From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, pp. 133-134 |
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Biṭrūjī: Nūr al‐Dīn Abū Isḥāq [Abū Jaʿfar] Ibrāhīm ibn Yūsuf al‐Biṭrūjī
Julio Samsó
Alternate
name
Alpetragius
Flourished Andalusia (Spain),
1185–1192
Biṭrūjī
was a famous Andalusian (Arab) cosmologist who wrote an astronomical work
that was quite influential in Latin Europe, where he was known as Alpetragius.
Little is known of his life. He was probably a disciple of the philosopher
Ibn Ṭufayl (died: 1185/1186), who
was already dead when Biṭrūjī wrote his Kitāb fī al‐hayʾa.
On the other hand, an anonymous treatise on tides (Escorial MS 1636, dated
1192) contains ideas seemingly borrowed from Biṭrūjī's work. A more
definitive guide to dating is Michael
Scot, who finished his Latin translation of Biṭrūjī's work in Toledo in 1217. His book was also
translated into Hebrew by Mosheh ben Tibbon in 1259, and one of the manuscripts
of this Hebrew translation states that he was a judge. A late 15th‐century
Moroccan source calls him faqīh (jurist). His name, al‐Biṭrūjī, may be a corruption of al‐Biṭrawshī,
derived from Biṭrawsh,
a village in Faḥṣ
al‐Ballūṭ
(Cordova province).
Biṭrūjī's
only extant work bears the title Kitāb [murtaʿish] fī
al‐hayʾa (A [revolutionary] book on cosmology), which is extant
in two Arabic manuscripts, the Latin translation of Scot, the Hebrew translation
of ben Tibbon, and the Latin by Calo Calonymos (1286–circa 1328) from
the Hebrew. A modern English translation and commentary can be found in Goldstein
(1971).
Biṭrūjī's
book is the final result of the efforts made by Andalusian Aristotelian philosophers
of the 12th century (Ibn Bājja,
Ibn Ṭufayl, Ibn
Rushd, and Maimonides) to
overcome the physical difficulties inherent in the geometrical models of Ptolemy's
Almagest and to describe the cosmos in agreement with Aristotelian
or Neoplatonic physics. It is a book on hayʾa (theoretical astronomy/cosmology).
Earlier Andalusian work in this genre include two books by Qāsim
ibn Muṭarrif al‐Qaṭṭān (10th
century), who followed the line of Ptolemy's Planetary Hypotheses,
and an anonymous Toledan author of the second half of the 11th century who
seems to represent the earliest Andalusian attempt to criticize the Almagest
from a physical point of view. Despite these precedents in the Islamic west,
Biṭrūjī seems to be the
first to present alternatives to Ptolemy's models. His knowledge of the astronomical
literature, though, was limited; he had probably read the Almagest,
but he does not seem to have understood it completely. According to Biṭrūjī,
Ptolemy was the archetypical mathematical astronomer who created imaginary
models that were successful in their ability to predict planetary positions
but were totally unreal.
Besides
Ptolemy, Biṭrūjī
may have read Theon
of Alexandria's Commentary to the Almagest. He also was well
acquainted with the treatise on the motion of the fixed stars by Zarqālī.
Furthermore, he quotes Jābir ibn
Aflaḥ's Iṣlāḥ al‐Majisṭī
(Revision of the Almagest) regarding the problem of the order of the
planets in the Solar System but rejects Jābir's proposal to put both
Mercury and Venus above the Sun, opting instead to make only Venus a superior
planet. Jābir had argued that proposal on the basis of a lack of records
of Mercury or Venus transits, but Biṭrūjī
suggested that this might be because of both Mercury and Venus being self‐luminous.
Biṭrūjī
presented the first non‐Ptolemaic astronomical system after Ptolemy,
although he admits that the results are only qualitative. As a follower of
Aristotle,
his system is homocentric, the celestial bodies being always kept at the same
distance from the center of the Earth. Despite this, Biṭrūjī employs mathematical
eccentrics and epicycles, which are placed on the surface of the corresponding
sphere and in the area of the pole. Apparently, he has adapted ideas derived
from Zarqālī's trepidation models or perhaps from Eudoxus.
One of the most original aspects of Biṭrūjī's system is his proposal of a physical cause
of celestial motions. Biṭrūjī uses the idea
of impetus, originally put forth by John
Philoponus (6th century) to deal with forced motion in the sublunar
world, to account for the transmission of energy from a first mover that is
placed in the ninth sphere. The motion of the ninth sphere, which rotates
uniformly once every 24 hours, is transmitted to the inner spheres, and it
becomes progressively slower as it approaches the Earth. The velocity of rotation
of each sphere is used by Biṭrūjī to establish the order of the planets. It
is noteworthy that Biṭrūjī is applying the same dynamics to the sublunar
and the celestial worlds, contradicting the Aristotelian idea that there is
a specific kind of dynamics for each world. Indeed, the force of the first
mover reaches the sublunary world causing the rotation of comets in the upper
atmosphere as well as the tides. Similar ideas can also be found in Ibn Rushd.
Both Ibn Rushd and Biṭrūjī
use another idea to explain this transmission of motion: the celestial spheres
feel a “passion” or “desire” (shawq, desiderium) to imitate
the sphere of the first mover, which is the most perfect one. Thus the spheres
closer to the first mover are most like the ninth sphere and therefore move
faster, while those farther away move slower. This use of shawq seems
to derive from Neoplatonic notions developed by the philosopher Abū al‐Barakāt
al‐Baghdādī (died: 1164), whose ideas may have been introduced
into Andalusia by his disciple Abū Saʿd
Isaac, the son of Abraham ibn ʿEzra.
Impetus
and shawq were used by Biṭrūjī in his attempt
to solve a puzzling problem: How can one explain that the unique first mover
can produce both the daily east–west motion and the longitudinal (zodiacal)
west–east motions in the planetary spheres? Biṭrūjī's explanation
is that the motions in longitude can be explained as a “delay” (taqṣīr,
incurtatio) in the perfect daily motion being transmitted by the first
mover; this delay becomes progressively more noticeable in the planetary spheres
further away from the first mover.
Biṭrūjī
builds his geometrical models on this theoretical basis. Taqṣīr corresponds to the
planetary motion in longitude while Biṭrūjī seems to identify
shawq with the anomaly. In the case of the planets, each one of them
moves near the ecliptic but its motion is regulated by the pole of each planet,
placed at a distance of 90° from the planet itself. This pole rotates on a
small polar epicycle whose center moves, as a result of taqṣīr, on a polar deferent. This use of a type of
deferent and epicycle (within the context of homocentric astronomy) allows
Biṭrūjī to
explain, in a way similar to Ptolemy, the irregularities of planetary motions
(direct motion, station, retrogradation). The problem is that Biṭrūjī
also tries to explain, using the motion in anomaly (rotation of the pole of
the planet on the polar epicycle), the changes in planetary latitude. This,
however, does not really work since the periods of recurrence in anomaly and
in latitude are not the same. Other problems result due to Biṭrūjī's ambiguity regarding
the direction of motions and the fact that shawq does not diminish,
as claimed, in the planetary spheres as they are further removed from the
first mover. Thus, despite their ingenuity, Biṭrūjī's models are unable to provide the predictive
accuracy of Ptolemy's models, and there are inconsistent aspects to them as
well. In the case of the fixed stars, he proposes a model that results in
a variable velocity in the precession of equinoxes, which echoes earlier Andalusian
theories of the trepidation of the equinoxes. The geometrical model for the
fixed stars is not easy to understand as preserved in the extant texts. A
recent paper by J. L. Mancha (2004) gives a new and sophisticated interpretation,
based on the Latin translation, which supports the hypothesis formulated by
E. Kennedy in 1973 that Biṭrūjī's
homocentric system is an updating and reformulation of the system of Eudoxus.
For the motion of the fixed stars the Zarqālian tradition would be combined
with aspects of Eudoxus's models, i. e., he uses a Eudoxan couple that
results in a hippopede. With Mancha's interpretation, Biṭrūjī's
model for the fixed stars makes sense, but we have the problem of establishing
which sources available to the Andalusian cosmologist gave him information
on Eudoxus's models.
Despite its scientific failings, the Kitāb fī al‐hayʾa
was quite successful. The Latin translation by Michael Scot contributed to
its European diffusion between the 13th and the 16th centuries. It was accepted
in scholastic circles where it was considered a valid alternative to Ptolemy's
Almagest. The work was also known in the Islamic East, perhaps introduced
in Egypt by Maimonides. The Damascene astronomer Ibn
al‐Shāṭir
mentions a certain al‐Majrīṭī as having presented non‐Ptolemaic
models; this may be a corruption of al‐Biṭrūjī's
name.
Abattouy, Mohammed (2001). “Au dessus ou au‐dessous du Soleil: Prolégomènes
sur la position de Mercure et Vénus dans la tradition astronomique andalouse.”
In Science et pensée scientifique en Occident Musulman au Moyen Age,
edited by Bennacer El Bouazzati, pp. 19–42. Rabat: Faculty of
Letters of Rabat.
Avi‐Yonah, Reuven S. (1985). “Ptolemy vs al‐Biṭrūjī:
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internationales d'histoire des sciences 35: 124–147.
Carmody,
Francis J. (1952). Al‐Biṭrūjī: De motibus celorum. Critical
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California Press. (See review by E. S. Kennedy in Speculum
29 (1954): 246–251.)
Casulleras, Josep (1998). “The Contents of Qāsim ibn Muṭarrif
al‐Qaṭṭān's
Kitāb al‐hayʾa.”
In The Formation of al‐Andalus, Part 2:
Language, Religion, Culture and the Sciences, edited by Maribel
Fierro and Julio Samsó,
pp. 339–358. Aldershot: Ashgate.
Cortabarría,
Angel (1982). “Deux sources de S. Albert le Grand: Al‐Bitruji et al‐Battani.”
Mélanges de l'Institut
dominicain d'etudes orientales 15: 31–52.
Forcada,
Miquel (1999). “La ciencia en Averroes.” In Averroes y los averroísmos: Actas del III Congreso Nacional de Filosofía Medieval. Zaragoza:
Sociedad de Filosofía
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Goldstein, Bernard R. (1971). Al‐Biṭrūjī:
On the Principles of Astronomy. 2 Vols. New Haven: Yale University Press.
(See the reviews by Kennedy and Lorch.)
Kennedy, E. S. (1973). “Alpetragius's Astronomy.” Journal for the History of Astronomy
4: 134–136.
Lorch, Richard
(1974). “Review of Al‐Biṭrūjī: On the
Principles of Astronomy” by Bernard Goldstein. Archives internationales d'histoire des sciences 24: 173–175.
Mancha, J.
L. (2004). “Al‐Biṭrūjī's Theory of the Motion of the Fixed
Stars.” Archive for the History of the Exact Sciences 58: 143–182.
Sabra,
A. I. (1984). “The Andalusian Revolt against
Ptolemaic Astronomy: Averroes and al‐Biṭrūjī.”
In Transformation and Tradition in the Sciences, edited by Everett
Mendelsohn, pp. 133–153. Cambridge: Cambridge University Press.
(Reprinted in Sabra, Optics, Astronomy and Logic,
XV. Aldershot: Ashgate, 1994.)
Saliba, George
(1994). A History of Arabic Astronomy: Planetary Theories during the Golden
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——— (1999). “Critiques of Ptolemaic
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——— (1994). Islamic Astronomy
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