In the final chapter of Book VIII, Ptolemy discusses the first and last visibilities of stars. Before diving into the text, let’s first take a moment to think about what this means.
The description is, in some ways, self evident – we’re considering when, based on the movement of the sun, are stars first and last visible.
Recall that, when viewed from inside the celestial sphere (i.e., on earth), the sun appears to move west to east along the ecliptic. Thus, as time passes, stars that were washed out by the sun will, at some point, become visible because they have risen while the sun has moved far enough along the ecliptic that it will be sufficiently far below the horizon as to not wash the star out in the morning glow before the sun actually crosses the horizon. This point in time is known as their helical rising and describes when they are first visible in the sky throughout the year.
These stars will, of course, initially be only briefly visible as the daily rotation of the sun will quickly cause them to be lost in the morning glow, but as the sun continues to progress eastward along the ecliptic, the star will be visible for longer.
Similarly, we can ask at what point the sun has moved sufficiently close to a star that, even after the sun has set, the star will be lost in the evening glow and thus, it is effectively invisible for the season until its next helical rising.
In case you haven’t been checking the dates on these posts, you may well notice that it’s been nearly a month and a half since my last post. It’s because this section has been quite challenging as I find Ptolemy’s descriptions here are an absolute mess. This is probably among the least sensical sections I’ve yet encountered. I think you’ll see why as we go through Ptolemy’s commentary. Continue reading “Almagest Book VIII: On the First and Last Visibilities of the Fixed Stars”
