Qasiyum outside Damascus. Afterwards several other observatories were erected at Wasit, Apamea, etc. Musa bin Shakir was a well-known engineer during the reign of Harun-ar-Rashid. His sons, specialised in astronomical researches and earned a great reputation as astronomers during the reign of Mamun and his two successors. Their research regarding the movements of solar and other astral bodies, was remarkable. They ascertained the size of the earth, the obliquity of the ecliptic, the variations of the lunar latitudes and the precession of the equinoxes! The work of the sons of Musa bin Shakir was continued by Al-Naziri and Muhammad bin Isa Abu Abdulla, who made notable additions to it. The invention of the telescope by Abul Hasan forms a landmark in the advancement of astronomical observations and, in improved form, was used with remarkable success in the observatories of Maragha, Cairo and Seville. A number of Mamun's astronomers headed by Musa Khwarizmi and Musa Ibn Shakir successfully engaged in one ofthe most oblicate geodetic operations, i.e., the determination of the size and the circumference of the earth. The measurement carried out in the plain of Sanjar and also at Palmyra, "yielded 56 2/3 Arabic miles as the length of a degree of the meridian--a remarkably accurate result, exceeding the real length of the degree at the place by about 2,877 feet" says C. A. Nallino.--"This would make the diameter of the earth 6,500 miles and its circumference 20,400 miles".' Muhammad Bin Musa al-Khwarizmi, a versatile genius of Islamic history translated the Sidhanta or Indian tables and wrote a commentary on it. He has written a valuable treatise on astronomy and has compiled his own tables (zij) which after two centuries were revised by the Spanish astronomer Majriti and translated into Latin by Adelard of Bath in 1126 A.D. These formed the basis of later astronomical works in the East and the West, replacing all earlier tables by Greek and Indian astronomers. These tables were also used in China. Ibrahim al-Fazari was the first Muslim to construct an astrolabe. He wrote on the use of the armillary sphere and prepared tables in accordance with the Arabic years. One of the earliest Arabic treatises on this instrument was written by Isa-al-Asturlabi who resided in Baghdad at about 830 A.D.

Abul Abbas al-Farghani (Alfraganus), a resident of Farghana in Central Asia, was an astronomer of repute, who in 861 A.D., according to Ibn Abi Usabiyah (Vol. I, page 207), supervised for the Abbasid Caliph Al-Mutawakkil the erection of a Nilometer at Fustat.

His well-known work AI-Mudkhil-ila-ilm-hayat-al-aflak (compendium of astronomy) was translated into Latin in 1135 A. D. by John of Seville and Gerard of Cremona. It was also rendered into Hebrew. "The introduction of Astronomy into Christian Europe", says J. W. Draper, "has been attributed to the translation of the works of Muhammad Fargani. In Europe, also, the Arabs were the first to build observatories; the Giralda, or Tower of Seville was erected under the superintendence of Geber, the mathematician"

The Buwayhid Sultans were also great patrons of learning and were surrounded by a galaxy of talented scholars invited from the four corners of the Islamic world. The Buwayhid Sultan Sharaf al-Daulah (982--89, A.D.) founded a good observatory in his palace at Baghdad where such celebrated astronomers as Abdur Rahman al-Sufi, Ahmad al-Saghani and Abul Wafa carried on their research. Abdur Rahman al-Sufi wrote al-kawakib al-Sabitah (fixed stars) which is known as a masterpiece of observational astronomy. Alkohi, another astronomer attached to the same dynasty, studied the movements of the planets and his research regarding the summer solstice and the autumnal equinox were of considerable value. Abul Wafa, born in 939 A. D. in Khorasan and established in Iraq was an outstanding mathematician and astronomer, who introduced the use of the secant as well as the tangent into astronomical observations. Another Buwayhid ruler, Rukn al-Daulah, (932--76 A. D.) patronised Abu Jafar al-Khazin, a well-known astronomer of Khorasan, who ascertained 'the obliquity of the ecliptic and solved a problem of Archimedes which leads to a cubic equation'.By the close of the tenth century A.D., Baghdad was thronged with hundreds of astronomers including Ali Ibn Amajur and Abul Hasan Ali Ibn Amajur who are known for their accurate calculation of the lunar movements.

Abu Abdulla Ibn Muhammad Ibn Jabir-al Battani (Albategnius-877-91 8 A.D.) a Sabian from Harran was one of the most illustrious astronomers of the East who is known as the Ptolemy of the Arabs. His tables translated into Latin formed the basis for astronomical work in Europe for several centuries. He also wrote a voluminous treatise on the subject and his astronomical tables were an advance over those of Khwarizmi and the Indians. He carried on his studies and observations in al-Raqqah. He was an outstanding original writer and a research scholar of repute who made several emendations to Ptolemy and rectified the calculations for the orbits of the moon and certain other planets. According to Philip K. Hitti, "He’s proved the possibility of annular eclipses of the sun and determined with greater accuracy the obliquity of the ecliptic, the length of the tropical year, end of the seasons and the mean orbit of the sun"."