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 astrolabe is phenomenally well-preserved. It consists of an alidade – a horizontal bar across the front of the astrolabe, with two pinprick holes for sighting – and a rete, the moveable disk with star-pointers, which both sit atop two double-sided plates indicating azimuths and star trajectories, all of which rests inside the mater, or body, of the astrolabe. While it may be a small ‘pocket-sized’ version, there is no skimping on detail. The engraving is especially fine and clear. Inside the astrolabe are four plates, indicating the position of stars in various locations. On one of the plates, the position for Mecca is marked, allowing the user to precisely align themselves with the holy city. The rim of the mater is marked with a 360° altitude scale numbered in alphanumeric notation. On the inside of the rim, at 12 o’clock, is a recess in which the tongues of the plates sit, to hold them stable. The plates themselves are marked with circles for the equator and tropics, as well as azimuths for every 10° and almucantars for every 5°, some marked with alphanumeric notation.

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.