In the history of astronomy, Islamic astronomy or Arabic astronomy refers to the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age (8th–15th centuries), 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 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 (see Latin translations of the 12th century) as well as Chinese astronomy and Malian astronomy.
A significant number of stars in the sky, such as Aldebaran and Altair, and astronomical terms such as alhidade, azimuth, and almucantar, 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.
Islam and astronomy
Islam has affected astronomy directly and indirectly. A major impetus for the flowering of astronomy in Islam came from religious observances, which presented an assortment of problems in mathematical astronomy, specifically in spherical geometry.
Background
In the 7th century, both Christians and Jews observed holy days, such as Easter and Passover, whose timing was determined by the phases of the moon. Both communities had confronted the fact that the approximately 29.5-day lunar months are not commensurable with the 365-day solar year. To solve the problem, Christians and Jews had adopted a scheme based on a discovery made in circa 430 BC by the Athenian astronomer Meton. In the 19-year Metonic cycle, there were 12 years of 12 lunar months and seven years of 13 lunar months. The periodic insertion of a 13th month kept calendar dates in step with the seasons.
On the other hand, astronomers used Ptolemy's way to calculate the place of the moon and stars. The method Ptolemy used to solve spherical triangles was a clumsy one devised late in the 1st century by Menelaus of Alexandria. It involved setting up two intersecting right triangles; by applying Menelaus' theorem it was possible to solve one of the six sides, but only if the other five sides were known. To tell the time from the sun's altitude, for instance, repeated applications of Menelaus' theorem were required. For medieval Islamic astronomers, there was an obvious challenge to find a simpler trigonometric method.
Islamic attitude towards astronomy
Islam advised Muslims to find ways of using the stars. The Qur'an says: "And it is He who ordained the stars for you that you may be guided thereby in the darkness of the land and the sea." On the basis of this advice Muslims began to develop better observational and navigational instruments, thus most navigational stars today have Arabic names.
Other influences of the Qur'an on Islamic astronomy included its "insistence that the Universe is ruled by a single set of laws" which was "rooted in the Islamic concept of tawhîd, the unity of God", as well its "greater respect for empirical data than was common in the preceding Greek civilization" which inspired Muslims to place a greater emphasis on empirical observation, in contrast to ancient Greek philosophers such as the Platonists who expressed a general distrust towards the senses and instead viewed reason alone as being sufficient to understanding nature. The Qur'an's insistence on observation, reason and contemplation ("see", "think" and "contemplate"), on the other hand, led Muslims to develop an early scientific method based on these principles, particularly empirical observation. Muhammad Iqbal writes:
“The general empirical attitude of the Qur'an which engendered in its followers a feeling of reverence for the actual, and ultimately made them the founders of modern science. It was a great point to awaken the empirical spirit in an age that renounced the visible as of no value in men's search after God.”
Several hadiths attributed to Muhammad also show that he was generally opposed to astrology as well as superstition in general. An example of this is when an eclipse occurred during his son Ibrahim ibn Muhammad's death, and rumours began spreading about this being God's personal condolence. Muhammad is said to have replied:
"An eclipse is a phenomenon of nature & has no relation to the death or birth of a human being."
From the 12th century onwards, Islamic astronomy began becoming a science primarily dependant upon observation rather than philosophy, primarily due to religious opposition from Islamic theologians of the Ash'ari school, most prominently Al-Ghazali, who opposed the interference of Aristotelian physics and Aristotelian cosmology in astronomy, opening up possibilities for an astronomy unrestrained by Aristotelian philosophy. For example, the Ash'ari doctrine influenced the theologian Fakhr al-Din al-Razi (1149–1209) to reject the Aristotelian notion of the Earth's centrality within the universe and instead propose the notion of a multiverse consisting of countless worlds and universes, "such that each one of those worlds be bigger and more massive than this world as well as having the like of what this world has." Al-Razi also criticized the Aristotelian notion of solid celestial spheres and suggested these may be "merely the abstract orbit traced by the stars." Later in the century, the Islamic theologian Adud al-Din al-Iji (1281–1355), under the influence of the Ash'ari doctrine of occasionalism, which maintained that all physical effects were caused directly by God's will, rejected the Aristotelian principle of an innate principle of circular motion in the heavenly bodies,[18] and maintained that the celestial spheres were "imaginary things" and "more tenuous than a spider's web". Under such influences, Ali Qushji (d. 1474) rejected Aristotelian physics and completely separated it from astronomy, allowing astronomy to become a purely empirical and mathematical science. This allowed him to explore alternatives to the Aristotelian notion of a stationery Earth, as he explored the idea of a moving Earth. He concluded, on the basis of empirical evidence rather than speculative philosophy, that the moving Earth theory is just as likely to be true as the stationary Earth theory and that it is not possible to empirically deduce which theory is true (see Astronomical physics and Earth's motion section below).[15][16]
Islamic rules
Arabic manuscript illumination from the 12th century CE showing various historical figures and scholars in medieval Islam
There are several rules in Islam which lead Muslims to use better astronomical calculations and observations.
The first issue is the Islamic calendar. The Qur'an says: "The number of months in the sight of Allah is twelve (in a year) so ordained by Him the day He created the heavens and the earth; of them four are sacred; that is the straight usage."[2][19] Therefore Muslims could not follow the Christian or Hebrew calendars and they thus had to develop a new one.
The other issue is moon sighting. Islamic months do not begin at the astronomical new moon, defined as the time when the moon has the same celestial longitude as the sun and is therefore invisible; instead they begin when the thin crescent moon is first sighted in the western evening sky. The Qur'an says: "They ask you about the waxing and waning phases of the crescent moons, say they are to mark fixed times for mankind and Hajj." This led Muslims to find the phases of the moon in the sky, and their efforts led to new mathematical calculations and observational instruments, as well as a special science being formed specifically for moon sighting.
Muslims are also expected to pray towards the Kaaba in Mecca and orient their mosques in that direction. Thus they need to determine the direction of Mecca from a given location. Another influencing factor is the time of Salah. Muslims need to determine from celestial bodies the proper times for the prayers at sunrise, at midday, in the afternoon, at sunset, and in the evening
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