The little-known GPS of Ancient Times. Perhaps the most used multipurpose astronomical instrument in history, the astrolabe is known to have been in use since at least 200BC and rose to prominence in the Middle Ages. Essentially, it’s an elaborate inclinometer-cum-slide rule, used to measure the altitude above the horizon of a celestial body. It can be used to identify stars or planets, to determine local latitude, to survey, and even determine your horoscope. The arrival, in the 17th-18th centuries, of accurate sextants and reliable portable chronometers rendered the astrolabe obsolete for navigation at sea, but it remains a fascinating and little-understood instrument. This replica, in polished brass, sits in a wooden plinth and stands 18cm tall.