The literature of the astrolabe to 1450

Abstract In the astrolabe, a flat circular plate (the rete) carrying a representation of the ecliptic and some of the stars, is rotated against another circular plate (the plate), on which the horizon and parallels to the horizon (the almucantars) are marked. The instrument may thus imitate the rotation of the celestial sphere against the horizon co-ordinates and, in particular, the Sun and the stars can be seen moving over the altitude circles. Normally, there are other markings on the plates, such as the division of the ecliptic on the rete and unequal-hour lines on the plate. (An ‘unequal’ or ‘seasonal’ hour (there were other terms for this) was one-twelfth of the daylight hours or night hours; they were commonly used for civil purposes in preference to ‘equal’ or ‘astronomical’ hours.) Any method of flattening the sphere onto the plane will do, if it is sufficiently even, but usually a stereographic projection is used — from the south pole for the northern parts of the celestial sphere (northern projection) and from the north pole for the southern regions (southern projection). One reason for this choice is to facilitate the work of the instrument-maker, for in stereographic projection, circles on the sphere are transformed into circles (or straight lines) on the plane. On the back of the instrument, there is an alidade or rule with sights, with which the altitude of the Sun and the stars may be measured on a scale on the rim of the instrument, which is sus- pended so as to hang vertically. Altitudes form the link between the sky and the astrolabe. In general, writings on the astrolabe may be classified according to the following three types: those on the construction of the instrument; those on its use; and those on the theory of stereographic projection.