Continued from last post but one

After this observation, the haze closed in again and no observation was possible for a while. When the sky cleared again, the sunspot had moved. Moved much further in an hour or so than a sunspot ordinarily moved in a day, but just about as much as the planet Mercury would have moved.

To understand what Gassendi was expecting to see, we must cast our minds back some 1500-odd years before Gassendi, to about the year 150 AD. At that time, the astronomer we know as Claudius Ptolemy published a book which came to be known by the name Almagest, from an Arabic expression meaning the greatest, although Ptolemy himself modestly called it simply a Mathematical Treatise. By whatever name, the book was a set of mathematical procedures for predicting the future location of planets in the sky based on observations of their past locations. This was a difficult task, since the motion of the planets is strangely irregular, or so it must have seemed to the ancient Greek astronomers.