Introduction.
In Almagest VIII 2 Ptolemy gives a description of the Milky Way, as seen from his Alexandrian observing location. In it, he mainly focuses on three aspects of his theme: first, the location of the limits of the bands which constitute the Milky Way. He points these limits by selecting certain stars which are at or close to, the point in which the milk ceases to be perceptible. Second, he gives us some vague indications about the density of the milk along its path. These variations are conveyed through ambiguous phrases such as the indication that the milk is “somewhat denser” (ἠρέμα πυκνότερα) in some place, or with a “smoky” (καπνώδης) appearance in other. While we cannot get a very precise image from these sentences, they nevertheless allow us to have a general picture of what he was looking at, and sometimes even relate his description to modern labels given to some regions of the galactic belt.1 Third, in his description Ptolemy also gives us a general description of the structure of the Milky Way, distinguishing between two main sections. One continuous to the south, and one which is located between the regions of Ara and Cygnus. This second, northern belt is itself cut in two parts by a dark region, which today we call “the Great Rift”, and Ptolemy described as “[…] considerable gap of open space […]”2.
The description of the Milky Way is given after Ptolemy’s stellar catalogue in VIII 1, and before his instructions to build a solid globe in VIII 3. The order makes sense: he needs to have the stellar labels and coordinates in order to use the stars themselves to describe and locate the milk in a meaningful way. Because the milk itself moved together with the rest of the fixed stars, it was natural to depict it together with them in the physical representations of the celestial sphere, such as the solid globes which were crafted in those days.
Although much work has been devoted to the stellar catalogue of the Almagest, and also to the extant evidence of ancient celestial spheres, there is no substantial scholarly work devoted to Ptolemy’s description of the Milky Way. We can just barely, for example, find a couple of mentions in (Pedersen, 2010: 259 and Neugebauer, 1975: 890). I expect that this work will serve as a monograph that makes an initial survey of the topic, and provides the necessary groundwork to foster future studies.
The first section of the paper deals with the location of the Milky Way in Ptolemy’s times for an Alexandrian observer. One of the more striking features of the text is that Ptolemy describes the entire Milky Way, beginning in Centaurus, moving towards the galactic center in Sagittarius, and going all the way round up to Argo. This would not be possible for an observer today. I will show, via a brief argument, how the effect of precession account for the difference, and how the milk was positioned in ancient times with respect to an observer in those parts of the world.
The second section, which is the longest, is focused on Ptolemy’s description properly. First, I will give a general account of the structure Ptolemy gives: the aforementioned two main belts, and how they are divided. Then, following the order of the Almagest, I will begin by the southern belt in Centaurus, and continue towards Sagittarius, all the way around. After that, I will focus on the two main parts of the northern belt, one on each side of the Great Rift. Finally, I will give some concluding remarks.
The paper follows Ptolemy’s order, describing the appearance of the milk in successive constellations, starting in Centaurus. My analysis is made according to the following pattern: first, I reproduce Ptolemy’s description in a given region. Ptolemy indicates the stars (mostly) following the labels he assigned to them in his catalogue. Therefore, I in turn indicate the corresponding star number for the catalogue.3 In most cases this is a straightforward procedure. Then, I also give modern identifications for the stars in Ptolemy’s catalogue. For this, I mostly use the identification given in Toomer’s edition. After this, I identify the location of each of the stars in a high-resolution image of the Milky Way. To avoid excessively big labels which would hinder a clear visualization, I number them in the image following the order of appearance in Ptolemy’s text. The caption of these images correlates these numbers with the modern names of the stars. Then, I do the same thing in Pannekoek’s diagrams of the Milky Way (more on this below). Finally, I compare Ptolemy’s description with the depictions in the high-resolution photograph and Pannekoek’s diagrams and analyze how they relate to each other.
The use of images deserves some additional clarifications. The high-resolution photograph was produced by the European Southern Observatory as part of their GigaGalaxy Zoom Project, in association with astrophotographer Serge Brunier. Regarding Pannekoek’s diagrams, they are taken from Die Nördliche Milchstrasse (1920) and from Die Südliche Milchstrasse (1928). They are isophotic charts4 that depict the constellations around the galactic equator, together with a numeric representation of the brightness of each region. The method he uses to measure the brightness of the milk is an adaptation of Argelander’s method for measuring the brightness of stars (Pannekoek, 1920: 2; 1928: 11). It is based on a fairly sophisticated procedure that takes many normalpunkte or normalstelle, and produces a common measurement system, which is later combined with visual descriptions to produce accurate diagrams. A full explanation of the reasoning behind the values given to each region is beyond the scope of this paper. It can be found in (Pannekoek, 1920: 2-5). The inclusion of Pannekoek’s diagrams in this paper serves as an additional support to the presence of modern photographs, and as an objective instrument to evaluate the consistency of Ptolemy’s indications of brightness, particularly regarding the determination of the limits of the milk.
Finally, I will assume a division of the celestial sphere by the galactic equator, where the north will be the hemisphere in which the north galactic and celestial poles are located, and south will be the other hemisphere. This is not always the convention used by Ptolemy, who is not consistent in this regard. Thus, while he sometimes talks about north and south, in other instances he refers to my northern and southern limits of the milk as the eastern and western limits, or the equivalent more advanced or rearmost limits.
Position of the Milky Way as seen from Alexandria in Ptolemy’s times.
The equatorial position of the galactic north for B1950.0 is 192;15° for RA, and 27;24° north for declination (Meeus, 1998: 94). This means that the northernmost latitude from which the entire Milky Way’s equator is visible today is about 27;30°. Because Alexandria’s latitude is 31;12° north, this shows that today it is not the case that the entire galactic plane is visible from Alexandrian latitudes: the declination of the galactic north pole would have to be at least around 31°.
While the effect of the variation of the obliquity of the ecliptic is negligible when it comes to the inclination of the galactic equator to the celestial equator, this is not the case with the effect of precession. The B1950.0 ecliptic position of the galactic north is 179;19,29° for longitude, and 29;48,43° north for latitude. If we assume a value of 50’’ per year for precession, we get a longitude for 140 of 154;11,9°. This means a declination of the galactic north for 140 of 37;23,50°.
So, in Ptolemy’s times, an observer at Alexandria would have been able to observe the entire equator of the Milky Way (“[…] the great circle drawn approximately along the middle of it […]” Toomer, 1984: 400) throughout one year. Nevertheless, because the varying width of the Milky Way spans several degrees on both sides of the galactic equator, the visibility of the entire galactic equator does not assure that the Milky Way itself will be wholly observable in all its details, particularly some sections near its rims. As we saw, Ptolemy would have been able to see up to about 37.5° - 31° = 6° to the south of the southernmost point of the galactic equator. This is a region some 4° southeast of the intersection of the two lines in Crux, almost in the center of the Coalsack (point SP in Figure 1). The relation between maximum altitude and distance to this southernmost point in the galactic equator follows a sinusoidal curve, with a maximum altitude of about 37.5° + 31° = 68.5° at the northernmost point of the galactic equator, which is about 1.5° to the northwest of the middle star in the “W” of Cassiopeia (point NP in Figure 1). However, the altitude reached by each point of the galactic equator is not the only variable relevant to study the visibility of the belt of the Milky Way. The angle between the galactic equator and the horizon varies throughout the day. For an observer at Alexandria in 140 the maximum would have been, of course, 68.5°. This means that while some points of the galactic equator might never rise more than 8° or 12° above the horizon, the band of the Milky Way which is about 15° in width on each side might still be mostly or entirely visible, due to the fact that at that moment the angle between the galactic equator and the horizon is large enough. In Figure 1 points I (in the constellation of Vela) and II (in the constellation of Ara) indicate the limits of the southern region where the visibility of the band is always less than 15° to the south of the galactic equator.
One last variable to remember is extinction, which is the absorbing and scattering effect that the atmosphere has on the light coming from the sky. As it is natural, the closer the object is to the horizon, the more relevant this effect becomes.
Ptolemy’s description.
Ptolemy begins his description by indicating the general structure of the Milky Way. He says that “[…] 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 […] The bifurcated part of the belt has one of its ‘forks’, so to speak, near Ara, and the other in Cygnus. But, whereas the advance [part of the] belt is in no way attached to the other part, since it forms gaps both at the fork by Ara and at the fork by Cygnus, the rearmost part is joined to the remainder of the Milky Way and forms [with it] a single belt […] It is this belt which we shall describe first, beginning with its southernmost section.”6
The description is simple. Throughout most of the Milky Way, there is a single belt. But when we approach the center in Sagittarius from each side, we find that it bifurcates, with one belt being continuous with the rest, and another going towards the north. As Ptolemy will comment later on, this second belt is itself divided into two sections. Refer to Figure 2.