The science of celestial objects (including the earth in relation to them), of space, and of the universe as a whole.
Useful information: In astronomy the Muslims integrated the astronomical traditions of the Indians, Persians, the ancient Near East and especially the Greeks into a synthesis which began to chart a new chapter in the history of astronomy from the 8th century onward. The Almagest of Ptolemy, whose very name in English reveals the Arabic origin of its Latin translation, was thoroughly studied and its planetary theory criticized by several astronomers of both the eastern and western lands of Islam leading to the major critique of the theory by Nasir al-Din al-Tusi and his students, especially Qutb alDin al-Shirazi, in the 13th century.
The Muslims also observed the heavens carefully and discovered many new stars. The book on stars of 'Abd al-Rahman al-Sufi was in fact translated into Spanish by Alfonso X el Sabio and had a deep influence upon stellar toponymy in European languages. Many star names in English such as Aldabaran still recall their Arabic origin. The Muslims carried out many fresh observations which were contained in astronomical tables called zij. One of the acutest of these observers was al-Battani whose work was followed by numerous others. The zij of al-Ma'mun observed in Baghdad, the Hakimite zij of Cairo, the Toledan Tables of alZarqali and his associates, the ll-Khanid zij of Nasir al-Din al-Tusi observed in Maraghah, and the zij of Ulugh-Beg from Samarqand are among the most famous Islamic astronomical tables. They wielded a great deal of influence upon Western astronomy up to the time of Tycho Brahe. The Muslims were in fact the first to create an astronomical observatory as a scientific institution, this being the observatory of Maraghah in Persia established by al-Tusi. This was indirectly the model for the later European observatories . Many astronomical instruments were developed by Muslims to carry out observation, the most famous being the astrolabe. There existed even mechanical astrolabes perfected by Ibn Samh which must be considered as the ancestor of the mechanical clock.
Astronomical observations also had practical applications including not only finding the direction of Makkah for prayers, but also devising almanacs (the word itself being of Arabic origin). The Muslims also applied their astronomical knowledge to questions of time-keeping and the calendar. The most exact solar calendar existing to this day is the Jalali calendar devised under the direction of 'Umar Khayyam in the 12th century and still in use in Persia and Afghanistan.