Finally we’re on Book VI. So far in the Almagest, we’ve had a few books which laid out some preliminary tables and concepts, a book on the sun, and two on the moon1. Now it’s time to put the sun and the moon together to start looking at some of the most dramatic astronomical phenomena: eclipses. To introduce this topic, Ptolemy begins with an uncharacteristically short chapter which is a single paragraph.
There are only a few simple things that Ptolemy refers to in this. The first is that a reminder that eclipses, whether lunar or solar, occur at syzygies, which is to say either conjunctions or oppositions. However, these are determined by the motions composed of both the mean and anomalistic motion of both the sun and moon. Ptolemy notes that this is indeed a complicated task to work out but is possible “if [one] does not shrink from [the labor of] comparing the individual positions of the luminaries at every appropriate occasion.”
Those “appropriate occasions” are, of course, the syzygies. So when are they? Well, we could calculate the position of the sun and moon every day to determine this, but that sounds extremely complicated2. Instead, Ptolemy promises “a more convenient way of finding these.” This will be a table of mean syzygies which we will explore in the next chapter.
Sorry for disappearing for two months without a post! As I did with Book III for the sun, I’m working on a paper using Ptolemy’s methodology to construct a lunar model suitable for modern day. This means reviewing a lot of the posts I’ve done in the past year to remember how it all goes and inevitably, I find things that I didn’t quite understand right the first time around. So I’ve been busy editing old posts and working on that paper instead of working on moving forward in the Almagest.
