The Brass Instrument

The title of The Brass Instrument (Keli ha-neḥoshet) refers to the astrolabe, a tool used for astronomical measurements and calculations, thought to have been first invented around the second century BCE. An astrolabe is made up of a circular body, called the mater (mother), which holds removable plates that are engraved with a stereographic projection of the celestial sphere as it appears in the latitude where the astrolabe is to be used. Over the plates sits the rete (net), which indicates the positions of the stars and the ring of the ecliptic and can be rotated. Aside from calculating star positions, the astrolabe can also be used for observations, by sighting an object through the alidade. As Ibn Ezra acknowledges, knowledge of the astrolabe stemmed from Greek writings transmitted by Arabic scientists. He therefore sought to explain this instrument for a Hebrew-reading audience. Three Hebrew translations and one Latin one, which Abraham wrote with a student, survive to this day. The excerpts that follow are drawn from the beginning of the work.