A Brief History of Clocks: from Thales to Ptolemy

Topics: Celestial coordinate system, Celestial sphere, Earth Pages: 7 (2970 words) Published: October 8, 1999
The clock is one of the most influential discoveries in the history of western science. The division of time into regular, predictable units is fundamental to the operation of society. Even in ancient times, humanity recognized the necessity of an orderly system of chronology. Hesiod, writing in the 8th century BC., used celestial bodies to indicate agricultural cycles: "When the Pleiads, Atlas' daughters, start to rise begin your harvest; plough when they go down" ( Hesiod 71). Later Greek scientists, such as Archimedes, developed complicated models of the heavens-celestial spheres-that illustrated the "wandering" of the sun, the moon, and the planets against the fixed position of the stars. Shortly after Archimedes, Ctesibus created the Clepsydra in the 2nd century BC. A more elaborate version of the common water clock, the Clepsydra was quite popular in ancient Greece. However, the development of stereography by Hipparchos in 150 BC. radically altered physical representations of the heavens. By integrating stereography with the Clepsydra and the celestial sphere, humanity was capable of creating more practical and accurate devices for measuring time-the anaphoric clock and the astrolabe. Although Ptolemy was familiar with both the anaphoric clock and the astrolabe, I believe that the development of the anaphoric clock preceded the development of the astrolabe.The earliest example, in western culture, of a celestial sphere is attributed to the presocratic philosopher Thales. Unfortunately, little is known about Thales' sphere beyond Cicero's description in the De re publica:For Gallus told us that the other kind of celestial globe, which was solid and contained no hollow space, was a very early invention, the first one of that kind having been constructed by Thales of Mileus, and later marked by Eudoxus with the constellations and stars which are fixed in the sky. (Price 56)This description is helpful for understanding the basic form of Thales' sphere, and for pinpointing its creation at a specific point in time. However, it is clearly a simplification of events that occurred several hundred years before Cicero's lifetime. Why would Thales' create a spherical representation of the heavens and neglect to indicate the stars? Of what use is a bowling ball for locating celestial bodies? Considering Eudoxus' preoccupation with systems of concentric spheres, a more logical explanation is that Thales marked his sphere with stars, and Eudoxus later traced the ecliptic and the paths of the planets on the exterior. The celestial sphere in question probably resembled this early Persian example.Perhaps the most famous celestial sphere is the mechanized globe attributed to Archimedes. Cicero was especially impressed by this invention because of its ability to imitate "the motions of the sun and moon and of those five stars which are called wanderers" with a single rotational focus (Price 56). By turning a crank, one could observe the "natural" course of the sun, moon and planets around the earth. The sphere was also remarkable for a second reason. Unlike a stationary globe, like that of Thales' and Eudoxus, a mechanized sphere requires gears to accurately represent the motion of the heavens. According to Prof. Derek Price, the mean period of Saturn can be mechanically represented by a gear ratio of 30 to 1. In other words, for every revolution of the sun around the earth, Saturn will only accomplish 1/30th of its revolution around the earth. The mean period of Jupiter can be represented by a gear ratio of 12 to 1, and Mars can be represented by a gear ratio of 2.5 to 1.An interesting problem arises when one attempts to mechanically represent the synodic month. A gear ratio of 235 to 19 is required for an accurate representation. However, this is impossible to achieve directly, presenting a serious challenge to Archimedes and other Greek scientists. Prof. Price claims that two different gear...
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