Abstract
The theories of the Sun and Moon developed in the preceding chapters have the feature in common that they provide exactly geocentric co-ordinates. In other words they presuppose an ideal observer situated at the centre T of the Earth and observing the centre of the planet in the direction TP (see Figure 7.1). A real observer placed at the point A on the surface of the Earth will see P in the direction AP. Unless P is at the zenith of A, the lines of vision TP and AP will include an angle called the parallax of P (in modern terms the daily parallax). It is the difference between the observed and the geocentric zenith distances of P. If we want to know the actual position of P (the direction of AP) then it is necessary to adjust the geocentric position derived from theory with a certain correction for parallax.
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Pedersen, O. (2011). Parallaxes and Eclipses. In: Jones, A. (eds) A Survey of the Almagest. Sources and Studies in the History of Mathematics and Physical Sciences. Springer, New York, NY. https://doi.org/10.1007/978-0-387-84826-6_7
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DOI: https://doi.org/10.1007/978-0-387-84826-6_7
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