17th century armillary sphere is graduated for both ecliptic and equatorial coordinate systemsnotice how each sign of the zodiac contains thirty degrees of the circle. Measuring time on an armillary sphere is a simple matter. First, imagine that you live on the earth's equator. From this position, the ecliptic is almost a perfect arch over your head. As the earth rotates, the sun will rise and set in a twenty-four hour period. Please remember that this is not the eclipticthe ecliptic will only determine where, on the horizon, the sun will rise and set each day. In antiquity, every day is a complete rotation of the sun around the earth. Time may be measured simply by dividing this rotation into twenty-four hours. If the rotation is a circle of 360deg., dividing it into 24 sections results in hours that are 15deg. long. In other words, if we know where the sun will rise on the horizon, according to the ecliptic, every fifteen degrees that the sun travels across the sky marks the end of an hour. Given a constant source of motion, it is possible to create a clockan accurate representation of the heavens, from an armillary sphere. Although the Greeks had the means of producing the necessary motion, the shape and intricacy of an "armillary sphere clock" may have prevented rigorous experimentation until the development of stereography. Until the development of stereography by Hipparchos in the middle of the second century BC., the Greeks measured time with various types of water clocks. The most simple water clock consisted of a large urn that had a small hole located near the base, and a graduated stick attached to a floating base. The hole would be plugged while the urn was being filled with water, and then the stick would be inserted into the urn. The stick would float perpendicular to the surface of the water, and when the hole at the base of the urn was unplugged, the passage of time was measured as the stick descended farther into the u...
