Astrolabes: Tools for decoding the movement of the stars

In medieval Oxford, a student drunkenly stumbling back to his dorms attempted to use an astrolabe to navigate his route home. It would have certainly pointed him in the right direction, but a lack of attention to where he was putting his feet meant he found himself at the bottom of a ditch.  

‘Astrolabes can tell you exactly how the sky looked at any given time,’ says James Hyslop, Head of Christie’s Travel, Science and Natural History department. ‘You take the star’s height in the sky, set up that star’s altitude on the other side of the astrolabe, then, using one of the rulers on the front of it, you can read off the time for night and day. They can be used to complete hundreds of different calculations. Astrolabes were the latest gadget — the medieval equivalent of an iPhone.’ For their users, they imposed order on the night sky and explained the stars, once considered to be peepholes into heaven.

‘If you can understand the principles behind an astrolabe, you can understand the principles behind all of astronomy — the apparent movement of the stars and the course of the sun throughout the year,’ says Hyslop. Learning to work these complex objects was once part of a gentleman’s education, a fundamental part of university courses in Europe. Geoffrey Chaucer, author of The Canterbury Tales, wrote a treatise addressed to ‘little Lewis’, his then ten-year-old son, on how to use an astrolabe. ‘The mathematics behind them is so complicated that they became a teaching tool,’ says Hyslop.

The origin of these objects is disputed. One account records Claudius Ptolemy (90 — 168 AD), the Greco-Egyptian astronomer and mathematician, travelling from Alexandria when a stone in the road — or an unexpected jolt — caused the celestial globe he was carrying to slip from where it was secured. It fell to the ground and was promptly trodden on by his donkey, becoming completely flattened. And so was christened the first astrolabe — clearly an apocryphal tale, but it serves as a neat creation myth for what Hyslop calls ‘a star map combined with an analogue computer’.

Although the technology behind the astrolabe may have originated in the Greek-speaking world, it was preserved and perfected through Islamic science in the Middle Ages, before returning to Europe in the Renaissance period. ‘I find the Islamic and Persian astrolabes most beautiful,’ says Hyslop. ‘Engraved with lines to indicate the times of prayer, the floral decoration is more intricate and the star map tends to have a more pleasing symmetry.’ Astrolabes are incredibly tactile objects, since you can take them apart and adjust their plates according to the city in which you happen to find yourself.

Astrolabes that hail from Islamic Spain can be decorated by a combination of Hebrew, Latin, and Islamic scripts — serving as a testament to the exchange of knowledge between universities and cities. Manuscripts detailing how these objects worked made their way into the hands of European scholars. News of the objects prompted curiosity, and a number of astrolabe workshops sprung up to satisfy academics who hoped to construct their very own.

Rulers all over the world shared the same desire to gaze up to the heavens, seeking guidance on the events on earth below. In his travelogue, Marco Polo records seeing astrolabes in the imperial capital Beijing, while around a century later, Sir John Mandeville writing about his journey to Kublai Khan’s court, recorded his encounters with philosophers, astronomers, geometrists, pyromancers and necromancers. Their astrolabes made with gold, ‘beautiful spheres’, were positioned nearby.

The Holy Roman Emperor Rudolf II had a table teeming with instruments including a celestial globe, a glass harpsichord and a clock featuring an astrolabe. Reportedly incapable of making a decision without first consulting the court astronomers, he was said to have refused to host anyone at court who had not first had their horoscope cast. He was horrified by an unfavourable horoscope cast by an imperial astrologer, which predicted his own family would hatch a plot to depose him. Despite desperately scrabbling to avoid his fate, the prediction came to pass — the Emperor perhaps hastening its progress by shutting himself off from his courtiers and attempting suicide.

Forced to abdicate by his brother Mathias, Rudolf II spent his last days powerless in a castle in Prague, surrounded by his exotic menagerie and his courtiers. He died three days after the death of his favourite lion. For Rudolf II, astrolabes acted as a bridge to the mystical as well as a precise tool of science.

Although telescopes and other more high tech sky-scanning devices have since been invented, astrolabes have endured as a symbol of scientific excellence. They decorated mid 20th century Turkish postage stamps and feature in garden designs as well as corporate logos. These beautiful objects remain highly collectable. As Hyslop concludes, ‘an astrolabe sings to you when you have it in your hands.’