Author: VoijaRisa

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The Ptolemaic Zodiac Project

This past year, while working on researching the history of Ptolemy’s star catalog, I came across the following quote from Toomer:

Ideally one should provide a reconstruction of the outline of each constellation as it appears on Ptolemy’s star-globe. Unfortunately no one has done the necessary work of assembling and comparing all the literary and iconographic evidence from antiquity and from the derivative Arabic tradition (notably as-Sufi). This would be an interesting and valuable enterprise.

Essentially what Toomer is noting here is that, while historians of astronomy have spent considerable time trying to reconstruct what stars the catalog described, less attention has been paid to how the constellations looked.

Thus, I’m launching a spin-off from my main project to look at this topic. Continue reading “The Ptolemaic Zodiac Project”

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Almagest Book VIII: On the Location of the Milky Way

Having laid out the star catalog, Ptolemy next faces a somewhat more abstract challenge: attempting to describe the nebulous structure of the Milky Way. He first describes the general shape and color:

Now the Milky Way is not strictly speaking a circle, but rather a belt of a sort of milky colour overall (whence it got its name); moreover this belt is neither uniform nor regular, but varies in width, colour, density and situation, and in one section is bifurcated. [All] that is very apparent even to the casual eye, but the details, which can only be determined by a more careful examination, we find to be as follows.

There’s not really much to discuss here, so instead, I’ll combine Ptolemy’s descriptions with some screenshots from Stellarium to help highlight what he’s talking about.

Continue reading “Almagest Book VIII: On the Location of the Milky Way”

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Scholarly History of Commentary on Ptolemy’s Star Catalog: The Aratus Latinus & Codex Climaci Rescriptus

In a previous post, we discussed a bit about Aratus’ poem, the Phaenomena, to which the Aratus Commentary was a response.

What I haven’t mentioned yet is that this original poem is still around. In fact, we have numerous copies of it thanks to it exploding in popularity in the $8^{th}$ century. However, sometime before then the poem itself had evidently been padded with other astronomical works. Among them were some descriptions of constellations. And these descriptions would provide new insight to our conversation. Continue reading “Scholarly History of Commentary on Ptolemy’s Star Catalog: The Aratus Latinus & Codex Climaci Rescriptus”

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Scholarly History of Commentary on Ptolemy’s Star Catalog: Stars at the Southern Limit

Another argument over the authorship of the star catalog examines the stars near the southern horizon. Since Hipparchus observed from Rhodes with a latitude of $\approx 37º$ and Ptolemy from Alexandria which is $\approx 31º$ N, this means that Ptolemy could have observed starts $5º$ further south that were never above the horizon of Hipparchus.

However, Delambre notes that there’s not a single star in the catalog that could not have been observed by Hipparchus at his latitude. So if Ptolemy was the originator of the catalog then, for some unknown reason, he declined to observe stars near the horizon1.

Thus, the inclusion or exclusion of stars cannot settle the matter. But astronomers have turned to other questions regarding stars near the southern horizon to approach it. So in this post, we’ll explore three papers on this subject. Continue reading “Scholarly History of Commentary on Ptolemy’s Star Catalog: Stars at the Southern Limit”

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Scholarly History of Commentary on Ptolemy’s Star Catalog: Grasshoff (1990) – Ptolemy’s Phaenomena

Previously, we discussed Vogt’s attempt to reconstruct the Hipparchan catalog by reverse calculating its coordinates from Hipparchus’ Commentary on Aratus. This Commentary was Hipparchus’ response to a poem by Aratus entitled the Phaenomena. Grasshoff ultimately took issue with Vogt’s methods, finding them insufficiently explained given the number of assumptions required to perform the transformation, to put too much stock in. Although not overtly stated, the fact that no one else has attempted to reproduce Vogt’s methods with better explanations, including Grasshoff himself, implies that the uncertainty surrounding such assumptions are considered sufficiently prohibitive that it is not worth attempting to refine Vogt’s methods.

However, Grasshoff isn’t finished with the Aratus Commentary just yet. While the issues with the dates and longitudes may make the Aratus Commentary too messy to use to reverse calculate Hipparchus’ catalog from, Grasshoff instead proposes going the other way around – using Ptolemy’s catalog to calculate same rising/culminating/setting descriptions given in the Aratus Commentary. These can then be compared to those in the Aratus Commentary without needing to worry about recovering Hipparchus’ catalog. Continue reading “Scholarly History of Commentary on Ptolemy’s Star Catalog: Grasshoff (1990) – Ptolemy’s Phaenomena”