s an engraved plate, making it easier to use than the wire mesh of the anaphoric clock, but because the position of the horizon differs from place to place, each astrolabe typically contained a number of tympanum. Only one tympanum was used at a time, and the inclusion of several tympanum insured that the astrolabe could be used at a variety of positions on the earth. Second, a skeletal projection of the starscalled a retewas fastened over the tympanum. The third primary component of an astrolabe is a simple device for measuring the distance of a star above the horizonusually a rod attached to the back of the astrolabe. One could produce a map of the sky on any given night by locating a known star, measuring its angular distance above the horizon, and rotating the rete until the representation of the star was aligned with its angular distance on the tympanum. During the Renaissance, the astrolabe was also included in clock designs such as this one by Janus Reinhold. The evolution of the anaphoric clock depended on several hundred years of Greek science. Thales' crude, spherical representation of the heavens laid a foundation for other Greek scientists to build on. After the construction of the first celestial sphere by Eudoxus, Archimedes created the first mechanical representation of the heavens using a complicated series of gears. However, armillary spheres were more commonly used to study the heavens. Shortly after the construction of Archimedes' sphere, Ctesibus built the first clepsydra. Although it is possible to observe the time on an armillary sphere, it is quite difficult to perpetually mimic the motion of the sun around the earth. The invention of stereography by Hipparchos made the construction of a dynamic representation of the heavens possible through the combination of planispheric projections with the clepsydra. The anaphoric clock and its cousin, the astrolabe, not only helped Ptolemy create the extensive catalogue in th...
