A Possible Date for Ptolemy’s Development of a Model for the Second Lunar Anomaly

Abstract

Ptolemy explicitly claims in the Almagest to have made 36 observations. For most of them, the date of the observation is determined by the kind of phenomenon observed, for example, the observation of a lunar eclipse or of an equinox. There are six observations, however, in which a particular date is not required by the nature of the phenomenon under investigation. In five of them, Ptolemy measures the longitude of each planet, without any particular requirement. In the sixth, he measures the longitude of a star. The aim of this paper is to show that these six observations belong to the set of observations that Ptolemy could have used for discovering the second lunar anomaly. Because all of them took place in the first half of year 139, one can presume that it was then when Ptolemy developed his model for the second lunar anomaly.