To an observer standing on Planet Earth, our humble home appears as the centre of the universe, with the celestial sphere moving around it. For millennia, the movement of the stars in this celestial sphere has been used to aid navigation at sea. Various instruments were created to aid sailors in their navigation; among the most sophisticated of these was the astrolabe. Designed to show the relative positions of the stars in the sky at different times of the year and in different astral planes, the astrolabe was an essential part of the navigator’s and astronomer’s toolkit for centuries, since their invention by Apollonius of Perga around 220 BC. But the astrolabe went under its most thorough development during the Islamic Period, when the scientific principles of astronomy were first laid out in detail. For the Muslims, the position of the stars was centrally important; knowledge of these positions and their seasonal changes enabled a precise orientation towards Mecca (qibla), and a precise division of the day into hours, both of which were essential for Muslim prayer (salah).

This exceptional, and large, brass astrolabe represents a high-point of Islamic precision metalworking. Inscriptions on the rear of the mater (case) and on the rearmost of the plates indicate that this astrolabe dates to 1142 hijri, which in the Gregorian calendar equates to between July of AD 1729 and July of AD 1730. Additionally, a name is attached to this piece: ‘Umar bin Ahmed. It is unclear whether bin Ahmed is the maker of the astrolabe – which are almost always signed – or whether this is the name of the maker’s patron. This name may be linked to ‘Umar bin Ahmed bin Sa’di al-Kadusi, the author of an Eighteenth Century AD manual on astrolabe quadrants called al-Muqantarat, though this conjecture cannot be proven conclusively either way. The astrolabe consists, as do all astrolabes, of the mater, around the rim of which are labels for subdivisions of the year. The mater is topped with the ‘throne’, an elaborately-decorated protrusion to which is attached the ring from which the astrolabe hangs from a string during use. The weight of the mater keeps the string taught, ensuring that the astrolabe dangles exactly in the direction of gravity, in order to give accurate readings. Within the mater is housed the rete, a net-like rotating plate of pointers, in the form of small curvaceous daggers, which indicate the positions of important stars. Particularly prominent stars are labelled in silver inlay. Within the mater are five terrestrial plates, which co-ordinate the celestial elements (the star pointers) with earthly observations. One plate, for example, relates four quadrants labelled with the four cardinal directions, and four Islamic months (mujharram, Ramadan, rajab, and shewal), along with four qualities of the winds from those directions (dry, moist, cold, warm). On another plate, the position of Mecca is indicated. Each of the brass plates has a locking mechanism in the top, to prevent them from shifting during use, and is inlaid with silver kufic inscriptions. The reverse of the mater contains a number of inscriptions, some of which refer to the praises of Allah, or, in one case, cryptically to the date of the astrolabe itself (‘one thousand, added to ten, added to fifty, subtracting eight’ to give the hijri year 1142. Through the middle is a heavy pin, on which is fixed the alidade, a rotating rule which aids in measurements and alignments.

Astrolabes could have a multiplicity of uses, and mastering the astrolabe took years of practice. To measure, for example, the position of the sun, one would hold the astrolabe up (many, like this example, hang from a rope, to allow gravity to hold the astrolabe directly vertically), and, without looking directly at the sun, align the alidade with it. This astrolabe has two pinpricks in the alidade, which would allow one to project the sun’s rays onto clean object, rather than having to look at the sun itself. One would then read off the angle of the alidade on the reverse ring. Similarly, to tell the time, one would align the alidade with the sun as above, then turn the rule to match the current date on the ecliptic ring. Then, using your measurement for the angle of the sun, one finds the correct altitude line, and turns the rete and the rule together until they intersect at the correct altitude. The rule is then pointing at the correct time. All of this might seem very complex for something as simple as the time. But it is important to remember that, in antiquity, there was no accurate measure of the time. While a sun dial might do for day-to-day purposes, for important timekeeping purposes – such as for Muslim prayer, or during navigation – the astrolabe remained the best option until well into the Eighteenth Century AD. The hunt for accurate timekeeping at sea was so important, especially over long distances, that the British Government awarded £ 23,065 (roughly £ 2,000,000 in AD 2017) to John Harrison for his invention of a marine chronometer that kept time even on rough seas and long voyages.