Kitāb al-jadwal fī al-ḥisāb al-sinīn al-ʻArabīyah wa-al-Rūmīyah

The correct reckoning of time has been the object of study by many Arabic scientists. Its importance in Islam has to do with the proper calculation of the length of days, months, and years necessary for the performance of the five daily prayers and for the celebration of festivals at the correct hours, days, and months of the lunar year. This 19th-century copy of a work by Muḥammad ibn ʻAbd al-Laṭīf Thābitī opens with some remarks on the signs of the zodiac, followed by a discussion of the 28 lunar mansions--the portions of the ecliptic passed by the moon in its orbit around the Earth. Although the difference between astrology and astronomy was already well known in ancient times, the presence of elements of these two fields side by side in the same work, as in this manuscript, was very common in Medieval treatises. The text explores the use of the solar and lunar systems to calculate the months of the Islamic calendar. It includes tables showing the correspondence of the Arabic and Christian years and examines discrepancies in dates. The text that surrounds these tables deals in detail with the movements of the sun and the moon and the times of dusk, dawn, and prayers, and it incorporates some poetical passages. The manuscript, from an unknown hand, is written in Naskhī script in black ink on the title page and in the text in black, red, and brown ink. The text is framed by a double red line and there are diacritical marks and marginal notes. The last pages of the manuscript include calculations in purple pencil in a modern hand. It previously was owned by a scholar active in the 1720s, Muḥammad ʻAlī ibn Ẓāhir Watarī, and later was in the collection of Shaykh Maḥmūd al-Imām al-Manṣūrī, professor of religion at Al-Azhar University in Cairo, which was purchased by the Library of Congress in 1945. World Digital Library. The text is a treatise on using the solar and lunar systems to calculate the months. It includes tables showing the correspondence of the Arabic and Christian years with a preface and it explains the positions and movement of the sun and the moon, and the discrepancies of the dates. Poem on fol. 14b and 15a.