Arab and Islamic Astronomy

During the period when Western civilization was experiencing the dark ages, between 700-1200 A.D., an Islamic empire stretched from Central Asia to southern Europe. Scholarly learning was highly prized by the people, and they contributed greatly to science and mathematics. Many classical Greek and Roman works were translated into Arabic, and scientists expanded on the ideas. For instance, Ptolemy's model of an earth-centered universe formed the basis of Arab and Islamic astronomy, but several Islamic astronomers made observations and calculations which were considerably more accurate than Ptolemy's. Perhaps the most fascinating aspect of Islamic astronomy is the fact that it built on the sciences of two great cultures, the Greek and the Indian. Blending and expanding these offen different ideas led to a new science which later profoundly influenced Western scientific exploration beginning in the Renaissance.
Purposes of Islamic Astronomy Perhaps the most vital reason that the Muslims studied the sky in so much detail was for the purpose of time-keeping. The Islamic religion requires believers to pray five times a day at specified positions of the sun. Astronomical time-keeping was the most accurate way to determine when to pray, and was also used to pin-point religious festivals.  The Muslim holy book, the Koran, makes frequent reference to astronomical patterns visible in the sky, and is a major source of the traditions associated with Islamic astronomy. Another important religious use for astronomy was for the determination of latitude and longitude. Using the stars, particularly the pole star, as guides, several tables were compiled which calculated the latitude and longitude of important cities in the Islamic world. Using this information, Muslims could be assured that they were praying in the direction of Mecca, as specified in the Koran. Aside from religious uses, astronomy was used as a tool for navigation. The astrolabe, an instrument which calculated the positions of certain stars in order to determine direction, was invented by the Greeks and adopted and perfected by the Arabs (see picture below). The sextant was developed by the Arabs to be a more sophisticated version of the astrolabe. This piece of technology ultimately became the cornerstone of navigation for European exploration.
Great Islamic Astronomers Science was considered the ultimate scholarly pursuit in the Islamic world, and it was strongly supported by the nobility. Most scientists worked in the courts of regional leaders, and were financially rewarded for their achievements. In 830, the Khalifah, al-Ma'muun, founded Bayt-al-Hikman, the 'House of Wisdom', as a central gathering place for scholars to translate texts from Greek and Persian into Arabic. These texts formed the basis of Islamic scientific knowledge. One of the greatest Islamic astronomers was al-Khwarizmi (Abu Ja'far Muhammad ibn Musa Al-Khwarizmi), who lived in the 9th century and was the inventor of algebra. He developed this mathematical device completely in words, not mathematical expressions, but based the system on the Indian numbers borrowed by the Arabs (what we today call Arabic numerals). His work was translated into Latin hundreds of years later, and served as the European introduction to the Indian number system, complete with its concept of zero. Al-Khwarizmi performed detailed calculations of the positions of the Sun, Moon, and planets, and did a number of eclipse calculations. He constructed a table of the latitudes and longitudes of 2,402 cities and landmarks, forming the basis of an early world map. Another Islamic astronomer who later had an impact on Western science was al-Farghani (Abu'l-Abbas Ahmad ibn Muhammad ibn Kathir al-Farghani). In the late 9th century, he wrote extensively on the motion of celestial bodies. Like most Islamic astronomers, he accepted the Ptolemaic model of the universe, and was partially responsible for spreading Ptolemaic astronomy not only in the Islamic world but also throughout Europe. In the 12th century, his works were translated into Latin, and it is said that Dante got his astronomical knowledge from al-Farghani's books. In the late 10th century, a huge observatory was built near Tehran, Iran by the astronomer al-Khujandi. He built a large sextant inside the observatory, and was the first astronomer to be capable of measuring to an accuracy of arcseconds. He observed a series of meridian transits of the Sun, which allowed him to calculate the obliquity of the ecliptic, also known as the tilt of the Earth's axis relative to the Sun. As we know today, the Earth's tilt is approximately 23o34', and al-Khujandi measured it as being 23o32'19". Using this information, he also compiled a list of latitudes and longitudes of major cities. Omar Khayyam (Ghiyath al-Din Abu'l-Fath Umar ibn Ibrahim al-Nisaburi al-Khayyami) was a great Persian scientist, philosopher, and poet who lived from 1048-1131. He compiled many astronomical tables and performed a reformation of the calendar which was more accurate than the Julian and came close to the Gregorian. An amazing feat was his calculation of the year to be 365.24219858156 days long, which is accurate to the 6th decimal place! Western science owes a large debt to Islamic and Arab scientists, whose contributions range from the Arabic names of stars which we still use today to the mathematical and astronomical treatices used by Europeans to enter our modern world of science.

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