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Almagest Book VII: On Precession

At the beginning of the last chapter, Ptolemy noted that the celestial sphere appears to have a rearward motion of its own known as precession of the equinoxes. This motion means that the position of the sun in the zodiacal signs slowly increase in advance as time passes. In particular, the sun was at the beginning of Aries in Ptolemy’s time, but since then has advanced such that today it lies near the beginning of Pisces.

We can see this mainly from the fact that the same stars do not maintain the same distance with respect to the solstical and equinocital points in our times as they had in former times: rather, the distance [of a given star] towards the rear with respect to [one of] the same points is found to be greater in proportion as time [of observation] is later.

Continue reading “Almagest Book VII: On Precession”

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Almagest Book VII: Hipparchus’ Star Alignments

Ptolemy lays out the arrangements described by Hipparchus by going through stars in various constellations:

Stars in Cancer. [Hipparchus] records that the star in the southern claw of Cancer [α Cnc], the bright star which is in advance of the latter and of the head of Hydra [β Cnc], and the bright star in Procyon [α CMi] lie almost on a straight line. For the one in the middle lies $1 \frac{1}{2}$ digits to the north and east of the straight line joining the two end ones, and the distances [from it to each of them] are equal.

First, we should discuss the identification of the stars. The designations of the stars in the brackets are the modern Bayer designations first created in $1603$. For Ptolemy’s part, he described the positions of the stars based on how they should appear in the representation of the constellation. For example, we see him here describing α Cnc as the “southern claw of Cancer”. However, because the depiction of the figures in constellations is somewhat subjective, this leads to some ambiguity in which stars are which, making assessing the validity of Ptolemy’s statements somewhat difficult. Continue reading “Almagest Book VII: Hipparchus’ Star Alignments”

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Almagest Book VII: On the Relative Positions of the Stars

Now, to deal with the next part of the theory, we shall begin discussing the stars, and first, in accordance with the logical order, the so-called fixed stars.

To introduce the next book of the Almagest, Ptolemy informs the reader that we will be taking some time to understand the fixed stars. There are two important things to note about this sentence. The first is that Ptolemy is taking extra care to make sure the reader knows we are discussing the fixed stars. This is because the Greek concept of “stars” could include the planets. Continue reading “Almagest Book VII: On the Relative Positions of the Stars”

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Almagest Book VI: Determining the Inclinations

And here we are. Final chapter of Book VI. Here, we’ll finish out Ptolemy’s discussion of the inclinations of eclipses. In the previous post, we discussed a few things we’ll need to determine that1, but we really haven’t discussed is what the “inclination” means. So I’ll begin by doing that as Ptolemy doesn’t really do a clear job of doing so. Continue reading “Almagest Book VI: Determining the Inclinations”

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Almagest Book VI: Horizon Diagram

In the previous chapter, we mentioned that one of the things we’ll need for Ptolemy’s weather prediction is the angle between the ecliptic and celestial equator as measured along the horizon. This is something we calculated back in II.2 and only now are we getting around to displaying a diagram to summarize it. So I’ll display the diagram here1 and discussion about it will be beneath the fold.

Continue reading “Almagest Book VI: Horizon Diagram”

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Almagest Book VI: Angles of Inclination at Eclipses

The final few chapters of Book VI are rather odd. Now that we’ve completed the discussion of eclipse prediction, Ptolemy wants to do an “examination of the inclination which are formed at eclipses.” However, he doesn’t appear to provide any motivation for doing so. Toomer and Neugebauer both indicate that the actual reason was likely weather prediction1, but the Almagest doesn’t contain any information on how this is to be used. Neugebauer indicates that,

[T]he technical term connected with this problem is “prosneusis”…developed from the original meaning of the verb νευειν (to nod, to incline the head, etc…). According to the terminology of hellenistic astrology, the planets or moon can, e.g., give their consent by “inclining” toward a certain position, i.e., by being found in a favorable configuration.

However, aside from these astrological purposes, these last few chapters are essentially left as a free-floating bit of material. Continue reading “Almagest Book VI: Angles of Inclination at Eclipses”

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Almagest Book IV: Adjustments to Intervals for Parallax

I stopped my previous post where I did because the material it covered is the end of the example problem Toomer provided. However, Ptolemy still has a few more paragraphs to go because

there is, in fact, a noticeable inequality in these intervals [of immersion/emersion] due, not to the anomalistic motion of the luminaries1, but to the moon’s parallax. The effect of this is to make each of the two intervals, separately, always greater than the amount derived by the above method, and, generally, unequal to each other.

In short, because the parallax changes over the course of the eclipse, it will cause the immersion and emersion durations to be longer than they would otherwise be.

We shall not neglect to take this into account, even if it is small.

Then let’s get to it. Continue reading “Almagest Book IV: Adjustments to Intervals for Parallax”