Astrolabe

An astrolabe (

The astrolabe, which is a precursor to the sextant,^[1] is effective for determining latitude on land or calm seas. Although it is less reliable on the heaving deck of a ship in rough seas, the mariner's astrolabe was developed to solve that problem.

Applications

A 10th-century astronomer Abd Al-Rahman Al-Sufi wrote a massive text of 386 chapters on the astrolabe. His worked described more than 1000 applications for the astrolabe's various functions,^[2]. These ranged from the astrological, the astronomical and the religious, to navigation and seasonal and daily time-keeping and tide tables. At the time of their use, astrology was widely considered as much of a serious science as astronomy, and study of the two went hand-in-hand. The astronomical interest varied between folk astronomy (of the pre-Islamic tradition in Arabia) which was concerned with celestial and seasonal observations, and mathematical astronomy, which would inform intellectual practices and precise calculations based on astronomical observations. In regard to the astrolabe's religious function, the demands of Islamic prayer times were to be astronomically determined to ensure precise daily timings, and the qibla, the direction of Mecca towards which Muslims must pray, could also be determined by this device. In addition to this, the lunar calendar that was informed by the calculations of the astrolabe was of great significance to the religion of Islam, given that it determines the dates of important religious observances such as Ramadan.^{[citation needed]}

Etymology

The Oxford English Dictionary gives the translation "star-taker" for the English word astrolabe and traces it through medieval Latin to the Greek word ἀστρολάβος : astrolábos,^[3][4] from ἄστρον : astron "star" and λαμβάνειν : lambanein "to take".^[5]

In the medieval Islamic world the

An early astrolabe was invented in the

Theon of Alexandria (c. 335 – c. 405) wrote a detailed treatise on the astrolabe, and Lewis^[12] argues that Ptolemy used an astrolabe to make the astronomical observations recorded in the Tetrabiblos. The invention of the plane astrolabe is sometimes wrongly attributed to Theon's daughter Hypatia (c. 350–370; died AD 415),^{[13][14][15][16]} but it's known to have been used at least 500 years earlier.^[14][15][16] The misattribution comes from a misinterpretation of a statement in a letter written by Hypatia's pupil Synesius (c. 373 – c. 414),^[14][15][16] which mentions that Hypatia had taught him how to construct a plane astrolabe, but does not say that she invented it.^[14][15][16]

Astrolabes continued in use in the Greek-speaking world throughout the Byzantine period. About AD 550, Christian philosopher John Philoponus wrote a treatise on the astrolabe in Greek, which is the earliest extant treatise on the instrument.^[a] Mesopotamian bishop Severus Sebokht also wrote a treatise on the astrolabe in the Syriac language in the mid-7th century.^[b] Sebokht refers to the astrolabe as being made of brass in the introduction of his treatise, indicating that metal astrolabes were known in the Christian East well before they were developed in the Islamic world or in the Latin West.^[17]

The first Renaissance treatises dealing with scientific problems were based on earlier classical works and were often concerned with Ptolemaic doctrines.^{[citation needed]}

Medieval era

Astrolabes were further developed in the

The

The first known metal astrolabe in Western Europe is the Destombes astrolabe made from brass in the eleventh century in Portugal.[25]^[26] Metal astrolabes avoided the warping that large wooden ones were prone to, allowing the construction of larger and therefore more accurate instruments. Metal astrolabes were heavier than wooden instruments of the same size, making it difficult to use them in navigation.^[27]

A simplified astrolabe, known as a balesilha, was used by sailors to get an accurate reading of latitude while at sea. The use of the balesilha was promoted by Prince Henry (1394–1460) while navigating for Portugal.^[34]

The astrolabe was almost certainly first brought north of the Pyrenees by Gerbert of Aurillac (future Pope Sylvester II), where it was integrated into the quadrivium at the school in Reims, France, sometime before the turn of the 11th century.^[35] In the 15th century, French instrument maker Jean Fusoris (c. 1365–1436) also started remaking and selling astrolabes in his shop in Paris, along with portable sundials and other popular scientific devices of the day.

Thirteen of his astrolabes survive to this day.^[36] One more special example of craftsmanship in early 15th-century Europe is the astrolabe designed by Antonius de Pacento and made by Dominicus de Lanzano, dated 1420.^[37]

In the 16th century,

Mechanical astronomical clocks were initially influenced by the astrolabe; they could be seen in many ways as clockwork astrolabes designed to produce a continual display of the current position of the sun, stars, and planets. For example, Richard of Wallingford's clock (c. 1330) consisted essentially of a star map rotating behind a fixed rete, similar to that of an astrolabe.^[39]

Many astronomical clocks use an astrolabe-style display, such as the famous

An astrolabe consists of a disk, called the mater (mother), which is deep enough to hold one or more flat plates called tympans, or climates. A tympan is made for a specific latitude and is engraved with a stereographic projection of circles denoting azimuth and altitude and representing the portion of the celestial sphere above the local horizon. The rim of the mater is typically graduated into hours of time, degrees of arc, or both.^[42]

Above the mater and tympan, the rete, a framework bearing a projection of the ecliptic plane and several pointers indicating the positions of the brightest stars, is free to rotate. These pointers are often just simple points, but depending on the skill of the craftsman can be very elaborate and artistic. There are examples of astrolabes with artistic pointers in the shape of balls, stars, snakes, hands, dogs' heads, and leaves, among others.^[42] The names of the indicated stars were often engraved on the pointers in Arabic or Latin.^[43] Some astrolabes have a narrow rule or label which rotates over the rete, and may be marked with a scale of declinations.

The rete, representing the sky, functions as a star chart. When it is rotated, the stars and the ecliptic move over the projection of the coordinates on the tympan. One complete rotation corresponds to the passage of a day. The astrolabe is, therefore, a predecessor of the modern planisphere.

On the back of the mater, there is often engraved a number of scales that are useful in the astrolabe's various applications. These vary from designer to designer, but might include curves for time conversions, a calendar for converting the day of the month to the sun's position on the ecliptic, trigonometric scales, and graduation of 360 degrees around the back edge. The alidade is attached to the back face. An alidade can be seen in the lower right illustration of the Persian astrolabe above. When the astrolabe is held vertically, the alidade can be rotated and the sun or a star sighted along its length, so that its altitude in degrees can be read ("taken") from the graduated edge of the astrolabe; hence the word's Greek roots: "astron" (ἄστρον) = star + "lab-" (λαβ-) = to take. The alidade had vertical and horizontal cross-hairs which plots locations on an azimuthal ring called an almucantar (altitude-distance circle).

An arm called a radius connects from the center of the astrolabe to the optical axis which is parallel with another arm also called a radius. The other radius contains graduations of altitude and distance measurements.

A shadow square also appears on the back of some astrolabes, developed by Muslim astrologists in the 9th Century, whereas devices of the Ancient Greek tradition featured only altitude scales on the back of the devices.^[44] This was used to convert shadow lengths and the altitude of the sun, the uses of which were various from surveying to measuring inaccessible heights.^[45]

Devices were usually signed by their maker with an inscription appearing on the back of the astrolabe, and if there was a patron of the object, their name would appear inscribed on the front, or in some cases, the name of the reigning sultan or the teacher of the astrolabist has also been found to appear inscribed in this place.^[46] The date of the astrolabe's construction was often also signed, which has allowed historians to determine that these devices are the second oldest scientific instrument in the world. The inscriptions on astrolabes also allowed historians to conclude that astronomers tended to make their own astrolabes, but that many were also made to order and kept in stock to sell, suggesting there was some contemporary market for the devices.^[46]

Mathematical basis

The construction and design of astrolabes are based on the application of the stereographic projection of the celestial sphere. The point from which the projection is usually made is the South Pole. The plane onto which the projection is made is that of the Equator.^[47]

Designing a tympanum through stereographic projection

The tympanum captures the celestial coordinate axes upon which the rete will rotate. It is the component that will enable the precise determination of a star's position at a specific time of day and year.

Therefore, it should project:

1. The zenith, which will vary depending on the latitude of the astrolabe user.
2. The horizon line and almucantar or circles parallel to the horizon, which will allow for the determination of a celestial body's altitude (from the horizon to the zenith).
3. The celestial meridian (north-south meridian, passing through the zenith) and secondary meridians (circles intersecting the north-south meridian at the zenith), which will enable the measurement of azimuth for a celestial body.
4. The three main circles of latitude (Capricorn, Equator, and Cancer) to determine the exact moments of solstices and equinoxes throughout the year.

The tropics and the equator define the tympanum

On the right side of the image above:

1.   The blue sphere represents the celestial sphere.
2.   The blue arrow indicates the direction of true north (the North Star).
3.   The central blue point represents Earth (the observer's location).
4.   The geographic south of the celestial sphere acts as the projection pole.
5.   The celestial equatorial plane serves as the projection plane.
6. Three parallel circles represent the projection on the celestial sphere of Earth's main circles of latitude:
   •   In orange, the celestial Tropic of Cancer.
   •   In purple, the celestial equator.
   •   In green, the celestial Tropic of Capricorn.

When projecting onto the celestial equatorial plane, three concentric circles correspond to the celestial sphere's three circles of latitude (left side of the image). The largest of these, the projection on the celestial equatorial plane of the celestial Tropic of Capricorn, defines the size of the astrolabe's tympanum. The center of the tympanum (and the center of the three circles) is actually the north-south axis around which Earth rotates, and therefore, the rete of the astrolabe will rotate around this point as the hours of the day pass (due to Earth's rotational motion).

The three concentric circles on the tympanum are useful for determining the exact moments of solstices and equinoxes throughout the year: if the sun's altitude at noon on the rete is known and coincides with the outer circle of the tympanum (Tropic of Capricorn), it signifies the winter solstice (the sun will be at the zenith for an observer at the Tropic of Capricorn, meaning summer in the southern hemisphere and winter in the northern hemisphere). If, on the other hand, its altitude coincides with the inner circle (Tropic of Cancer), it indicates the summer solstice. If its altitude is on the middle circle (equator), it corresponds to one of the two equinoxes.

The horizon and the measurement of altitude

On the right side of the image above:

1.   The blue arrow indicates the direction of true north (the North Star).
2.   The central blue point represents Earth (the observer's location).
3.   The black arrow represents the zenith direction for the observer (which would vary depending on the observer's latitude).
4.   The two black circles represent the horizon surrounding the observer, which is perpendicular to the zenith vector and defines the portion of the celestial sphere visible to the observer, and its projection on the celestial equatorial plane.
5.   The geographic south of the celestial sphere acts as the projection pole.
6.   The celestial equatorial plane serves as the projection plane.

When projecting the horizon onto the celestial equatorial plane, it transforms into an ellipse upward-shifted relatively to the center of the tympanum (both the observer and the projection of the north-south axis). This implies that a portion of the celestial sphere will fall outside the outer circle of the tympanum (the projection of the celestial Tropic of Capricorn) and, therefore, won't be represented.

Additionally, when drawing circles parallel to the horizon up to the zenith (almucantar), and projecting them on the celestial equatorial plane, as in the image above, a grid of consecutive ellipses is constructed, allowing for the determination of a star's altitude when its rete overlaps with the designed tympanum.

The meridians and the measurement of azimuth

On the right side of the image above:

1.   The blue arrow indicates the direction of true north (the North Star).
2.   The central blue point represents Earth (the observer's location).
3.   The black arrow represents the zenith direction for the observer (which would vary depending on the observer's latitude).
4.   The two black circles represent the horizon surrounding the observer, which is perpendicular to the zenith vector and defines the portion of the celestial sphere visible to the observer, and its projection on the celestial equatorial plane.
5.   The five red dots represent the zenith, the nadir (the point on the celestial sphere opposite the zenith with respect to the observer), their projections on the celestial equatorial plane, and the center (with no physical meaning attached) of the circle obtained by projecting the secondary meridian (see below) on the celestial equatorial plane.
6.   The orange circle represents the celestial meridian (or meridian that goes, for the observer, from the north of the horizon to the south of the horizon passing through the zenith).
7.   The two red circles represent a secondary meridian with an azimuth of 40° East relative to the observer's horizon (which, like all secondary meridians, intersects the principal meridian at the zenith and nadir), and its projection on the celestial equatorial plane.
8.   The geographic south of the celestial sphere acts as the projection pole.
9.   The celestial equatorial plane serves as the projection plane.

When projecting the celestial meridian, it results in a straight line that overlaps with the vertical axis of the tympanum, where the zenith and nadir are located. However, when projecting the 40° E meridian, another circle is obtained that passes through both the zenith and nadir projections, so its center is located on the perpendicular bisection of the segment connecting both points. In deed, the projection of the celestial meridian can be considered as a circle with an infinite radius (a straight line) whose center is on this bisection and at an infinite distance from these two points.

If successive meridians that divide the celestial sphere into equal sectors (like "orange slices" radiating from the zenith) are projected, a family of curves passing through the zenith projection on the tympanum is obtained. These curves, once overlaid with the rete containing the major stars, allow for determining the azimuth of a star located on the rete and rotated for a specific time of day.

See also

• Armillary sphere
• Astronomy in the medieval Islamic world
• Philippe Danfrie, designer and maker of mathematical instruments, globes and astrolabes
• Equatorium
• List of astronomical instruments
• Mariner's astrolabe
• Yantraraja
• Zeiss-Planetarium Jena
• Hamburg Planetarium
• Astronomical clock
• Hypatia
• Planetarium
• Prague astronomical clock

References

Footnotes

Notes

1. ^ "HISTORIANS' HOME YIELDS RICH LODE; New York Society Searches Its Own Building for Items to Mark Anniversary; SHOW OPENS THURSDAY; Portrait of Stuyvesant and Champlain's Astrolabe Will Be on Display". The New York Times. May 18, 1964. Retrieved February 4, 2024.
2. ISBN 978-1-4129-4164-8
   .
3. ^ "Astrolabe". Oxford English Dictionary (2nd ed.). 1989.
4. Oxford Dictionaries. Archived from the original
   on October 22, 2013.
5. ^ "Online Etymology Dictionary". Etymonline.com. Retrieved 2013-11-07.
6. ^ ^{a b} King 1981, p. 44.
7. ^ King 1981, p. 51.
8. ^ King 1981, p. 45.
9. ISSN 2076-0752
   .
10. S2CID 194101486
    .
11. ^ Wolf 2003, pp. 64–69.
12. ^ Lewis 2001.
13. ^ Michael Deakin (August 3, 1997). "Ockham's Razor: Hypatia of Alexandria". ABC Radio. Retrieved July 10, 2014.
14. ^
    ISBN 978-1-137-56997-4
    .
15. ^
    ISBN 978-1-59102-520-7
    .
16. ^
    ISBN 9780816054237
    .
17. ^ Sebokht, Severus. "Description of the astrolabe". Tertullian.org.
18. ISSN 1475-4878
    .
19. ISBN 978-0-691-11485-9
20. Richard Nelson Frye
    : Golden Age of Persia. p. 163
21. ^ Nizamoglu, Cem (2005-08-10). "Using an Astrolabe". Muslim Heritage. Retrieved 2023-10-16.
22. ISBN 978-0-521-80040-2
    .
23. ^ O'Connor, John J.; Robertson, Edmund F., "Sharaf al-Din al-Muzaffar al-Tusi", MacTutor History of Mathematics Archive, University of St Andrews
24. JSTOR 1006002
    .
25. ^ "Qantara – 'Carolingian' astrolabe". Qantara-med.org. Retrieved 2013-11-07.
26. ISBN 978-0-465-00950-3
27. ISBN 9780802779786
    ..
28. OCLC 889717964
    .
29. ^ "Introduction". The Astrolabe: an Online Resource. 2006. Retrieved 2020-05-15.
30. ISBN 0-226-31635-1
    .
31. ^ Kunitzsch, Paul (1981). "On the authenticity of the treatise on the composition and use of the astrolabe ascribed to Messahalla". Archives Internationales d'Histoire des Sciences Oxford. 31 (106): 42–62.
32. ISBN 978-1-4020-4559-2
    . Paul Kunitzsch has recently established that the Latin treatise on the astrolabe long ascribed to Ma'sh'allah and translated by John of Seville is in fact by Ibn al-Saffar, a disciple of Maslama al-Majriti.
33. ISBN 0-415-96930-1
34. OCLC 889717964
    .
35. ISBN 978-0-465-00950-3
36. ISBN 978-0-387-31022-0
    . Retrieved August 22, 2012.
37. ISBN 978-3-86560-865-9
38. doi:10.1484/J.ALMAGEST.5.114932
    .
39. ^ North 2005.
40. ^ "Astrolabium G. Galilei". Ulysse Nardin. Archived from the original on 2 January 2011.
41. ^ "Christaan van der Klauuw".
42. ^
    ISBN 0-521-79143-X
    .
43. ^ "Star Names on Astrolabes". Ian Ridpath. Retrieved 2016-11-12.
44. ^ King, David A. Some Medieval Astronomical Instruments and Their Secrets, in Mazzolini, R. G. (ed.), Non-Verbal Communication in Science prior to 1900. Florence. p. 30.
45. ^ King, David A. (2018). The Astrolabe: What it is & what it is not. Frankfurt, Germany: Frankfurt.
46. ^
    Bibcode:1956iatw.book.....M
    .
47. S2CID 211008813
    .

Bibliography

• Evans, James (1998), The History and Practice of Ancient Astronomy, Oxford University Press,
  ISBN 0-19-509539-1
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• Gunella, Alessandro; Lamprey, John (2007), Stoeffler's Elucidatio (translation of Elucidatio fabricae ususque astrolabii into English), John Lamprey
• King, D. A. (1981), "The Origin of the Astrolabe According to the Medieval Islamic Sources", Journal for the History of Arabic Science, 5: 43–83
• King, Henry (1978), Geared to the Stars: the Evolution of Planetariums, Orreries, and Astronomical Clocks, University of Toronto Press
• Krebs, Robert E.; Krebs, Carolyn A. (2003), Groundbreaking Scientific Experiments, Inventions, and Discoveries of the Ancient World, Greenwood Press.
• Laird, Edgar (1997), Carol Poster and Richard Utz (ed.), "Astrolabes and the Construction of Time in the Late Middle Ages.", Constructions of Time in the Late Middle Ages, Evanston, Illinois: Northwestern University Press: 51–69
• Laird, Edgar; Fischer, Robert, eds. (1995), "Critical edition of Pélerin de Prusse on the Astrolabe (translation of Practique de Astralabe)", Medieval & Renaissance Texts & Studies, Binghamton, New York,
  ISBN 0-86698-132-2
• Lewis, M. J. T. (2001), Surveying Instruments of Greece and Rome, Cambridge University Press.
• Morrison, James E. (2007), The Astrolabe, Janus,
  ISBN 978-0-939320-30-1
  .
• Neugebauer, Otto E. (1975), A History of Ancient Mathematical Astronomy, Springer
• North, John David (2005), God's Clockmaker: Richard of Wallingford and the Invention of Time, Continuum International Publishing Group,
  ISBN 978-1-85285-451-5
• Wolf, Markus (2003). Die Häuser von Solunt und die hellenistische Wohnarchitektur (in German). Mainz: P. von Zabern.
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External links

Wikimedia Commons has media related to:
Astrolabe (category)

Wikisource has the text of the 1911 Encyclopædia Britannica article "Astrolabe".

Look up astrolabe in Wiktionary, the free dictionary.

• Interactive digital astrolabe by Alex Boxer
• A digital astrolabe (HTML5 and javascript)
• Astrolabe Tech Made ... Not So Easy
• paper astrolabe generator, from the ESO
• "Hello World!" for the Astrolabe: The First Computer Video of Howard Covitz's Presentation at Ignite Phoenix, June 2009. Slides for Presentation Licensed as
  Creative Commons by-nc-nd
  .
• Video of Tom Wujec demonstrating an astrolabe. Archived 2012-03-23 at the
  Creative Commons by-nc-nd
  .
• Archive of James E. Morrison's extensive website on Astrolabes
• A working model of the Dr. Ludwig Oechslin's Astrolabium Galileo Galilei watch
• Ulysse Nardin Astrolabium Galilei Galileo: A Detailed Explanation
• Fully illustrated online catalogue of world's largest collection of astrolabes
• Mobile astrolabe and horologium
• Medieval equal hour horary quadrant
• A Beginner's Guide to Basic Construction and Use of the Astrolabe (using ruler, protractor and compasses) (PDF), archived from the original (PDF) on 2015-06-17, retrieved 2018-10-26