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... stant source of motion, it is possible to create a clock-an 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 urn. These early clocks were used when equal measurements of time needed to be established. For example, if two orators were to be allotted the same amount of time to speak before an assembly, a water clock of this nature would have been constructed for the occasion. In the second century BC. , a man named Ctesibus created a more elaborate water clock for measuring the time of day. The Clepsydra, as it is called, consisted of four major parts: a vessel for providing a constant supply of water (B), a reservoir and notched floatation rod (F), a display (G), and a device for adjusting the flow of water into the vessel (D). Water was continually poured into the vessel (B), with the overflow escaping from a pipe (I).
Water flowed from this vessel into the reservoir at a constant rate. As the reservoir filled with water, the floating, notched rod ascended at a constant rate. This rod was attached to the display (G), which indicated the time of day. The Greeks divided the day into twelve hours of unequal length to insure an equal division of day and night. Because the Greeks divided the day into hours of unequal length, it was necessary to include a device (D) to regulate the flow of water from the vessel (B) into the reservoir (F). By raising the flat, circular cap in the conical vessel (B), the flow of water could be increased, decreasing the length of an hour.
In the summer, the day is longer than the night, and in the winter the reciprocal is true. Therefore, in the summer, the clock would be adjusted to extend the length of each day hour. A second way the Greeks standardized the length of a day was by modifying the clock display. A cylinder with sloping hour lines was used instead of a circular face. The mechanism worked as follows: as water collected in the reservoir, a pointer would raise as the cylindrical display rotated. In this manner, the pointer would gradually trace the course of the adjusted hours on the cylindrical display.
However, 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 model-a 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 disks-the 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 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 is 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 stars-called a rete-was fastened over the tympanum.
The third primary component of an astrolabe is a simple device for measuring the distance of a star above the horizon-usually 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's phone, 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 the Almagest, but also established the foundation of modern time keeping.
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Research essay sample on Time Of Day Century Bc
