Some medieval Muslims took a keen interest in the study of astrology, despite the Islamic prohibitions: partly because they considered the celestial bodies to be essential, partly because the dwellers of desert-regions often travelled at night, and relied upon knowledge of the constellations for guidance in their journeys. After the advent of Islam, the Muslims needed to determine the time of the prayers, the direction of the Kaaba, and the correct orientation of the mosque, all of which helped give a religious impetus to the study of astronomy and contributed towards the belief that the heavenly bodies were influential upon terrestrial affairs as well as the human condition. The science dealing with such influences was termed astrology (Arabic: علم النجوم Ilm an-Nujūm), a discipline contained within the field of astronomy (more broadly known as علم الفلك Ilm al-Falak ‘the science of formation [of the heavens]’). The principles of these studies were rooted in Arabian, Persian, Babylonian, Hellenistic and Indian traditions and both were developed by the Arabs following their establishment of a magnificent observatory and library of astronomical and astrological texts at Baghdad in the 8th century.
Throughout the medieval period the practical application of astrology was subject to deep philosophical debate by Muslim religious scholars and scientists. Astrological prognostications nevertheless required a fair amount of exact scientific expertise and the quest for such knowledge within this era helped to provide the incentive for the study and development of astronomy.
Medieval Islamic astrology and astronomy continued Hellenistic and Roman era traditions based on Ptolemy‘s Almagest. Centres of learning in medicine and astronomy/astrology were set up in Baghdad and Damascus, and the Caliph Al-Mansur of Baghdad established a major observatory and library in the city, making it the world’s astronomical centre. During this time knowledge of astronomy was greatly increased, and the astrolabe was invented by Al Fazari. Many modern star names are derived from their Arabic and Persian names.
Albumasur or Abu Ma’shar (805 – 885) was one of the most influential Islamic astrologers. His treatise Introductorium in Astronomiam (Kitab al-Mudkhal al-Kabīr) spoke of how ‘”only by observing the great diversity of planetary motions can we comprehend the unnumbered varieties of change in this world”. The Introductorium was one of the first books to find its way in translation through Spain and into Europe in the Middle Ages, and was highly influential in the revival of astrology and astronomy there.
Persians also combined the disciplines of medicine and astrology by linking the curative properties of herbs with specific zodiac signs and planets. Mars, for instance, was considered hot and dry and so ruled plants with a hot or pungent taste, like hellebore, tobacco or mustard. These beliefs were adopted by European herbalists like Culpeper right up until the development of modern medicine.
The Persians also developed a system, by which the difference between the ascendant and each planet of the zodiac was calculated. This new position then became a ‘part’ of some kind. For example, the ‘part of fortune’ is found by taking the difference between the sun and the ascendant and adding it to the moon. If the ‘part’ thus calculated was in the 10th House in Libra, for instance, it suggested that money could be made from some kind of partnership.
The Almagest, together with the original contributions of 9th to 10th century Persian astronomy such as the astrolabe, was introduced to Christian Europe beginning in the 11th century, by contact with Islamic Spain.
Another notable Persian astrologer and astronomer was Qutb al-Din al Shirazi born in Iran, Shiraz (1236–1311). He wrote critiques of Ptolemy’s Almagest and produced two prominent works on astronomy: ‘The Limit of Accomplishment Concerning Knowledge of the Heavens’ in 1281 and ‘The Royal Present’ in 1284, both of which commented upon and improved on Ptolemy’s work, particularly in the field of planetary motion. Al-Shirazi was also the first person to give the correct scientific explanation for the formation of a rainbow.
Ulugh Beyg was a fifteenth-century Timurid Sultan and also a mathematician and astronomer. He built an observatory in 1428 and produced the first original star map since Ptolemy, which corrected the position of many stars and included many new ones.