Several 17th century astronomers made the effort to learn Arabic and Persian to have access to observations that would help them settle topical astronomical debates. This included the first five Savilian professors of Astronomy at Oxford. Some, like John Greaves, travelled to the Arab world where they made observations, learned Arabic and collected manuscripts.
Roger Long, Astronomy, 1742 (larger version).
Analysing the earth's axial tilt
Establishing the precise axial tilt of the earth (also called the obliquity of the ecliptic) and whether it has changed over the years is important for many astronomical and calendrical calculations. To do this Edward Bernard, FRS and Savilian Professor of Astronomy, dug deep into a wealth of Arabic materials and discovered valuable measurements of this tilt that did not match the contemporary value. He initially interpreted this as observational error but others like Pierre-Simon Laplace confirmed what some Arab astronomers thought 800 years earlier - the tilt changed periodically.
Johannes Hevelius, Firmamentum Sobiescianum sive Uranographia, 1687 (larger version). Hevelius produced this atlas of constellations after years of observations. He used the observations of al-Sufi, Ulugh Beg and Tizini as guidance but also identified many new stars.
Johannes Hevelius and Ulugh Beg
Johannes Hevelius (1611-1687) was a renowned Polish astronomer and the first foreign FRS. He had great respect for Arab and Muslim scholars; he cited them and depicted them often in his books. As he set himself up to re-map the sky he focussed his attention on the well-known 15th century star catalogue of Ulugh Beg. Hevelius wrote to the Royal Society asking them to find and translate the catalogue. The Society appointed John Wallis to oversee the translation that was later sent to Hevelius.
Ulugh Beg (1394 -1449) was a Timurid ruler, astronomer and mathematician. He lived in Samarkand, which he turned into an intellectual centre that attracted many scholars to its Madrasa and observatory. He achieved remarkably accurate observations by building an enormous sextant in his observatory, parts of which still survive. He compiled his observations in a highly celebrated star catalogue in which he built upon the star atlas of al-Sufi and used al-Sufi's observations of the stars too far south to be observed from Samarkand.
Islamic planisphere astrolabe, Maghrib, 18th century (larger version). Credit: Science Museum, London.
Re-mapping the earth and the sky
The seventeenth-century quest to re-map the earth and the sky called for another quest - to find and translate the observations and measurements made previously by Muslim astronomers and geographers. Fellows of the Royal Society played an important part in this project.
Astrolabes were used throughout the Islamic Golden age to map the sky and the earth. Abulfeda used such an instrument to determine the coordinates of cities and other places.
Hevelius using a telescope fitted with a quadrant (larger version).
Edmund Halley FRS
Halley (1656–1742) used Arabic observations, and learnt Arabic himself in order to translate two books from that language. He admired the tenth century astronomer Al Battani for being a keen and very precise observer and the first to dare to criticize Ptolemy. Halley spotted unlikely observations in the Latin translations of al-Battani’s work, but he was confident that these were only translation errors. He wrote that he wished he could lay his hands on the original Arabic text ‘so as to be able to confirm our emendations’.
Acceleration of the moon
Halley began the research that led to the establishment of the secular acceleration of the moon, a possible increase in the Moon's mean rate of motion relative to the stars. Halley started by using al-Battani’s observations of the eclipses at al-Raqqa (in modern Syria) and comparing them to ancient and modern observations. The debate was followed up by other astronomers, including Richard Dunthorne, Long and Laplace. All used al-Battani’s observations and those of other Babylonian, Greek and Muslim astronomers in addressing this question, which was finally settled in 1853.
From astrolabes to telescopes
Astrolabes and quadrants gradually gave way to the newly-invented telescope, but this was not a swift change. Telescopes offered a closer look at stars, but mapping the coordinates of these stars remained the task of the traditional quadrant. Hevelius was reluctant to use the telescope and claimed he could do as well with his quadrant and alidade (Arabic for ruler), a claim that was verified by Halley when he visited Hevelius. Eventually Hevelius fitted his telescope with a quadrant to get the best of both worlds.