Al–Battani

One of the most important astronomers and mathematicians of this time in either the Eastern or the Western world was al–Battani (c. 858–929), whose full name was Abu Abdallah Muhammad ibn Jabir ibn Sinan al–Raqqi al–Harrani al–Sabi al–Battani.

As in other branches of science and mathematics, figures from the Arab world made key contributions in astronomy during the period of time that Europeans refer to as the Middle Ages. Many of these Arab investigators were fully aware of the fund of scientific knowledge that had come down from the Greeks and Romans of antiquity, and in many cases they improved on the principles and observations they had inherited. Al–Battani's work grew to a position of renown among European astronomers in later centuries, among whom he was known under various Latin (or Italian) forms of his name: Albatenius, Albategnius, or Albategni. His estimates of the length of the year, and of other less familiar numerical underpinnings of the modern science of astronomy, turned out to be strikingly accurate in an age that knew no telescopes or other modern astronomical equipment. He is regarded as one of the greatest astronomers in the entire tradition of the Islamic world.

Family Worshipped Stars

The facts of al–Battani's life are known only in outline. Based on the 877 date that he himself attached to his earliest astronomical observations, he is believed to have been born in the decade of the 850s in the European calendar, perhaps in the year 858. He was a native of Harran, in what is now Turkey, southeast of the modern city of Urfa. At the time, the area was part of the Mesopotamian lands whose cultural and administrative center was Baghdad. Although the "Abdallah Muhammad" component of his full name indicates that he converted to the Islamic faith at some point, his family adhered to the Sabian religion, a local sect in whose belief system the stars played a central role. Members of this sect had transmitted and cultivated astronomical data and stories dating back to the culture of ancient Mesopotamia, centuries before. The area spawned other important astronomers and mathematicians including Thabit ibn Qurra, who was slightly older than al–Battani and would have lived in Harran during al–Battani's youth.

It is unclear where al–Battani acquired that name; it might have referred to a place where he lived or worked later in life. The "al–Harrani" ("Resident of Harran") portion of his name was shared with another famous scientific figure of the time, Jabir ibn Sinan al–Harrani, who was a maker of scientific instruments and who may well have been al–Battani's father. Al–Battani himself was noted for his skill in this trade, and some of the accuracy of the measurements for which he became famous was due to the superior quality of the instruments he made for himself. Among these was an ingenious type of astronomical model called an armillary sphere; mounted like a modern globe, it contained rings representing the movements of celestial bodies. Like a globe, the hollow sphere could be rotated on a central axis, and the individual rings could also be rotated. The whole sphere was encircled by a larger ring whose circumference was divided into degrees.

Al–Battani did not invent the armillary sphere, but his sphere was more precise than earlier versions. Modeling of this kind helped al–Battani make several important astronomical calculations regarding the sun's relationship to the earth. Although the realization that the earth orbited around the sun rather than the other way around awaited the discoveries of Copernicus, published in 1543, al–Battani accurately observed that the distance between sun and earth varies rather than remaining constant. One correct conclusion al–Battani drew from this observation was that annular eclipses of the sun, in which the moon interposes itself exactly between earth and sun but leaves a bright ring around its edge, would occur occasionally, when the sun was at its greatest distance from the earth.

Another important and accurate observation al–Battani made regarding the earth and sun pertained to the fact that the plane formed by the earth–sun orbit does not match that formed by an imaginary slice through the earth's equator. Al–Battani's calculation of the angle between these two planes, known as the obliquity or inclination of the ecliptic, resulted in the figure of 23 degrees and 35 minutes, remarkably close to the actual figure of 23 degrees, 27 minutes, and 8.26 seconds. He also made important discoveries concerning the so–called precession of the equinoxes, the changes in the time of the annual equinox as reckoned against the positions of bodies in the sky. All of this information was well known to the European astronomers of the Renaissance who laid the foundations for the modern understanding of the physical world.

Lived in Present – Day Syria

Al–Battani apparently spent much of the roughly 40 years of his astronomical career in the city of al–Raqqa, on the Euphrates River in what is now Syria (hence the "al–Raqqi" in his name). He may have chosen that place because several other families from Harran had moved there. Much of his time was spent in making astronomical observations and in compiling the data that underlay his major work, known as the Kitab al–Zij (The Book of Astronomy) or simply as the Zij (a word originally derived from the Persian language, where it denoted a certain strand used in weaving a rug. After issuing the Zij in one version before the year 900, he revised it sometime after 901, taking into account two eclipses, one solar and one lunar, that he had had the chance to see that year on a visit to the city of Antioch in Syria.

The Zij had 57 chapters, plus a preface in which al–Battani exhorted future generations to improve upon his own results. It ranged widely over what was known in his time of the heavens and the structure of the universe. The first part of the book described the celestial sphere and divided it in two ways, into degrees and into signs of the Zodiac—in the ancient world, astrology was considered an important and fully valid science, and several of al–Battani's shorter writings took up aspects of the subject. Al–Battani went on to lay out the mathematical underpinnings of his work, and then to take up specific astronomical problems.

Along the way he included a catalogue of stars that he had made in the year 880, naming 489 stars and creating one of the most valuable star registries of the era before telescopes (although the Greek astronomer Ptolemy had named 1,022). He also estimated the length of the year at 365 days, 5 hours, 48 minutes, and 24 seconds, an error of slightly less than seven one–hundredths of one percent. In the middle of the book, al–Battani explained his theory of the motion of the sun, the moon, and the five known planets (Mercury, Venus, Mars, Jupiter, and Saturn). He gave directions for using the tables in the book, returned to astrology toward the end, and concluded the book with several chapters on the construction of astronomical instruments, including a sundial. The Arab bookseller Ibn an–Nadim, writing in the year 988 (and quoted in a review of al–Battani's work shown on the website of St. Andrews University of Scotland) opined that "Nobody is known in Islam who reached similar perfection in observing the stars and scrutinizing their motions."

Several other specific aspects of al–Battani's work exerted a strong influence over later generations of scholars. The most important was that he used what we would now call trigonometry—the study of the ratios pertaining to the sides and angles of right triangles—in making his calculations. Trigonometry had roots in ancient India and spread from there to the Arab world. Al–Battani made use of such concepts as the sine, cosine, tangent, and cotangent, an improvement over the methods used by the greatest of the Greek astronomers, Ptolemy.

Ptolemy (active beginning around the year 127) was widely revered by Arab astronomers of the day, including al–Battani. In fact, just as younger scholars today may be reluctant to dismantle the work of their mentors, al–Battani was circumspect about criticizing Ptolemy even where he had clearly improved on the Greek astronomer's work. He tended rather to correct Ptolemy in a tacit way, without referring to Ptolemy's errors. In general, al–Battani's place in history is that of a refiner of Ptolemy's investigations.

Died after Petitioning Baghdad Government

Al–Battani was active as an astronomer until about the year 918. In the year 929 he accompanied a group of townspeople from al–Raqqa, who may have included some of his own descendants, to Baghdad as part of what was likely a tax protest. The aging astronomer survived to plead his cause but died on the journey home at Qasr al–Jiss, near the present–day Iraqi city of Samarra.

After his death, al–Battani's influence was magnified. The praise of the bookseller Ibn an–Nadim cited above attests to his fame in the Arab world. During the later medieval era in Europe, Western scholars turned to the Islamic world as they attempted to reconstruct the foundations of sciences that had been buried since the decline of the Roman empire centuries before. Not only astronomy and mathematics but also music, medicine, history, and linguistic studies in the West bear strong Arabic imprints at their deepest levels. Al–Battani's Zij was twice translated into Latin in the twelfth century, but only one translation, made by Plato of Tivoli in 1116 under the title De motu stellarum (On the Motion of the Stars), survived. The lost translation was by one Robert Retinensis, probably Robert of Chester, the first scholar to translate the Koran into Latin. King Alfonso X of Spain ordered another translation made in the thirteenth century, this one into Spanish. This also survives today. Al–Battani's original manuscript is housed in the Vatican Library.

At a time when very few books were selected to appear in printed form, al–Battani's Zij made the list. The translation by Plato of Tivoli was published in Nürnberg, Germany, in 1537 and another edition appeared in Bologna, Italy, in 1645. Thus the treatise became known to astronomers and mathematicians all over central and northern Europe. Spanish Jewish astronomers, too, knew al–Battani's work. The greatest astronomers of the European Renaissance—Nicholas Copernicus, Johannes Kepler, and Tycho Brahe—all explicitly acknowledged al-Battani's influence, and as late as 1749 his observations of eclipses were still being cited by astronomers.

Al–Battani's observations of solar motion, in fact, were more accurate than those of the great Copernicus himself, perhaps because al–Battani worked at a more southerly latitude and did not have to factor into his calculations certain types of atmospheric refraction that become more pronounced closer to the poles. The European historian of Islam C.A. Nallino published a gigantic Arabic edition of al–Battani's Zij in three volumes between 1899 and 1907, and science historians since then have noted his role as a follower of Ptolemy and as a bridge–builder between the ancient world and the foundations of modern astronomy. Like much of the fascinating history of the Arab world's influence on Western science and culture, however, al–Battani's name is little known even among general readers with a scientific background in Western countries.

Books

Biographical Dictionary of Mathematicians, edited by Charles Coulston Gillispie, Scribner's, 1991.

Daintith, John, et al., Biographical Encyclopedia of Scientists, 2nd. ed., Institute of Physics Publishing, 1994.

Encyclopedia of the Institute of Physics Publishing, 2nd. ed., 1994. edited by H.A.R. Gibb et al., Brill (Leiden, Netherlands), 1960.

Notable Mathematicians, Gale, 1998.

Sarton, George, Introduction to the History of Science, Volume I: From Homer to Omar Khayyam, Robert E. Krieger Publishing Company, 1975.

Online

"Abu Abdallah Mohammad ibn Jabir Al–Battani," School of Mathematics and Statistics, University of St. Andrews, Scotland, http://www–gap.dcs.st–and.ac.uk (December 13, 2004).

al-Battani

c. 858-929

Arab Astronomer and Mathematician

Al-Battani is considered the greatest astronomer of the medieval Islamic world. He is best known for his astronomical handbook Kitab al-Zij, which introduced new trigonometric methods for performing astronomical computations. He devised improved instruments and made accurate observations that allowed him to give corrected values for several astronomical constants, including the obliquity of the ecliptic and time of the equinoxes.

Known to the West as Albategnius, al-Battani was born in or near Harran in northwestern Mesopotamia (modern Turkey) around 858. His father was the noted instrument-maker Jabir ibn Sinan al-Harrani. No information exists regarding al-Battani's formal education. However, it seems reasonable to assume that his facility in devising improved instruments, which included a new type of armillary sphere, was nurtured through technical training provided by his father. His ancestors were Sabians, a religious sect adhering to a mixture of Christian and Islamic doctrines, but al-Battani was fully committed to Islam. Most of his research was conduct at al-Raqqa on the Euphrates River, where his family had moved when he was a youth. In 929 he journeyed to Baghdad to petition the caliph regarding some injustice to the people of al-Raqqa. He died at Qasr al-Jiss on the return trip.

Of the many works al-Battani wrote on astronomy and mathematics, his best known, Kitabal-Zij, contains most of his important findings. The Zij consists of a star catalog and trigonometric tables, as well as solar, lunar, and planetary tables together with canons for their use. The treatise was written from a Ptolemaic perspective. However, al-Battani turned a critical eye toward Ptolemy's (second century a.d.) practical results, and designed improved instruments to collect more accurate data. These observations revealed errors in Ptolemy's Almagest and allowed al-Battani to provide corrected values for many of the main parameters of planetary motion.

Al-Battani's careful observations showed that the solar apogee, the point at which the Sun is smallest in apparent size and farthest from Earth, had shifted from the position indicated by the Almagest. Contradicting Ptolemy, this implied that the solar apogee was slowly moving. Though al-Battani did not explicitly state this, he has often been credited with the discovery. Thabit ibn Qurra (836-901) had earlier found the solar apogee to be moving at a rate of 1° every 60 years, but when he found the rate of precession (the conical gyration of a spinning body's axis of rotation) to be the same, he concluded they were identical. Al-Battani's value for the solar apogee was no better than Ibn Qurra's. Thus, he was in no better position to distinguish between the Sun's motion and precession. In fact, the first to correctly and unambiguously state the proper motion of the solar apogee was al-Zarqali (1029?-1087?). Al-Battani also provided better values for the obliquity of the ecliptic and the precession constant.

Among his many other accomplishments, al-Battani rectified the Moon's mean motion in longitude, provided better estimates of the apparent solar and lunar diameters and their annual variation, and demonstrated the possibility of annular solar eclipses. He also developed a sophisticated method for determining the magnitude of lunar eclipses. His redetermination of the time of equinox allowed him to make an improved estimation of the tropical year, which he calculated as 365 days, 5 hours, 46 minutes, and 24 seconds—short by only 2 minutes and 22 seconds. Al-Battani also improved astronomical computations through his introduction of the trigonometric sine function and formulae for the solution of problems involving spherical triangles.

The Zij proved influential in the development of European astronomy. It was originally translated into Latin during the twelfth century by Robert of Chester (fl. c. 1141-1150). However, the only extant Latin version was made about the same time by Plato of Tivoli. This version was first printed in Nuremberg (1537) under the title De motu stellarum (On Stellar Motion).

STEPHEN D. NORTON