Astrolabe Glossary and Notes


Ibn az-Zarqāllu lived and worked first in Toledo and later in Córdoba during the eleventh century. He wrote a number of works on astronomy — including the Suma referente al movimeinto del sol on the proper motion of the solar apogee, the Tratado relativo al movimiento de las estrellas fijas on trepidation theory, and his Tratado de la lámina de los siete planetas that seems to be the first text to recognize that the orbit of a planet (in this case Mercury) is not a circle.

Az-Zarqāllu also wrote an important treatise on stereographic projection used to construct astrolabes. In his Tratado de la azafea, az-Zarqāllu described a new method of projecting the celestial sphere onto a plane. Andalusian astronomers had taken up the problem of developing a universal projection, one that would avoid the need to engrave a new plate for each latitude. Az-Zarqāllu's answer to this problem was a method that projected 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.

This method of projection enjoyed much success both in further Arabic treatises and in translation — it was translated into various vernacular languages as well as Latin. Although comparitively few astrolabes survive that bear az-Zarqāllu's projection, a large number of manuscripts testify to its popularity well into the sixteenth century. By the late fifteenth century, the saphea Azarchelis as it was called in Latin, had become a common part of the astronomical curriculum at European universities. In the middle of the sixteenth century, Gemma Frisius developed his astrolabium catholicum that resembled closely az-Zarqāllu's projection and extended the popularity of this instrument yet further.

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