Contributions of Ancient Arabian and Egyptian Scientists on Astronomy - Part 5

By: Md. Wasim Aktar
Deptt. of Agril. Chemicals, Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, Nadia, West Bengal, India.

A famous astronomer of the 11th century, who belonged to Cordova (Spain), was Abu Ishaq Ibrahim Ibn Yahya al-Naqqàsh, commonly known as Ibn al-Zarqàli or al-Zarqàli (Latin: Arzachel). He was also an eminent astronomer of this century. He lived from 1029 to 1087. He was the best observer of his time, who made astronomical observations for about 19 years (1061—1080). He invented an improved astrolabe called Safihah (Saphaea Arzachelis) on which he also wrote a treatise. It was translated into Latin, Hebrew and many vernaculars. Al-Zarqàli was the first to prove explicitly the motion of the solar apogee with reference to the stars. According to his calculations it was equal to 12.04” per year (the real value being 11.8”). He edited the planetary tables called Toledan Tables. These tables were probably the result of the observations made in Toledo by him and by a great observer Ibn Said in collaboration with other Muslim and Jewish astronomers. They were translated into Latin and enjoyed much fame. (25)

A famous astronomer, mathematician and poet, ‘Umar Ibn al-Khayyãm, reformed the old Persian calendar which had been replaced by the Islamic calendar after the Muslim conquest of Persia. This reformed calendar was called Al-Tàrikh al-Jalãli after the name of the Saljuq Sultan Malik Shah Jalal al-Din who in 1074-75 called ‘Umar Ibn al-Khayyãm to his observatory for making this reform. Many interpretations have been given to it. Each interpretation is accurate to a certain degree, but at any rate ‘Umar’s calendar was probably more accurate than the Gregorian (Christian) calendar. Three interpretations, the second of which seems to be the most accurate, are being quoted here along with the authority giving the interpretation and the resulting error.

1. Al-Shirãzi’s interpretation: 17 intercalary days in 70 years;’ error. 1 day in about 1540 years.
2. Ulugh Beg’s interpretation: 15 intercalary days in 62 years; error, 1 day in about 3770 years.
3. Modern interpretation: 8 intercalary days in 33 years: error, 1 day in about 5,000
(in the Gregorian calendar there is an error of 1 day in 3330 years). (26)

The greatest astronomer of the 12th century, who also belonged to Spain, was Abu Muhammad Jãbir Ibn Aflah. He was born or lived in Seville. He vigorously criticized the Ptolemaic theory of planets, and wrote a book on astronomy entitled Islah al-Majisti (the correction of the Almagest). He was of the view that the lower planets Mercury and Venus), at least, must have visible parallaxes. Venus may happen to be exactly on the line joining the sun and the earth. The most important part of his book is the introduction on trigonometry. The book was soon translated into Latin and Hebrew. Jãbir Ibn Aflah is said to be the inventor of the astronomical instrument called turquet (torquetum) which contains two graduated circles in two perpendicular planes. The same invention has also been ascribed to two other persons, namely, Frances of Leige (11th century) and Näsir al-Din Tusi (13th century). The turquet was introduced into the Latin West by Regionomentus. It gained a great popularity in the 15th and 17th centuries. (27)

Another astronomer of the time was Abu’l Qãsim Hibat Allah Ibn Husain al-Badi’ al-Asturlãbi. He was also a physician, mathematician, poet and litterateur. He was the greatest expert of his time in the knowledge and construction of astrolabes; hence his nickname al-Asturlãbi. In 1120—30 astronomical observations were made under his direction, and astronomical tables were compiled. The observations were carried out in the palace of the Saljuq Sultan of Iran, Mughith al-Din Mahmud (1117—1131). The tables were dedicated to the Sultan, and were called after him the Mahmudic tables. Al-Asturlãbi was very much praised by Muslim biographers. He died in Baghdad in 1139-40. (28)

In the 13th century there flourished in the East a great scholar of Persian origin, named Abu Ja’far Muhammad Ibn Muhammad Ibn al-Hasan, Näsir al-Din al-Tusi al-Muhaqqiq, (the researcher). He was born in Tus (Khurasan) in 1201, and died in Baghdad in 1274. He was a philosopher, mathematician, astronomer and physician. He was one of the greatest Muslim mathematicians and scientists. He wrote both in Arabic and Persian. It is said that he knew Greek as well. He joined the Mongol service, and was later made administrator of the Waqf revenues.

While he was administrator he resided at Maragha in Asia Minor (1259—1274). Here he made astronomical observations in an observatory established by the Mongol ruler Hulagu Khan II after he had defeated the last ‘Abbasi Caliph, al-Mu’tasim, in 1258. A library was attached to it. It is said to have contained 4, 00,000 volumes which the Mongol armies had collected in Syria, Mesopotamia and Persia. Näsir al-Din was the first director of this observatory. He was succeeded by two of his sons.
Näsir al-Din was well acquainted with the knowledge of the Greeks. He wrote about 64 works on many subjects. Here we shall, consider only some of his astronomical and astrological works. The most important astronomical work of Näsir al-Din is the Tadhkirah fi ‘Ilm al-Hay’ah (The description of astronomy) which is a condensed summary of astronomy. To explain it many commentaries and super commentaries have been written. The work enjoyed much popularity, it consists of four chapters. The second chapter, beside other things, contains interesting criticism of the Ptolemy’s Almagest in which he showed a great ingenuity. The criticism chiefly concerns the anomalies of the moon, and the motion in the latitude of the planets (particularly Mercury and Venus) ; also the proposition of a new system to replace the complicated Ptolemaic machinery of deferents and epicycles. His new and forceful criticism of astronomy as well as of other Muslim astronomers helped Copernicus in making his reform’. Näsir al-Din wrote one treatise on the five quadrants and two treatises on astrolabe. He also wrote two treatises on calendar.