruments himself, but he designed many. It is not clear whether he managed a workshop or inspired his nephew to establish one, but the resulting instruments set a new and enduring standard for quality and artistic workmanship. Arsenius clearly ran his own operation after Gemma's early death at the age of 47.

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Some of the workers from the Arsenius shop revolutionized calligraphic engraving and artistic design and raised European map and instrument making to an esthetic level that is still admired. Thomas Gemini, a Protestant who worked with Gemma in Louvain as a journeyman , migrated (or was banished for heresy) to England where he had enormous influence on the English instrument making tradition. The beautiful maps, globes and instruments made by Gerard Mercator, who began his career in Louvain, are still admired as among the finest ever made. The elegant and artistic Louvain style introduced by Gemma Frisius and Arsenius was adopted by instrument makers in England, Spain, Italy and Germany. A notable practitioner was the German, Erasmus Habermel, whose beautiful and delicate instruments rivaled and even exceeded the Louvain products in quality and style. Artisans such as Gemini, Habermel and Humphry Cole had dramatic influence on the future of European instrument making.

It should be noted that the arcs on the saphea tend to be rather close together and congested. A rather large instrument is required in order to achieve anywhere near the accuaracy of a normal plane astrolabe. Surviving universal astrolabes of this type were exquisitely executed and were quite large and heavy. They must have been incredibly expensive; another reason why this type of instrument never gained a lot of popularity.

Two other forms of universal astrolabe were developed. In 1550, Juan de Rojas Sarmiento of Monzón, Spain, published a commentary on a universal astrolabe based on the orthographic projection of the celestial sphere onto the solsticial colure. Rojas had studied under Gemma Frisius in Louvain where he met Hugo Helt who assisted in the commentary and wrote the section on the instrument's construction. Astrolabes based on the orthographic construction had been discussed earlier and Rojas did not claim to invent the method or assert that he was the first to apply it to the astrolabe, but this form did not gain much attention until Rojas' publication. Phillipe de la Hire (1640-1718) published a third form of universal astrolabe in late 17th century that attempted to resolve some difficulties of both the saphea and Rojas versions. This method was described by Nicholas Bion (1652-1733) in 1702 but apparently no la Hire astrolabes were ever made of metal as interest in the astrolabe had declined by this time.