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| Program > Proof, Prediction and Mathematics in Ancient and Islamic Science |
| Programme > Preuve, prédiction et mathématiques dans les sciences antiques et islamiques |
| Alexander
Jones
(University of Toronto) Frames of Reference in Ancient Astronomy |
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Modern astronomy follows Ptolemy (mid second century A.D.) in taking the Spring Equinoctial Point (one of the intersections of the ecliptic--the plane of the sun's orbit--with the plane of the earth's equator) as the zero point for measuring planetary positions along the ecliptic. In Ptolemy's astronomy this frame of reference is clearly distinguished from the more "natural" frame of reference of the stars, which Ptolemy considers unsuitable on theoretical grounds. We know that in Babylonian astronomy the two frames of reference (respectively called "tropical" and "sidereal") are not treated as distinct. An examination of computed positions of the moon and planets in Babylonian and Greek astronomical texts under certain conditions makes it possible to find out which was actually being used. One might have expected a fairly neat transition from purely sidereal, observation-based positions in Babylonian texts to purely tropical, computation-based positions after Ptolemy; the reality turns out to have been considerably more complicated.
University
of Toronto
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