© Museum of the History of Science, Oxford
Very few people today know what an astrolabe is. But if we look back over the last 2000 years, the astrolabe maybe by far the most important mathematical instrument that there has ever been.
Nobody knows exactly who invented the first astrolabe. Astrolabes seem to have originated in the ancient Greek world about AD 300. Most of the mathematical theories which the astrolabe needs in order to work were developed by the ancient mathematician Ptolemy in about AD 150. Ptolemy worked in Alexandria now in Egypt, which was a very important place for the study of mathematics in ancient times.
An astrolabe is basically a map of the stars. It is very like a modern plastic 'planisphere' star map that you can buy in book shops. Unlike some star maps which are globes, an astrolabe is completely flat. This was useful because it meant that astrolabes were easy to carry around, to make, and to use. A popular story says that Ptolemy invented the astrolabe when the donkey that he was riding on trod on his star globe and flattened it.
Over time astrolabes became much more than just star maps. For Medieval and Renaissance mathematicians they were like multimedia PCs. They did all the calculations that mathematicians needed and allowed many practical problems to be solved that needed maths.
This astrolabe is very beautiful and very well made. It is a mathematical instrument that has been turned into a work of art. The fact that so much care and attention has gone into making it shows how important astrolabes used to be.
This astrolabe was actually made in Sind, now in Pakistan, although at the time it was part of northern India. This area was a very culturally rich and important centre for making mathematical instruments in the 17th and 18th century. The astrolabe has words engraved on it which say that it was made in AH 1077. However, this date is the year counted according to the Islamic calendar which is different from the Christian calendar that we now use in most parts of Europe. AH marks the start of the Muslim era, and stands for Anno Hegirae, the year in which Mohammed fled from Mecca to Medina, and equates to AD 622. The year AH 1077. corresponds to AD 1666 or 1667.
Astrolabes show the position of the stars in the sky by using pointers. These pointers move over a grid of lines which are used as a system of reference co-ordinates. By seeing where the star pointers point to on the grid you can tell how high in the sky the star is and in what direction it is.
The star pointers are cut out of a sheet of metal called the 'rete', which means 'net' in Latin. This is the very thin network of curved lines and circles that can be seen in the picture of the astrolabe. It is supposed to look like a complicated and beautiful network. It was common to decorate the pointers on astrolabes so that they looked artistic. For astrolabes made in Christian places, star pointers were often made as human and animal shapes. For example, very often the rete of an astrolabe would contain a picture of a dragon, and the end of the dragon's tail would be a pointer which showed the position of a star.
This astrolabe doesn't have any human or animal figures because it was made in a part of India that was Islamic in the 17th century. It is forbidden to show humans and animals in Islamic art. Instead, the star pointers in this astrolabe have been made into a very beautiful and elegant pattern of leaves.