The Astrolabe

The astrolabe, sometimes called the slide rule of the heavens, can trace its history back to Hellenistic times. The smart phone of its day, it could perform numerous functions: calculate the time of day or night; determine your position; show the movement and identify of heavenly bodies; cast horoscopes; help you navigate the oceans, and survey all the land you can see.

Among numerous other advances in the sciences, and mathematics, the early Islamic scholars were responsible for a spectacular leap forward in astrolabe design - the invention of the ‘universal’ astrolabe - also known as the ‘Saphea’ or catholic astrolabe.

Whereas the classical astrolabe required a specific plate - a disc that would sit in the body of the astrolabe or ‘mater’ - for each latitude, its universal cousin could be used at any given latitude. While the idea of the Saphea originated in Baghdad during the ninth century CE, actual instruments would not be produced until the eleventh century CE in Toledo, Spain.

Spain under Islamic rule was, for the time, a beacon of religious tolerance in Europe, with Muslims, Christians, and Jews, living relatively harmonious, multicultural lives. It was within this culture that one of the greatest mathematicians and instrument makers of their (or any other) generation was born: Abu Ishaq Ibrahim ibn Yahya a-Naqqash al-Tujibi al-Zarqali, better known as Al-Zarqali or al-Zarqalluh (c420-480 H / c1029-1087 CE), which literally translates as ‘engraver’, as he was so proficient at the craft. In the west his name would become Latinized as Azarquiel.

Azarquiel devised a new stereographic projection in which he cast both the equatorial and ecliptic coordinate systems on to a vertical plane that cut the celestial sphere at the solstices. Adding a selection of important stars to this grid system produced a universal projection that was valid for every latitude without sacrificing any of the functionality of a standard projection.

Although the new instrument was a significant step forward, it did require the user to have a much better understanding of mathematics in order to use it effectively, and thus the classical astrolabe would continue to be the more popular instrument. This is reflected in the number of surviving examples from this period. The universal astrolabe would not catch on, in Western Europe, until some 500 years after the first example was made by Azarquiel.

Dating

Although this Saphea is not dated, an approximate date can be ascertained by the Zodiac scale and Julian calendar - to the verso - and the placement of the stars on the projection on the recto.

The scale and calendar show that 0-degree Aries corresponds with approximately 13.8 March. This puts the possible date of construction between 1150 and 1250. The position of the stars corroborates this, with the closest match being from the thirteenth century.