motion for two disks by using a clepsydra. By incorporating planispheric projections of the heavens into the clepsydra, the Greeks created the first anaphoric clocks. The anaphoric clock consists of a rotating star map behind a fixed, wire representation of the meridian, the horizon, the equator and the two tropics. The fixed disk consists of several concentric circles, divided into twenty-four sections by a series of small arcs. Each section represents one hour of the day. Because the long arc extending from one end of the disk to the other is the horizon, the first hour of the day begins on the right side of the disk at the horizon. The twelve hours of the day are above the horizon, and the twelve hours of the night are below the horizon. A stereographic map of the ecliptic was attached behind this fixed representation. Although circular in shape, the ecliptic did not rotate around its center. To accurately represent the daily path of the sun, the ecliptic rotated around a point approximately halfway between the center and the bottom edge of the circle. The ecliptic would complete one rotation around this point every day. Furthermore, the ecliptic was fashioned with 365 holes around its circumference, one for every day of the year, in which was placed a peg to represent the sun. The year began at the vernal equinox, and after each daily rotation of the ecliptic the peg would advance to the next hole along the perimeter of the ecliptic. However, the ecliptic was reset each day so that the peg always began at the horizon. The anaphoric clock was both a clock and a calendar, illustrating the both the time of day and the progression of the sun along the ecliptic. A second product of stereography is the astrolabe, a device for locating the position of the stars at any point in time. The astrolabe consists of three major parts: First, there is a fixed disk called a tympanum on which one can measure the position of the stars. The tympanum i...
