Astronomical Observatories in the Classical Islamic Culture

The modern astronomical observatory as a research institute (as opposed to a private observation post as was the case in ancient times) is a creation of the Islamic scientific tradition. Since the early 9th century, the astronomers of Islamic lands worked in astronomical observtories in which they performed precise observations of the skies and produced accurate astronomical tables. The Islamic observatory was a dynamic scientific specialized institution with its own scientific staff, director, astronomical program, large astronomical instruments and building. Islamic observatories were also the earliest institutions to emphasize group research and in them theoretical investigations went hand in hand with observations.

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FSTC Research Team

*Table of contents

1. Founders of a Dynamic Tradition of Astronomy
2. Al-Biruni Astronomer
3. Scientific Investigation in the Islamic Observatories
4. Instrumentation in Islamic Observatories
5. The Impact of Al-Battani on European Astronomy
6. The Legacy of Ulugh Beg
7. Final Remarks
8. Bibliography
9. Notes

Note of the editor

This article has been published on in August 2002 in PDF format. We present it to our readers in a new editing in HTML, with illustrations. © FSTC 2002-2009.

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1. Founders of a Dynamic Tradition of Astronomy

In the history of astronomy, the tradition of Islamic or Arabic astronomy was one of the most thriving and dynamic. Its contribution to astronomical knowledge extend over several centuries during the Islamic classical age, from the 8th century until at least the late 16th century. Its written corpus was mostly written in the Arabic language and the areas of its production took place in the Middle East, Central Asia, Al-Andalus, and North Africa, and later in China and India. The tradition of astronomy in Islamic science closely parallels the genesis of other sciences in its assimilation of foreign material and the amalgamation of the disparate elements of that material to create a science. These included Sassanid, Hellenistic and Indian works in particular, which were translated and built upon. In turn, Islamic astronomy later had a significant influence on Indian, Byzantine and European astronomy as well as Chinese astronomy.

Figure 1: The figure of Ka'ba in Ibn Al-Saffâr's Al-Zîj al-Mukhtasar, Istanbul, Süleymaniye Library, MS Sehit Ali Pasha 2776/2.

A significant number of stars in the sky, such as Aldebaran and Altair, and astronomical terms such as alhidade, azimuth, and almucantar, are still today recognized with their Arabic names. A large corpus of literature from Islamic astronomy remains today, numbering approximately 10,000 manuscripts scattered throughout the world, many of which have not been investigated yet. Even so, the historians of science since the 19th century devoted a remarkabe work of study, edition and commentary that gave rise to a reasonably accurate picture of Islamic activity in the field of astronomy [1].

The early eminent Muslim astronomers include Al-Battani, al-Sufi, al-Biruni and Ibn Yunus. Al-Battani (d. 929), known to the Latins as Albategni or Albatenius, was the author of the Sabian Tables (Al-Zij al-Sabi), a work which had great impact on his successors [2]. His improved tables of the sun and the moon comprise his discovery that the direction of the sun's eccentric as recorded by Ptolemy was changing. This, in modern astronomy, means the Earth is moving in varying ellipse [3]. He also worked on the timing of the new moons, the length of the solar and sideral year, the prediction of eclipses, and the phenomenon of parallax, carrying us "to the verge of relativity and the space age", Wickens asserts [4].

Al-Battani also popularised, if not discovered, the first notions of trigonometrical ratios as we use them today [5]. During the same period, Yahya Ibn Abi Mansur had completely revised the astronomical table of Ptolemy's Almagest after meticulous observations and tests, which lead him to produce the famous Al-Zij al-Mumtahan (the validated astronomical table) [6].

Belonging to the same era, Abd-al Rahman al-Sufi (903-986) made several observations on the obliquity of the ecliptic and the motion of the sun (or the length of the solar year [7].) He became renowned for his observations and descriptions of the stars, their positions, their magnitudes (brightness) and their colour, setting out his results constellation by constellation, for each constellation, providing two drawings, one from the outside of a celestial globe, and the other from the inside (as seen from the sky) [8]. Al-Sufi also wrote on the astrolabe, finding thousands of uses for it. En par with other learned Muslims, he also pinpointed shortcomings of Greek astronomy.

Ibn Yunus (d. 1009), in his observation endeavours used, amongst others, a large astrolabe of nearly 1.4 m in diameter, and made observations which included more than 10, 000 entries of the sun's position throughout the years [9]. His work, in French edition [10], inspired Laplace in his determination of the ‘Obliquity of the Ecliptic' and the ‘Inequalities of Jupiter and Saturn's'. Newcomb also used his observations of eclipse in his investigations of the motions of the moon [11].

2. Al-Biruni Astronomer

Al-Bīrūnī (973-circa 1050) was one of the most accomplished scientists of the entire Middle Ages, and his interests extended to almost all branches of science. The total number of his works, mostly in Arabic, is 146, of which only 22 are extant. Approximately half of these writings are in the exact sciences. In addition to mathematics, astronomy, and astrology, he was accomplished in the fields of chronology, geography, pharmacology, and meteorology.

Figure 2: The drawing of deferents by Taqi al-Din ibn Ma'ruf in his treatise of astronomy Sidrat muntaha al-afkar, Nuruosmaniye library, MS 2308.

Since his young age, Al-Bīrūnī studied Greek science, especially astronomy. He was convinced of the importance of observation, and he recorded many of his own observations in his books. One of these works is his Tahdīd al-amākin (Determination of coordinates of cities). In this book, Al-Bīrūnī mentions a lunar eclipse of 997 that he observed in Khwārizm, having arranged a simultaneous observation with Abū al-Wafā al-Būzjānī who was residing in Baghdad. Al-Bīrūnī's aim was to find the difference in longitude of the two cities.

Al-Bīrūnī's The Chronology of the Ancient Nations, written in about 1000, is a mine of information on calendars used by the Persians, Sogdians, Kwārizmians, Jews, Syrians, Harrānians, Arabs, and Greeks. This is still one of the most reliable sources on ancient and medieval chronology.

In the second half of his life, Al-Bīrūnī became more and more interested in Indian culture. This change came as a result of his accompanying Sultan Mahmūd on several expeditions to India. Going deep in his knowledge of the Indian legacy, Al-Biruni was able to accumulate much knowledge of Indian culture, especially that of the exact sciences written in Sanskrit. His studies on India resulted in his masterpiece called India, completed in 1030.

Al-Bīrūnī was most productive in the years around 1030, after Mahmūd died and the throne passed on to his elder son Mas'ūd, to whom Al-Bīrūnī dedicated his magnum opus on astronomy, al-Qānūn al-Mas'ūdī. The book consists of 11 treatises, each containing several chapters. Treatise I is an introduction, dealing with the principles and basic concepts of astronomy as well as cosmology, time, and space. Treatise II deals with calendars, the three best known being the Hijra, Greek (i. e., Seleucid), and Persian. Treatise III is on trigonometry. Treatise IV takes up spherical astronomy. Treatise V discusses geodesy and mathematical geography. Treatise VI is on time differences, the solar motion, and the equation of time. Treatise VII deals with the lunar motion. Treatise VIII is on eclipses and crescent visibility. Treatise IX is on the fixed stars. Treatise X is on the planets. Treatise XI describes astrological operations.

Al-Qānūn al-Mas'ūdī is primarily based on Ptolemy's Almagest, but many new elements, of Indian, Iranian, and Arabic origin, are added. Al-Bīrūnī also tried to improve Ptolemy's astronomical parameters using the observations that were made by his predecessors and by himself. He refers to the elements of Indian calendar and chronology in Treatises I and II. In Treatise III, after explaining the chords according to Ptolemy, he offers a table of sines as well as a table of tangents (gnomon shadows). The 1,029 fixed stars are tabulated in Table IX.5.2 following the model of those in the Almagest (where the number is 1,022). To the longitude of the stars in the Almagest, Al-Bīrūnī added 13° according to the increase from Ptolemy's time due to the precession of equinoxes. The magnitudes of the stars are given in two columns, one based on the Almagest and the other from Sūfī's book on the 48 constellations. Al-Bīrūnī's planetary theory, which is found in Treatise X, is essentially the same as Ptolemy's, with some modifications in the parameters. The last treatise is on the topic of astrology, which require highly advanced knowledge of mathematics; these include the equalization of the houses and the determination of the length of one's life by means of the computation of an arc called tasyīr.

Although al-Qānūn al-Mas'ūdī did not have much influence in medieval Europe, the book was well read in the eastern half of the Muslim world and indeed further east. One example of this is that a very peculiar irregularity in Mercury's first equation table in the al-Qānūn can be attested to in the Chinese text Huihui li (composed in 1384).

Another major work of Al-Bīrūnī is Kitāb al-tafhīm li-awā'il sinā'at al-tanjīm. This book is divided into three parts with the subject areas being mathematics, astronomy, and astrology. Al-Bīrūnī's aim is very clearly stated by himself: "I have begun with geometry and proceeded to arithmetic and the science of numbers, then to the structure of the Universe, and finally to judicial astrology, for no one is worthy of the style and title of astrologer who is not thoroughly conversant with these four sciences [12]."

3. Scientific Investigation in the Islamic Observatories

Observation of the sky had begun in earnest in Islam. The observatory [13] as a distinct scientific institution for observation, and where astronomy and allied subjects were researched, also owes its origin to Islamic science [14]. The first to be set up was the Shammasiyah observatory, which Caliph Al-Mamun had built in Baghdad around 828. It was associated with the scientific academy of Bayt al-Hikma (House of Wisdom) (also set up by Al-Mamun.) The astronomers made observations of the sun, the moon and planets, and results were presented in a book called the Mumtahan (Validated or Tested) Zij. In the same century, more observations were made by the Banu Musa brothers mostly in Baghdad. Their accomplishments included the study of The Ursa Major (or the Great Bear). They also measured maximum and minimum altitudes of the sun, and observed lunar eclipses. Ibn Sina, Al-Battani, Al-Fargani and other scholars also devoted much of their attention and focus to observation and study of the sky.

Figure 3: The drawing of a sundial and its table by Taqi al-Din in his book Rayhanat al-ruh fi rasm al-sa'at 'ala mustawa 'l-sutuh, Suleymaniye Library, MS Esad Efendi 2055.

In the 11th century, the Seljuk Sultan Malik Shah (ruled 1072-1092) built a more advanced observatory, which functioned for almost 20 years. Two centuries later, approximately, was built the Maragha Observatory in Azerbaidjan. It was fitted with a large library (over 400,000 books) and also with instruments capable of greater performance (hence of large size). Maragha observatory was managed by no less than Nasir Al-Din Al-Tusi (d. 1274) and Qutb Al-Din Al-Shirazi. Al-Tusi was the author of the Ilkhanid Tables and the catalogue of fixed stars that were to rule for several centuries throughout the world. Maragha observatory also became an institution for research, and an academy for scientific contacts and teaching. It lasted until at least the beginning of the 14th century. Today, however, all that remain are the foundations of it.

Further advance in the construction of observatories is observed at Samarqand where Ulugh Beg founded an observatory in around 1424. It was a ‘monumental' building equipped with a huge meridian, made of masonry, symbol of the observatory as a long lasting institution [15]. A trench of about 2 metres wide was dug in a hill, along the line of the meridian, and in it was placed the segment of the arc of the instrument. Built for solar and planetary observations, it was equipped with the finest instruments available, including a ‘Fakhri sextant', with a radius of 40.4 metres, which made it the largest astronomical instrument of its type. The main use of the sextant was to determine the basic constants of astronomy, such as the length of the tropical year.

Other instruments included an armillary and an astrolabe. Ulugh Beg also assembled the best-known mathematicians of his day among whom was al-Khashi, who wrote an elementary encyclopaedia on practical mathematics for astronomers, surveyors, architects, clerks and merchants [16]. Observations were quite advanced for their time, and so the stellar year was found to be 365 days, 6 hours, 10 minutes and 8 seconds (only 62 seconds more than the present estimation).

The observatory at Samarqand remained active until nearly 1500 [17], but was later reduced to ruins, and apparently disappeared, until the archaeologist V. L. Vyatkin found its remains in 1908. Amongst the remains was a fragment of the gnomon of large size used to determine the height of the sun from the length of the shadow. There were also remains of a building of cylindrical shape with a complex interior plan [18]. It is also known through Abd-al-Razak that one could see a portrayal of the ten celestial spheres with degrees, minutes, seconds and tenths of seconds, the spheres of rotation, the seven moving planets, the fixed stars and the terrestrial sphere, with climate, mountains, seas, deserts etc [19]. Samarqand, in the early decades of the 15th century, Krisciunas observes, was "the astronomical capital of the world." And for such, "it is deserving of further study [20]."

4. Instrumentation in Islamic Observatories

Some of the last observatories built by Muslims were by Jai Singh, Maharajah of Jaipur, who constructed observatories in Delhi, Jaipur, Ujjain and other Indian cities. The one in Delhi, the Jantar Mantar, was built in 1724 at the request of the Mughal ruler Muhammad Shah. Generally, the instruments found were based on those found at Maragha and Samarqand, although in architectural terms, the Indian observatory represented a major accomplishment as seen from current photographs.

Figure 4: Celestial Globe made by Ja'far ibn 'Umar ibn Dawlatshah al-Kirmani, Persian, 1362/3. © Museum of History of Science, Oxford, 2006. This celestial globe shows a view of the stars from ‘outside' the universe and was probably used for teaching purposes. The stars are represented by inlaid silver discs, their sizes corresponding to the magnitude of the stars. The axis of the globe can be adjusted for various latitudes in the northern hemisphere. The constellations appear to have been drawn according to the ‘Book of the Fixed Stars' of 'Abd al-Rahman ibn al-Sufi (903-986 A.D.), one of the greatest of the Muslim astronomers. (Source).

The construction of an advanced observatory was no mean undertaking; not least on the financial front. It was, hence, only natural that such an institution demanded the patronage of kings, princes or very wealthy people. As a matter of fact, the observatory soon took on the prerogative as a royal institution [21]. Al-Mamun gave the lead; Ulugh Beg, centuries later was wholly, and personally involved, in the undertaking (more than in the running of stately affairs). The Buwayyids, another instance, supported the use of advanced, larger and heavier equipment. Other than finance, observatories also required the cooperation of well trained astronomers and engineers, for the success of their operations [22].

In all cases, however, the instruments gradually became bulkier, the aim being to minimise error as much as possible [23]. Each piece was also devoted for a particular class of observations [24]. At Maragha, the ecliptical consisted of five rings, the largest of which being twelve feet across [25]. Included, too, was a meridian armillary consisting of a graduated bronze ring in the shape of an alidade set upon the meridian to measure solar altitudes in zenith distance; a large stone sundial accurately aligned to the meridian and used only for determining the obliquity of the ecliptic; an equatorial armillary made in the form of a bronze ring set firmly parallel to the plane of the equator; and a parallactic instrument, a type of transit used to measure the zenith distance of a star or the moon at culmination [26]. To gain the required rigidity, instruments were built of masonry when the foundations of the structure could be made secure, as at the Indian observatories.

5. The Impact of Al-Battani on European Astronomy

Observation in Islamic times reached beyond what much of scholarship gives it credit for. Many aspects of it were pioneering as can be observed from few extracts on the life and works of al-Battani by Carra de Vaux [27]. The merit of al-Battani, the author points out, is to pioneer the use of trigonometry in his operations. Al-Battani is also quoted saying:

Figure 5a-b: Astrolabe with geared calendar made by Muhammad b. Abi Bakr, Isfahan, 1221/2. © Museum of History of Science, Oxford, 2006. This early Persian astrolabe with a geared calendar movement is the oldest geared machine in existence in a complete state. It illustrates an important stage in the development of the various complex astronomical machines from which the mechanical clock derives. Islamic scholars learned of this design from a text by al-Bîrûnî, who explained how gearing might be used to show the revolutions of the sun and moon at their relative rates, and to demonstrate the changing phase of the moon. These phenomena were of fundamental importance in the lunar calendar used in Islam. (Source).

"After having lengthily applied myself in the study of this science, I have noticed that the works on the movements of the planets differed consistently with each other, and that many authors made errors in the manner of under taking their observation, and establishing their rules. I also noticed that with time, the position of the planets changed according to recent and older observations; changes caused by the obliquity of the ecliptic, affecting the calculation of the years and that of eclipses. Continuous focus on these things drove me to perfect