Eratosthenes. Posidonius. Hipparchus.
Eratosthenes (276-196 B.C.) was the first who reduced geography to a regular system, and laid its foundations on clear and solid principles. Under the patronage of the Ptolemies he had access to all the material collected by Alexander and his generals. The doctrine of sphericity of the earth had by this time been adopted, and the aim of his labours was to delineate, in conformity with this principle, the known parts of the earths surface. Founding his system on the use of the gnomon, he supposed a line to be traved through certain places, in all of which the longest day was known to be exactly of the same length. Such as line would evidently be a parallel to the equator. This first parallel passed through Rhodes, and was ever afterwards adopted as the basis of ancient maps. Eratosthenes continued his work by tracing other parallels at certain intervals form the first, one through Alexandria, another through Syene, a third through Merce. He also traced, at right angles to these, a meridian passing through Rhodes and Alexandria southwards to Syene and Meroe. As the progress which he thus made towards the completion of what he had so skillfully conceived naturally tended to enlarge his ideas concerning geographical science, he attempted next to determine the circumference of the globe by the actual measurement of a segment of one of its great circles. Posidonius made another measurement of an arc of the meridian between Rhodes and Alexandria about 170 years afterwards; but the amount of error in the calculations of Eratosthenes and Posidonius is uncertain, for want of a knowledge of the true length of the stadium in which their results are expressed. The ancients made their first meridian at the sacred promontory of Iberia, and their longitudinal error increased rapidly as they advanced eastwards. This is no doubt due to their longitudes being based entirely on distances calculated in the itineraries of travelers. Such data of course produced very great distortions in the representations given of the countries on the surface of the globe.
The improvements introduced by Eratosthenes were perfected in principles by Hipparchus, who flourished from 160 to 135 B.C. He was the first astronomer who undertook the arduous task of making a catalogue of the stars and fixing their relative positions. His object was to transmit to posterity a knowledge of the state of the heavens at the period of his observations. The extremities of the imaginary axis round which the heavens perform their diurnal revolutions suggest two fixed points by which the position of the great circle of the celestial sphere, called the celestial equator is determined. If a great circle be supposed to pass through these points and any star, the position of the star will be ascertained if we measure in degrees and parts of a degree the arc of the meridian circle intercepted between the star and the equator, and also the arc of the equator intercepted between a given point in it and the meridian circle passing through the star. Upon this principle Hipparchus arranged the stars according to their places in the heavens; and the great improvement which he introduced into geography consisted in this, that he applied to the determining of the position of any point on the surface of the earth the same rule which he had introduced in the arrangement of the constellations. Thus he furnished the means of ascertaining the relative positions of places with far greater accuracy than could be obtained from itinerary measurements. He made a considerable number of observations for latitude, and pointed out how longitudes might be determined by observing the eclipses of the sun and moon.
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