Astronomy in the medieval Islamic world
Medieval Islamic astronomy comprises the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age, and mostly written in the Arabic language. These developments mostly took place in the Middle East, Central Asia, Al-Andalus, and North Africa, and later in the Far East and India. It closely parallels the genesis of other Islamic sciences in its assimilation of foreign material and the amalgamation of the disparate elements of that material to create a science with Islamic characteristics. These included Greek, Sassanid, and Indian works in particular, which were translated and built upon.
Islamic astronomy played a significant role in the revival of ancient astronomy following the loss of knowledge during the early medieval period, notably with the production of Latin translations of Arabic works during the 12th century.
A significant number of stars in the sky, such as Aldebaran, Altair and Deneb, and astronomical terms such as alidade, azimuth, and nadir, are still referred to by 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 read or catalogued. Even so, a reasonably accurate picture of Islamic activity in the field of astronomy can be reconstructed.
History
Pre-Islamic Arabs
The Islamic historian Ahmad Dallal notes that, unlike the Babylonians, Greeks, and Indians, who had developed elaborate systems of mathematical astronomical study, the pre-Islamic Arabs relied upon empirical observations. These were based on the rising and setting of particular stars, and this indigenous constellation tradition was known as Anwā’. The study of Anwā’ was developed after Islamization when Arab astronomers introduced mathematics to their study of the night sky.Early period
The first astronomical texts that were translated into Arabic were of Indian and Persian origin. The most notable was Zij al-Sindhind, a zij produced by Muḥammad ibn Ibrāhīm al-Fazārī and Yaʿqūb ibn Ṭāriq, who translated an 8th-century Indian astronomical work after 770, with the assistance of Indian astronomers who were at the court of caliph Al-Mansur. Zij al-Shah was also based upon Indian astronomical tables, compiled in the Sasanian Empire over a period of two centuries. Fragments of texts during this period show that Arab astronomers adopted the sine function from India in place of the chords of arc used in Greek trigonometry.Ptolemy’s Almagest was translated at least five times in the late eighth and ninth centuries, which was the main authoritative work that informed the Arabic astronomical tradition.
The rise of Islam, with its obligation to determine the five daily prayer times and the qibla inspired intellectual progress in astronomy.
Astronomical methods
The philosopher Al-Farabi described astronomy in terms of mathematics, music, and optics. He showed how astronomy could be used to describe the Earth's motion, and the position and movement of celestial bodies, and separated mathematical astronomy from science, restricting astronomy to describing the position, shape, and size of distant objects. Al-Farabi used the writings of Ptolemy, as described in his Analemma, a way of calculating the Sun's position from any fixed location.Golden Age
was an academy that was open to the public and financially supported during the reign of the Abbasid caliph al-Ma'mun in the early 9th century in Baghdad. Astronomical research was greatly supported by al-Mamun through the House of Wisdom, al-Ma'mun also built the first observatory in Baghdad and subsequent observatories were built around regions of Iraq and Iran.The first major Muslim work of astronomy was Zij al-Sindhind, produced by the mathematician Muhammad ibn Musa al-Khwarizmi in 830. It contained tables for the movements of the Sun, the Moon, and the planets Mercury, Venus, Mars, Jupiter and Saturn. The work introduced Ptolemaic concepts into Islamic science, and marked a turning point in Islamic astronomy, which had previously concentrated on translating works, but which now began to develop new ideas. Another notable figure in al-Ma'mun’s court was Sind ibn 'Alī, a Jewish convert to Islam, who contributed to the Zīj al-Sindhind and was credited with constructing astronomical instruments.
Jewish scholars in the Islamic world also engaged with astronomical methods developed by their Muslim counterparts. In 931, Saadia Gaon used a zīj to calculate the positions of the sun, moon, and five visible planets at a specific time, as noted in his commentary on Sefer Yetzirah. He may have studied the zīj tradition in part to counter contemporaries who sought to use such calculations to determine and sanctify the new moon each month.
Doubts on Ptolemy
In 850, the Abbasid astronomer Al-Farghani wrote Kitab fi Jawami. The book gave a summary of Ptolemic cosmography. However, it also corrected Ptolemy based on the findings of earlier Arab astronomers. Al-Farghani gave revised values for the obliquity of the ecliptic, the precession of the apogees of the Sun and the Moon, and the circumference of the Earth. The book was circulated through the Muslim world, and translated into Latin.By the 10th century, texts had appeared that doubted that Ptolemy's works were correct. Islamic scholars questioned the Earth's apparent immobility, and position at the centre of the universe, now that independent investigations into the Ptolemaic system were possible.
The 10th century Egyptian astronomer Ibn Yunus found errors in Ptolemy's calculations. Ptolemy calculated that the Earth's angle of axial precession varied by one degree every 100 years. Ibn Yunus calculated the rate of change to be one degree every 70 years.
Between 1025 and 1028, the polymath Ibn al-Haytham wrote his Al-Shukuk ala Batlamyus. While not disputing the existence of the geocentric model, he criticized elements of the Ptolemy's theories. Other astronomers took up the challenge posed in this work, and went on to develop alternate models that resolved the difficulties identified by Ibn al-Haytham. In 1070, Abu Ubayd al-Juzjani published the Tarik al-Aflak, in which he discussed the issues arising from Ptolemy's theory of equants, and proposed a solution. The anonymous work al-Istidrak ala Batlamyus, produced in Al-Andalus, included a list of objections to Ptolemic astronomy.
Nasir al-Din al-Tusi also exposed problems present in Ptolemy's work. In 1261, he published his Tadkhira, which contained 16 fundamental problems he found with Ptolemaic astronomy, and by doing this, set off a chain of Islamic scholars that would attempt to solve these problems. Scholars such as Qutb al-Din al-Shirazi, Ibn al-Shatir, and Shams al-Din al-Khafri all worked to produce new models for solving Tusi's 16 Problems, and the models they worked to create would become widely adopted by astronomers for use in their own works.
Nasir al-Din Tusi wanted to use the concept of Tusi couple to replace the "equant" concept in Ptolemic model. Since the equant concept would result in the Moon distance to change dramatically through each month, at least by the factor of two if the math is done. But with the Tusi couple, the Moon would just rotate around Earth resulting in the correct observation and applied concept. Mu'ayyad al-Din al-Urdi was another engineer/scholar that tried to make sense of the motion of planets. He came up with the concept of lemma, which is a way of representing the epicyclical motion of planets without using Ptolemic method. Lemma was intended to replace the concept of equant as well.
Earth rotation
discussed the possibility of whether the Earth rotated about its own axis and around the Sun, but in his Masudic Canon, he set forth the principles that the Earth is at the center of the universe and that it has no motion of its own. He was aware that if the Earth rotated on its axis, this would be consistent with his astronomical parameters, but he considered this a problem of natural philosophy rather than mathematics.His contemporary, Abu Sa'id al-Sijzi, accepted that the Earth rotates around its axis. Al-Biruni described an astrolabe invented by Sijzi based on the idea that the earth rotates.
The fact that some people did believe that the Earth is moving on its own axis is further confirmed by an Arabic reference work from the 13th century which states:
According to the geometers , the earth is in a constant circular motion, and what appears to be the motion of the heavens is actually due to the motion of the earth and not the stars.
At the Maragha and Samarkand observatories, the Earth's rotation was discussed by Najm al-Din al-Qazwini al-Katibi, Tusi and Qushji. The arguments and evidence used by Tusi and Qushji resemble those used by Copernicus to support the Earth's motion. However, it remains a fact that the Maragha school never made the big leap to heliocentrism.
Alternative geocentric systems
In the 12th century, non-heliocentric alternatives to the Ptolemaic system were developed by some Islamic astronomers in al-Andalus, following a tradition established by Ibn Bajjah, Ibn Tufail, and Ibn Rushd.A notable example is Nur ad-Din al-Bitruji, who considered the Ptolemaic model mathematical, and not physical. Al-Bitruji proposed a theory on planetary motion in which he wished to avoid both epicycles and eccentrics. He was unsuccessful in replacing Ptolemy's planetary model, as the numerical predictions of the planetary positions in his configuration were less accurate than those of the Ptolemaic model. One original aspects of al-Bitruji's system is his proposal of a physical cause of celestial motions. He contradicts the Aristotelian idea that there is a specific kind of dynamics for each world, applying instead the same dynamics to the sublunar and the celestial worlds.