What Hipparchus is doing is similar to modern regression analysis, in which a straight line or other simple curve is made to give as good a fit as possible to a series of points representing observations. The line of best fit does not go through all the points exactly; the deviations of observed points from the true line are regarded as error (due either to observational error or to the intrinsic variability of the subject matter; the latter cause is not relevant in astronomy). It is an exercise in good probabilistic judgment to decide how much to complicate the model to provide a better fit
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