er, the former example, the circular face, is more important because of the modifications made to it after the discovery of stereography by Hipparchos. Stereography is a technique by which three dimensional objects are projected on two dimensional surfaces. Hipparchos used stereography to create a projection of the celestial sphere from its southern celestial pole to its equatorial plane. In other words, he created a two dimensional image of a three dimensional modela planispheric projection of the heavens. By separating the projection of the stars and the ecliptic from the projection of the horizon and the equator, Greek scientists could simultaneously represent the progression of the sun along the ecliptic and the daily rotation of the sun around the earth. In essence, by separating the two projections scientists recreated the rotational components of an armillary sphere on a two dimensional surface. By incorporating these two planispheric projections of the sky into the display of a clepsydra, the Greeks discovered a way for providing the constant source of motion necessary for an accurate representation of time. Recall that an armillary sphere can be used to tell time because it allows one to divide the daily rotation of the sun around the earth into 24 hours, with each hour equal to 15 degrees of the complete rotation. The problem with keeping time on an armillary sphere is that a constant source of motion is required for the sphere to mimic the actual motion of the sun around the earth. By using stereography, scientists were able to project the armillary sphere on two disksthe first provided the means for measuring sun's position in the sky, and the second disk illustrated the sun's actual path across the sky. There are two advantages to having the heavens projected on two disks, as opposed to a single sphere. First, it is easier to construct a two dimensional model than a complicated sphere. Second, it is easy to provide constant...
