Astrolabe Glossary and Notes

Abu Ma‘shar

Al-Bīrūnī

Al-Bīrūnī (البيروني) (b. 4 September 973; d. after 1050): Al-Bīrūnī was born and grew up in the Khwarazm region, south of the Aral Sea. He wrote more than 100 treatises on most technical and scientific subjects, including astronomy, astrology, medicine, geography magic, mathematics, meteorology and gems. He also wrote literature, histories, and books on religion and philosophy. Al-Bīrūnī wrote a valuable work on construction and use of the astrolabe as well as a book on shadows that contains descriptions of numerous sun dials. His The Book of Instruction in the Elements of the Art of Astrology was an important text and compendium on astrological practices. See E.S. Kennedy’s entry in the Dictionary of Scientific Biography, ed. Charles Coulston Gillispie, vol. 2 (New York: Charles Scribner’s Sons, 1970), 147-58.

Alidade

The alidade is the sighting rule often found on the back of an astrolabe that was used to ascertain the altitude of the sun or star. Pin-holes pierce the sighting vanes to allow for direct observation of the star or indirect observation of the sun. See also rule, bracchiolus and pointer.

Almucantar

Almucantars are the circles of equal altitude projected onto the celestial sphere. Almucantars are small circles that along with azimuths produce a grid on the celestial sphere analgous to the grid produced by lines of longitude and latitude projected onto the earth.

Altitude

The height — or vertical angular distance — in degrees of a celestial body (e.g., a planet or star) above an observer’s horizon. Altitudes are commonly measured above the horizon and reported as positive values. An object with an altitude of 0° is on the horizon; one with an altitude of 90° is directly overhead, at the zenith.

Az-Zarqāllu

Ibn az-Zarqāllu lived and worked first in Toledo and later in Córdoba during the eleventh century. He wrote a number of works on astronomy — including the Suma referente al movimeinto del sol on the proper motion of the solar apogee, the Tratado relativo al movimiento de las estrellas fijas on trepidation theory, and his Tratado de la lámina de los siete planetas that seems to be the first text to recognize that the orbit of a planet (in this case Mercury) is not a circle.

Az-Zarqāllu also wrote an important treatise on stereographic projection used to construct astrolabes. In his Tratado de la azafea, az-Zarqāllu described a new method of projecting the celestial sphere onto a plane. Andalusian astronomers had taken up the problem of developing a universal projection, one that would avoid the need to engrave a new plate for each latitude. Az-Zarqāllu's answer to this problem was a method that projected both the equatorial and ecliptic coordinate systems on to a vertical plane that cut the celestial sphere at the solstices. Adding a selection of important stars to this grid system produced a universal projection that was valid for every latitude without sacrificing any of the functionality of a standard projection.

This method of projection enjoyed much success both in further Arabic treatises and in translation — it was translated into various vernacular languages as well as Latin. Although comparitively few astrolabes survive that bear az-Zarqāllu's projection, a large number of manuscripts testify to its popularity well into the sixteenth century. By the late fifteenth century, the saphea Azarchelis as it was called in Latin, had become a common part of the astronomical curriculum at European universities. In the middle of the sixteenth century, Gemma Frisius developed his astrolabium catholicum that resembled closely az-Zarqāllu's projection and extended the popularity of this instrument yet further.

Azimuth

The horizontal angular distance in degrees along an observer’s horizon between the north or south point of the meridian line and the perpindicular line drawn from the celestial body to the horizon. On astrolabes, this distance is commonly measured from the closest meridian line and does not exceed 90°. In modern astronomy, the azimuth is the direction of a celestial body measured clockwise in degrees from the north. The azimuth is commonly used along with the altitude.

Back

The back or obverse of the astrolabe is opposite the hollowed-out womb of the instrument. A wide range of calendrical, observational and astrological tables appear on the backs of astrolabes. Occasionally, the back was used for a universal projection.

Brachiolus

A pointer used to indicate a position on a universal astrolabe.

Celestial equator

The great circle formed by the plane of the earth’s equator intersecting the celestial sphere.

Celestial latitude

The apparent angular distance between a celestial body and the ecliptic, measured along a perpindicular from the object to the eclipitic. Celestial latitude is reported as a positive value when the body is above the ecliptic and a negative value when it is below the eclipitic. Celestial latitude is often used with celestial longitude to locate the position of celestial objects.

Celestial longitude

The angular distance between the first point of Aries and the perpindicular line drawn from a celestial object to the ecliptic. This value is measured counter-clockwise from the first point of Aries and is reported in degrees of a sign. Today, the celestial longitude is reported in degrees from 0 - 360. The celestial longitude is often used with celestial latitude to locate a celestial object.

Celestial pole

The two celestial poles are the points on the celestial sphere at which the earth’s axis would intersect the celestial sphere, if this axis were extended.

Celestial sphere

Observered from the earth, the stars and planets appear to be located on the inner surface of an immense sphere. The motions of planets can be plotted against the backdrop of the zodiacal signs and fixed stars, as if all the celestial bodies were equally distant from the earth. This imaginary sphere is the celestial sphere.

Decans

The system of decans was originally associated with thirty-six constellations that rose at specified times of the year. Early examples of such lists can be found on coffin lids from the Egyptian Middle Kingdom and ceilings of tombs dating from the New Kingdom. Evidence suggests that their risings marked both the days of the year and the time of night. As early as 2100 B.C. the decans were associated with particular gods. This system evolved over time and became another scheme for dividing the zodiac.

Like the faces, the decans are even divisions of 10° each, starting with the first point of Aries and terminating at the last point of Pisces. Whereas the faces appear to have been indigenous to Arabic astrology, the system of decans is found first in Egyptian and later in Indian astrology (al- Bīrūnī asserts that the decans were a Hindu practice). By the time al- Bīrūnī wrote his The Book of Instruction in the Elements of the Art of Astrology in the early eleventh century, the decans were ruled not by Egyptian gods but were governed by a planet or luminary. Although the decans spread widely throughout the Islamic and Indian worlds, tables of decans are rare on astrolabes. The lords of the decans are given below.

Zodiacal Sign Planetary Lords
10° 20° 30°
Aries المراخ
Mars
الشمس
Sun
المشترى
Jupiter
Taurus الزهره
Venus
العؕارد
Mercury
الوحل
Saturn
Gemini العؕارد
Mercury
الزهره
Venus
الوحل
Saturn
Cancer القمر
Moon
المراخ
Mars
المشترى
Jupiter
Leo الشمس
Sun
المشترى
Jupiter
المراخ
Mars
Virgo العؕارد
Mercury
الوحل
Saturn
الزهره
Venus
Libra الزهره
Venus
الوحل
Saturn
العؕارد
Mercury
Scorpio المراخ
Mars
المشترى
Jupiter
القمر
Moon
Sagittarius المشترى
Jupiter
المراخ
Mars
الشمس
Sun
Capricorn الوحل
Saturn
الزهره
Venus
العؕارد
Mercury
Aquarius الوحل
Saturn
العؕارد
Mercury
الزهره
Venus
Pisces المشترى
Jupiter
القمر
Moon
المراخ
Mars

Declination

The vertical angular distance between a celestial body and the celestial equator. Unlike altitude, which is commonly reported as a positive number, i.e., the distance above the horizon, declination can be either a positive or a negative value. Declination is often used with mediation to locate the position of a star or planet.

Ecliptic

The apparent annual path of the sun against the backdrop of the fixed stars. This great circle is inclined 23°27’ degrees from the celestial equator and intersects it at the vernal and autumnal equinoxes, i.e., on 20 March when the sun enters the zodiacal sign of Aries and on 22 September when the sun enters the zodiacal sign of Libra.

Equinoxes

The points of intersection between the celestial equator and the ecliptic, the first point of Aries and the first point of Libra. At these points the sun’s apparent position is on the equator and represents the beginnings of spring and autumn. In 2005, the spring equinox occurred on 20 March, the autumnal equinox on 22 September. On these dates the length of day equals that of night, hence the name equinox.

Faces

The faces are one third of each sign, i.e., 10° of the zodiacal circle, beginning with the first point of Aries and continuing to the last point of Pisces. Each of the 36 resulting faces was said to be governed by a one of the five planets or either of the luminaries. Al-Bīrūnī, in his The Book of Instruction in the Elements of the Art of Astrology, provides an early codification of this scheme, identifying the lord of each face. On Islamic astrolabes, the list of faces is often labeled الوجوه (al- wujūh) and indicates the lords by the last letter of the governing planet or luminary. The lords of the faces are given in the table below.
Zodiacal Sign Lords of the Faces
10° 20° 30°
Aries المراخ
Mars
الشمس
Sun
الزهره
Venus
Taurus العؕارد
Mercury
القمر
Moon
الوحل
Saturn
Gemini المشترى
Jupiter
المراخ
Mars
الشمس
Sun
Cancer الزهره
Venus
العؕارد
Mercury
القمر
Moon
Leo الوحل
Saturn
المشترى
Jupiter
المراخ
Mars
Virgo الشمس
Sun
الزهره
Venus
العؕارد
Mercury
Libra القمر
Moon
الوحل
Saturn
المشترى
Jupiter
Scorpio المراخ
Mars
الشمس
Sun
الزهره
Venus
Sagittarius العؕارد
Mercury
القمر
Moon
الوحل
Saturn
Capricorn المشترى
Jupiter
المراخ
Mars
الشمس
Sun
Aquarius الزهره
Venus
العؕارد
Mercury
القمر
Moon
Pisces الوحل
Saturn
المشترى
Jupiter
المراخ
Mars

Fiducial edge

The edge of a rule against which a scale is read is called the fiducial edge. To facilitate the use of a scale, the fiducial edge is often graduated.

Fixed stars

The backdrop of stars that do not appear to move relative to one another. The brighter of these stars are often found on retes and grouped into constellations. Twelve of the constellations along the ecilptic have given their names to the zodiacal signs, although today these constellations have precessed more than a full sign from those which bear their names.

Gazetteer

A gazetteer is a list of cities along with their longitude and latitude. Frequently, the maker has added one, two or three additional geographical parameters: the inclination of the azimuth of the Qibla (the انحراف or "inḥirāf"), the direction of the azimuth of the Qibla in relation to the four cardinal points of the comapss (الجهات or "al-jihat"), and less frequently the distance from Mecca (المسافت "al-masāfat"). Gazetteers are most commonly engraved as concentric circles into the womb of Islamic astrolabes. There are, however, instruments that have additional gazetteers on the edge of the limb and on plates.

Although the order varies, all gazetteers begin with either the longitude (ؕول) or the latitude (عرض), given in degrees and minutes. If the maker chose to include the inḥirāf, this is usually presented next, again listed in degrees and minutes. The fourth common parameter is the jihat, which expresses the direction of the azimuth of the Qibla as northeast (abbreviated شش), southeast (abbreviated شج), southwest (abbreviated غج), south of the Qibla (abbreviated جق) or north of the Qibla (abbreviated شق).

The gazetteers in the collection have been transcribed faithfully reproducing the Arabic orthography. This will regularly reflect regional spelling variations. In translating the city names, however, we have tried to adopt a standard, recognizable spelling. Two primary reference tools have been used to assist in identifying cities: the list of cities included in Gibbs and Saliba (1984) and Kennedy and Kennedy (1987). The second of these two works is the definitive guide to Arabic place names and itself draws on Gibbs and Saliba (1984). Transcribing Arabic numbers presents a number of difficulties. In particular, certain Arabic numbers are easily confused, e.g., numbers between 11 and 19 are easily misread as numbers between 31 and 39 or numbers between 51 and 59. Where possible, the geographical parameters have been checked against the two resources cited above to see if they are reasonable. In cases of great discrepancy between the value read on the instrument and that found in printed catalogues, the instrument has been double-checked to ensure accuracy. Unusual or seeming aberrant values have not been discarded merely on the basis of their being unusual.

Great circle

A circle formed on the surface of a sphere by any plane that passes through the center of that sphere. The celestial equator and the eclipitic are great circles. On earth, the equator is a great circle.

Horizons

Series of lines, usually drawn on the back of an astrolabe or on a tympan, which enable the calculation of the time of sunrise and sunset at a particular latitude (on a particular day of the year).

Horse

The wedge that passes through the pin and holds the rete and plates in place is sometimes called the horse, after its characteristic horse shape. On many instruments, the horse is plainly cut, while on other instruments the horse is ornately carved and engraved. Occasionally, the horse is replaced by a threaded nut.

Limb

The raised circumference on the front of an astrolabe is called the limb. The limb often contains a scale of degrees and on Western astrolabes an hour scale. The limb was also used to secure the tympans from moving when the rete was rotated, either by means of a pin extending from the limb or a notch that accepted the tab from the tympan.

Luminary

The sun and the moon were commonly referred to as the two luminaries. Like the planets, the luminaries seem to move against the backdrop of fixed stars. However, the size and brightness of the sun and the moon distinguish them.

Lunar mansions

The set of lunar mansions are the divisions of the moon’s path into twetny-eight constellations, analogous to the zodiacal signs. Systems of lunar mansions were common in early Chinese, Indian and Arabic astronomy. Indeed, the names of the lunar mansions in each tradition often bear close similarities in meaning. Nevertheless, the origin and dissemination of the lunar mansions remains an open question. Like the images associated with the signs of the zodiac, a wide range of images came to signify the different constellations of the lunar mansions.

The lunar mansions were probably first used for calendrical as well as astrological and astronomical purposes. By the thirteenth century, however, they had acquired powers of celestial magic as their iconography came to represent celestial talismans. The Arabic texts were translated first into Spanish (Las ymagenes de las veynt & ocho mansiones de la Luna) and later into Latin (often with a title like De imaginibus caelestibus), producing a rich tradition that disseminated the talismans of the lunar mansions and associated them with other magical texts (e.g., the Hermetic text Picatrix).

According to al-Bīrūnī, there are two common variants in the names of the lunar mansions, notably for the first and the last mansions. He gives both of these in his list of lunar mansions contained in his The Book of Instruction in the Elements of the Art of Astrology. In addition to these differences, some instruments include the longer names for some of the mansion (e.g., فرغ المقدم or سعد بلع).

Lunar Mansion Transliteration Alternative Name
الشرؕين al-Sharaṭain نؕحا / Naṭḥā
البؕين al-Buṭain
الثريا al-Thurayya
الدبران al-Dabarān
الهقعه al-Haq‘ah
الهنعه al-Han‘ah
الذراع al-Dhirāh
النثره al-Nathrah
الؕرفه al-Ṭarfah
الجبهه al-Jabhah
الزبره al-Zubrah
الصرفه al-Ṣarfah
العوا al-‘Awwā
السماك al-Simāk
الغفو al-Ghafr
الزبانا al-Zubānā
الاكليل al-Iklīl
القلب al-Qalb
الشوله al-Shaula
النعايم al-Na‘ā’im
البلده al-Baldah
الذابح al-Dhābih سعد الذابح / Sa‘d al-Dhābih
البلع al-Bula‘ سعد البلع / Sa‘d al-Bula‘
السعود al-Su‘ūd سعد السعود / Sa‘d al-Su‘ūd
الاخبيه al-Akbiyyah سعد الاخبيه / Sa‘d al-Akbiyyah
المقدم al-Muqaddam فرغ المقدم / Fargh al-Muqaddam
المواخر al-Mu’akkhir فرغ المواخر / Fargh al-Mu’akkhir
بؕن الحوت Baṭn al-Ḥūt رشا / Rishā’

Mater

The main body of the astrolabe to which the tympans, rete and alidades are attached.

Mediation

The angular distance between the first point of Aries and the line from the celestial pole through the celestial object to the ecliptic. In other words, the mediation is the degree of the ecliptic that crosses the meridian at the same time as the celestial object. This value is measured counter-clockwise from the first point of Aries and is reported in degrees of a sign. Although mediation was formerly used with declination to locate a celestial object, astronomers now commonly pair right ascension with declination.

Meridian

The great circle on the celestial sphere that intersects an observer’s zenith and both celestial poles. Any object on the meridian will either be at an observer’s zenith, or due north or south from the observer.

Mudir

Mudir (مدير) is the Arabic term for the knob or button often found on retes of Islamic astrolabes. As many as four small knobs decorate the retes, are often made of silver and serve both a decorative and a practical purpose.

Pin

The pin passes through the center of the astrolabe and holds the various parts together. Although the pin is occasionally decorated and quite ornate, more often it is simply engraved. The horse or wedge passes through the pin and secures the rete, plates and any rules.

Planets

Until the late eighteenth century, when Sir William Herschel identified Uranus, there were only five known planets. These were Mercury, Venus, Mars, Jupiter and Saturn. Along with the sun and the moon, these were the seven wandering stars that seemed to move relative to one another against the backdrop of the fixed stars and constellations.

Plate

Plates are the plates that usually bear the stereographic projection of the heavens. The most common stereographic projection used in constructing astrolabes is one that projects onto a horizontal plane, usually coplanar with the cestial equator, as viewed from the south celestial pole. Such a projection varies with any change in the observer's latitude on earth. Thus, most instruments contain a number of plates, each engraved with a projection for a specific latitude. Because of this latitude dependence, plates have often offered some clue as to where the astrolabe might have been used. A person who lived in London would have certainly found a plate engraved for 52° (the value found on one of the instruments in the collection) more useful than one engraved for 40° (the latitude of Toledo Spain).

Occassionally, plates will contain other types of scales (e.g., Tablets of Horizons) or other information (e.g., astrological tables or gazetteers).

Pointer

Pointers are frequently found on the front of astrolabes and occasionally bear declination scales. Pointers facilitate using the front of the instrument, making it easier to set the rete to correct position.

Precession

Although the earth’s axis remains inclined 23°27’ from the ecliptic, its direction changes slowly. Over 25,800 years this slow change will describe a complete circle and produces the ‘precession of the equinoxes’. The equinoctial points, the first degrees of Aries and of Libra, drift slowly backwards (westwards) along the equator approximately 1°24’ every century. The precession of the equinoxes has caused the zodiacal constellations to drift one full sign west from their zodiacal signs.

Ptolemy, Claudius

Claudius Ptolemy (c. 85 B.C. - c. 165 A.D.) was a Greek astronomer who lived and worked in Alexandria, Egypt. Although he wrote extensively on the various mathematical sciences, including harmony, music and analema, he is most famous for his works on astronomy, astrology and geography.

His work on astronomy, the Almagest, a name it adopted from its Arabic translation (the Greek title is Τάδε ἔνεστιν ἐν τ῕ πρώτῳ τῆς � τολεμαίου μαθΕματικῆς συντάξεως) is the most detailed work on mathematical astronomy from antiquity. Although largely based on the work of Hipparchus, the Almagest codified the geo-static model of the solar system that came to bear Ptolemy's name.

Ptolemy also wrote the astrological Tetrabiblos (in Greek, ΚΛΑΥΔΙΟΥ � ΤΟΛΕΜΑΙΟΥ ΤΩΝ � ΡΟΣ ΣΥΡΟΝ Α� ΟΤΕΛΕΣΜΑΤΙΚΩΝ), which became the authoritative source on astrology until the sixteenth century.

Finally, his Geography, which was largely lost to the Latin West until the fifteenth century, provided a rational method for mapping the known world and extending that map to incorporate newly discovered lands.

Qibla

The direction towards Mecca.

Rete

The 'skeletal' rotating disc on the front of most planispheric astrolabes. It usually consists of the circle of the ecliptic, the tropic of cancer and the tropic of capricon, and contains pointers for major stars.

Right ascension

Ring

The ultimate piece of the suspension apparatus was the ring. The ring often passed through the top of the shackle and could rotate freely.

Rule

The rule resembles a pointer but is double-ended and bears a scale, often a declination scale.

Shackle

The shackle is the part of the suspension apparatus that connects the ring to the throne. The shackle is sometimes quite complex and often allows the astrolabe to rotate about the vertical axis.

Small circle

Small circles are produced by the intersection of a plane and a sphere if the plane does not pass through the center of the sphere. Examples of small cirlces include almucantars and lines of latitude. Compare small circles to great circles.

Star pointer

The variously shaped pointers on the rete of the astrolabe which are used to indicate or measure the position of a star.

Terms

Terms were another method of dividing the zodiacal signs. Like the faces and the decans, the system of terms started at the first point of Aries and ended at the last point of Pisces. Unlike these other two methods, however, term divisions were unequal, varying from 5° to 12° of a sign. Further, terms were associated only with planets that governed them, not the luminaries. By the time Ptolemy wrote his Tetrabiblos in the second century A.D., there were already multiple methods of dividing the zodiac. He presented in detail the Egyptian terms and mentioned the Chaldean terms. Ultimately, he preferred a scheme that he claimed he had recently found in an ancient manuscript. When al-Bīrūnī wrote his The Book of Instruction in the Elements of the Art of Astrology, he spoke disparingly of various methods devised by the Chaldeans, Hindu astrologers and a certain Asṭarātūs before presenting both the Egyptian and Ptolemaic terms. Most Islamic astrolabes makers preferred the Egyptian terms, probably agreeing with al-Bīrūnī that they are more correct.

Egyptian Terms

Zodiacal Sign Degrees Planetary Lord Zodiacal Sign Degrees Planetary Lord
Aries 6 المشترى
Jupiter
Libra 6 الوحل
Saturn
12 الزهره
Venus
14 العؕارد
Mercury
20 العؕارد
Mercury
21 المشترى
Jupiter
25 المراخ
Mars
28 الزهره
Venus
30 الوحل
Saturn
30 المراخ
Mars
Taurus 8 الزهره
Venus
Scorpio 7 المراخ
Mars
14 العؕارد
Mercury
14 الزهره
Venus
22 المشترى
Jupiter
19 العؕارد
Mercury
27 الوحل
Saturn
24 المشترى
Jupiter
30 المراخ
Mars
30 الوحل
Saturn
Gemini 6 العؕارد
Mercury
Sagittarius 7 المشترى
Jupiter
12 المشترى
Jupiter
17 الزهره
Venus
17 الزهره
Venus
21 العؕارد
Mercury
24 المراخ
Mars
26 الوحل
Saturn
30 الوحل
Saturn
30 المراخ
Mars
Cancer 7 المراخ
Mars
Capricorn 7 العؕارد
Mercury
13 الزهره
Venus
14 المشترى
Jupiter
19 العؕارد
Mercury
22 الزهره
Venus
26 المشترى
Jupiter
26 الوحل
Saturn
30 الوحل
Saturn
30 المراخ
Mars
Leo 6 المشترى
Jupiter
Aquarius 7 العؕارد
Mercury
11 الزهره
Venus
13 الزهره
Venus
18 الوحل
Saturn
20 المشترى
Jupiter
24 العؕارد
Mercury
24 المراخ
Mars
30 المراخ
Mars
30 الوحل
Saturn
Virgo 7 العؕارد
Mercury
Pisces 12 الزهره
Venus
17 الزهره
Venus
16 المشترى
Jupiter
21 المشترى
Jupiter
19 العؕارد
Mercury
28 المراخ
Mars
28 المراخ
Mars
30 الوحل
Saturn
30 الوحل
Saturn
Ptolemaic Terms
Zodiacal Sign Degrees Planetary Lord Zodiacal Sign Degrees Planetary Lord
Aries 6 المشترى
Jupiter
Libra 6 الوحل
Saturn
14 الزهره
Venus
11 العؕارد
Mercury
21 العؕارد
Mercury
19 المشترى
Jupiter
26 المراخ
Mars
24 الزهره
Venus
30 الوحل
Saturn
30 المراخ
Mars
Taurus 8 الزهره
Venus
Scorpio 6 المراخ
Mars
15 العؕارد
Mercury
12 الزهره
Venus
22 المشترى
Jupiter
21 العؕارد
Mercury
24 الوحل
Saturn
24 المشترى
Jupiter
30 المراخ
Mars
30 الوحل
Saturn
Gemini 7 العؕارد
Mercury
Sagittarius 8 المشترى
Jupiter
13 المشترى
Jupiter
14 الزهره
Venus
20 الزهره
Venus
19 العؕارد
Mercury
26 المراخ
Mars
25 الوحل
Saturn
30 الوحل
Saturn
30 المراخ
Mars
Cancer 6 المراخ
Mars
Capricorn 6 العؕارد
Mercury
13 الزهره
Venus
12 المشترى
Jupiter
20 العؕارد
Mercury
19 الزهره
Venus
27 المشترى
Jupiter
25 الوحل
Saturn
30 الوحل
Saturn
30 المراخ
Mars
Leo 6 المشترى
Jupiter
Aquarius 6 العؕارد
Mercury
13 الزهره
Venus
12 الزهره
Venus
19 الوحل
Saturn
20 المشترى
Jupiter
25 العؕارد
Mercury
25 المراخ
Mars
30 المراخ
Mars
30 الوحل
Saturn
Virgo 7 العؕارد
Mercury
Pisces 8 الزهره
Venus
13 الزهره
Venus
14 المشترى
Jupiter
18 المشترى
Jupiter
20 العؕارد
Mercury
24 المراخ
Mars
26 المراخ
Mars
30 الوحل
Saturn
30 الوحل
Saturn

Throne

The throne at the top of the instrument both offered a means of orienting the astrolabe and of suspending the instrument so that it could be used to make observations. The Arabic term is كرسي (kursi). Islamic astrolabe makers, particularly in the eastern part of the Islamic world, frequently decorated the كرسي with inscriptions from the Qur‘an or with their own signatures.

Tympan

The plates that are found in the womb of the mater. Usually marked with azimuth and almucantar lines for a specific latitude.

Womb

The hollow, circular depression in the centre of the mater formed by the raised limb and the back plate of the instrument. The various tympans and the rete fit inside the womb. The inside or back of the womb is often engraved. On Islamic instruments, the womb frequently bears a gazetteer. Astrological tables as well as additional stereographic projections are not uncommon on European astrolabes.

Zenith

The zenith is the point on the celestial sphere directly above the observer, the pole of the observer’s horizon.

Zodiac

The zodiac is the band on the celestial sphere that forms the backdrop for the motions of the sun, the moon, and the five planets known to the ancients. The zodiac is divided into twelve zodiacal signs of 30°: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces. Each sign was named after a prominent constellation. The precession of the equinoxes has caused these constellations to shift out of the signs named for them. Below is a table showing the symbols of the zodiacal signs as well as their Latin and Arabic names:
MHS Home | Contact Us | ©2005 Museum of the History of Science