Abstract
This paper analyses the horizon diagram in Ptolemy's Almagest and in the Handy Tables. In both works this diagram is used only for the calculation of the prosneusis in an eclipse. The aim of such a calculation is not clear : one has suggested some astrological purpose, or ancient meteorological theory. The paper examines the prosneusis calculations given by Theon of Alexandria and by some Byzantine authors : Stephanos, Barlaam, Nicephorus Gregoras, Theodorus Meliteniotes. For each calculation a figure is drawn showing the angles obtained for the prosneusis and the positions of Sun and Moon at the different moments of the eclipse. In each case this figure is very similar to a modern diagram showing the scheme of the eclipse, but inverted, as seen in a reflecting surface such as a basin full of some liquid. So one can suggest that the prosneusis calculation was intended to give a precise account of the phases of the eclipse, perhaps with help of a figure reproducing the phenomenon as seen by the observer.
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