Posted on by VoijaRisa

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.

Branches

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, through which the great circle is drawn approximately along the middle of it would pass. It is this belt which we shall describe first, beginning with the southernmost section.

First, Ptolemy discusses the plane of the milky way. What he is most notably describing is the lane of dust that runs down the center of much of it, noting that this belt appears to be disrupted in two places: Near Ara and Cygnus.

The first of these, near Ara, is quite easy to see in this image. The central dust lane has a diagonal gap in it from roughly Norma to Scorpius.

However, what he’s referring to in Cygnus is less obvious.

Here, I believe Ptolemy is noting that the dust lane, while not having a distinct break, does wander southwards in Cygnus allowing the background glow to be seen above its plane between Cygnus and Vulpecula which is uncommon for the region.

What Ptolemy appears to be getting at is that, aside from these two deviations thanks to the dust, the milky way’s plane is relatively consistent and a great circle can be drawn through it which we would describe as the plane of the galaxy.

Borders

In each of the following sections, Ptolemy’s goal is primarily to inform us which stars bound the milky way. Thus, he tends to give stars that lie near the northern and southern edges, if any such thing can be defined. After all, the milky way is diffuse and doesn’t have a hard edge.

Since there’s not much to be said, I’ll be light on commentary and just give images of each of the sections to which Ptolemy is referring with the stars in question labeled.

This [section] goes through the legs of Centaurus, and is rather less dense and less bright [than the rest]. The star on the knee-bend of the right hind leg1 is a little farther south than the line [bounding] the milk to the north, and so are the star on the left front knee2 and the star under the right hind hock1. But the star in the left hind lower leg1 and on the right front hock2 are to the north of its southern rim, by about $2º$ (where the great circle is $360º$. It is slightly denser in the region near the hind legs.

Centaurus is well away from the center of the milky way, so looking this direction is looking “out” of our galaxy. Thus, it’s relatively thin here, as Ptolemy notes.

The description of the milky way appearing denser behind (i.e., to the west or right in this picture) the back hooves (i.e., Crux) makes sense as there are several bright nebulae in this region including the Carina nebula. There is also notably less dust obscuring this region.

Next in order, the northern rim of the milk is about $1 \frac{1}{2}º$ from the star on the rump of Lupus3, and the southern rim encloses the burning-apparatus of Ara4, but just grazes the northernmost of the two stars close together in the brazier5 and the southernmost of the two stars in the base4, while the star in the northern part of the brazier and the one in the middle of the brazier5 lie right in the milk. These sections are rather less dense.

This description all seems pretty accurate to me with the exception that θ Ara looks further outside the milk than β and γ Ara. However, there is some bulge to the east (left) so I won’t discount Ptolemy’s description entirely.

Next, the northern part of the milk encloses the three joints before the sting of Scorpius6 and the nebulous mass to the rear of the sting7, while the southern rim touches the star in the right from hock of Sagittarius8 and encloses the star on his left hand8. The star on the southern portion of the bow8 is outside the milk, but the star on the point of the arrow8 lies in the middle of it, while the stars in the northern part of the bow9 also lie in it, each of them being a little more than $1º$ removed from one of the rims, the southern star from the southern rim, the northern star from the opposite rim. The area [of the Milky Way] near the three joins [of Scorpius]6 is somewhat denser, while the area round the point [of the arrow of Sagittarius] is very dense indeed and appears smoky.

Here we’re nearly looking towards the center of the galaxy10. There’s definitely some fuzziness in Ptolemy’s boundaries as he describes ε Sgr as outside the milk, but the star close to it, η Sgr, as touching it. In my mind, the brightness of the milky way is nearly identical near those stars.

The following section is a little less dense. It extends along [the constellation] Aquilla, maintaining about the same width throughout. The star on the tip of the tail of the snake11 held by Ophiuchus lies in the open, a little more than one degree away from the advance rim of the milk, while the two most advanced of the bright stars below it lie right in the milk: the southern one12 is $1º$ from the rear rim, and the northern one12, $2º$ [from it]. The rearmost of the [two] star in the right shoulder of Aquila12 touches the same rim, while the more advanced one12 is cut off inside it, as is also the more advanced bright star of those in the left wing12. Furthermore, the bright star on the place between the shoulders12 and the two star which lie on a straight line with it fall a little short of touching the same rim. Next, Sagitta is enclosed entirely within the milk. The star on the the arrowhead13 lies one degree from the eastern rim, while the star on the notch13 lies two degrees from the western rim. The section round Aquila is slightly denser, and the remainder slightly less dense.

Moving away from the center of the galaxy, we quickly fall off in brightness, helped by a large dust lane which is why Ptolemy describes θ Ser as lying “in the open”.

Next, the milk extends towards Cygnus. Its north-western rim is defined in a reentrant angle by the star in the southern shoulder of Cygnus14, the star under it in the same [southern] wing14, and the two stars on the southern leg15. Its south-eastern rim is defined by the star in the tip of the southern wing-feathers14, and encloses the two stars under the same wing outside the constellation15, which are about $2º$ from it [the rim – into the milk]. The section around the wing is slightly denser. The next section is continuous with that belt, but is much denser and seems to have a different starting-point. For it points towards the end parts of the other belt, but leaves a gap between it [and itself]: on its southern side it joins the belt which we are currently describing, which is very rarefied at the junction; but after the point where it forms a gap with the other belt it gets denser, beginning from the bright star in the rump of Cygnus14 and the nebulous mass in the northern knee. Then it makes a slight bend as far as the star on the southern knee, and continues, gradually diminishing in density, up to the tiara of Cepheus. The northern side is delimited by the southernmost of the three stars in the tiara16 and the star to the rear of those three, at which it also forms two outrunners, one verging to the north and east, the other to the south and east.

The term “reentrant angle” is not one I’ve encountered before but apparently does have a definition: in a polygon, an interior angle greater than 180°, with its point turning back into the figure rather than out from it.

This seems like a bit of an odd definition to apply here, but what Ptolemy is getting at is that there’s a wedge going rearwards (right) with its tip near λ Cyg. This is pretty easy to see as there’s a thick dust lane just over this star defining this bright wedge.

Using ζ Cyg as the southern border seems a bit generous as it appears to be rather northwards of there. But given this isn’t a firm edge I won’t disagree.

When Ptolemy gets to the “next section”, I imagine he’s referring to the fact that the dust lane creates a brief gap between α Cyg and ν Cyg. There, he describes it as getting denser which is a result of the reddish region just north of ξ Cyg, which is the North America nebula.

Next the milk encloses the whole of Cassiopeia except for the star in the end of the leg17. The southern rim is defined by the star in the head of Cassiopeia17, and the northern rim by the star in the foot of the throne17 and the star in the lower leg of Cassiopeia17. The other stars [of Cassiopeia] and all those round about this [constellation] lie in the milk. The area near the rims are of thinner consistency, but those at the middle of Cassiopeia display a dense patch running the length [of the Milky Way].

This constellation is pretty straightforward in this very light region. When observing, it’s quite rare that I can even notice the milky way going through Cassiopeia.

Next, the righthand parts of Perseus are enclosed in the milk. Furthermore, its northern edge, which is very rarefied, is defined by the lone star outside the right knee of Perseus18, and its southern edge, which is very dense, but the bright star on his right side19 and by the two rearmost stars of the three to the south of that20. Enclosed in it also are the nebulous mass on the hilt19, the star in the head19, the star in the right shoulder19 and the star on the right elbow19. The quadrilateral in the right knee21 and also the star on the same [right] calf22 lie in the midst of the milk, while the star in the right heel23 is also inside it, a little distance from the southern border.

This region is extremely faint. Ptolemy correctly identifies that the milky way does run through it, but much of it is obscured by dust cloud on the perimeter of which BSC $1314$ and γ Per lie.

The dense region to which Ptolemy refers is the Alpha Persei Cluster.

Next the belt goes through Auriga, displaying a slightly thinner consistency. The star on the left shoulder, called Capella24, and the two stars on the right forearm25 fall just short of touching the north-eastern rim of the milk, while the small star over the left foot in the lower hem [of the garment]26 defines the south-western edge. The star over the right foot 24 lies half a degree within the same edge, and the two stars close together on the left forearm, called Haedi 25, lie in the middle of the belt.

Not much to say here. There’s a nebula near $14$ Aur which is the Flaming Star Nebula but is not sufficiently bright that Ptolemy refers to the region as being more bright than the rest.

Next the milk goes through the legs of Gemini, displaying a certain amount of density in elongated form just over the stars at the ends of the legs. Now the advance edge of the milk is defined by the rearmost of the three stars on a straight line under the right foot of Auriga27, by the rearmost star of the two in the staff of Prion28 and by the northernmost [two] of the four stars on his hand29; the brilliant star under the right hand of Auriga30 and the star in the rear foot of the rear twin31 are approximately $1º$ inside the rear edge, while the stars in the other feet32 lie in the midst of the milk.

Ptolemy’s description of the milky way going through the legs of Gemini is generous here. The stars labeled in Gemini above are all the feet of Gemini, so the main band of the milky way is distinctly beneath Gemini. 

Ptolemy’s discussion of the “advance edge” is rather muddled the stars he gives do not so much define an “edge” of anything and are spread out over nearly $8;30º$. There is not an appreciable difference in the brightness of the milky way between Gemini and the previous constellation, Auriga33. Similarly, the two stars given in Orion, the ones in the Prion, are still well within the milk.

Then the belt passes by Canis Minor [Procyon] and Canis Major: it leaves the whole of Canis Minor outside the milk no small distance to the east, and leaves Canis Major too outside to the west, almost in its entirety; for the star on its ears34 is caught by a sort of cloud which projects [from the Milky Way] and which then almost touches the three stars in the neck of Canis Major next to that [star] towards the rear35, while the lone star over the head of Canis Major, outside it and at some distance36, is about $2 \frac{1}{2}º$ inside the eastern rim. The consistency in the whole region is somewhat thinner.

It’s an important note here that I have tried to align these images so that the milky way is running horizontally. However, this means that the direction of north is not necessarily always towards the top. This is why Ptolemy’s description of much of CMa being outside the milk to the east does not mean to the left in this image. Indeed, much of CMa is beneath it.

It’s hard to understand what Ptolemy was meaning by the cloud that seems to protrude, but my suspicion is that he’s referring to the bright region just above θ CMa which is IC $2177$ and NGC $2327$, often referred to as the parrot or seagull nebula for good reason.

After that the milk passes through Argo. The western rim of the belt is defined by the northernmost and most advanced of the stars in the little shield in the poop37. The star in the middle of the little shield38, the two stars together under it39, the bright star at the beginning of the deck near the steering-oar40 and the midmost of the three stars in the keel41 are just short of touching the same [western] edge. The northernmost of the three stars in the mast-holder42 defines the eastern rim, while the bright star in the stern-ornament40 is $1º$ within the same [eastern] edge, and the bright star under the rearmost little shield in the deck41 is the same amount, $1º$, outside the same [eastern] edge. The southernmost of the two brilliant stars in the middle of the mast43 touches the same edge, and the two bright stars at the point where the keel is cut off44 are about $2º$ inside the advance rim. At that point the milk joins the belt through the legs of Centaurus. The consistency in this area too, throughout Argo, is somewhat rarefied, but the sections of it around the little shield, the mast-holder and the point where the keel is cut off are more dense.

Many of the stars here don’t seem to follow Ptolemy descriptions well. For example, m Pup strikes me as nowhere near a rim as do most of the stars following, yet still described as “touching the same edge.” The only way I can make this portion makes sense is to ignore the brightness going through Canis Major and imagine the cloud extending upwards into the milky way from the bottom of this image, right under the label Puppis, extending further to the right.

For the eastern rim, the star, BSC $3535$ does do a good job of defining it, although that rim is disrupted due to a large cloud dimming that region. Similarly, ρ Pup also seems appropriate to define this rim.

However, Ptolemy immediately goes awry again, describing λ Vel as being near the same edge when it is, if anything, $1º$ from the opposite edge. However, by my estimation using Stellarium’s angle measurement tool, it is at least $3º$ from the drop in brightness from the lower edge in the above image. The star in the mast, β Pyx does correctly appear to be on the same edge, as does λ Vel, although I find it hard to define exactly how far.

The belt we mentioned previously forms a gap, as we said, between [itself and] the one we have [just] described, at Ara. Beginning at that point, it encloses the three joints of Scorpius’ [tail] nearest the body6, but leaves the rearmost star of the three in the body45 $1º$ outside its western rim. The star in the fourth joint45 lies in the open space between the two belts, about the same distance from each, a little more than $1º$.

We’ve already discussed this region so I’ll repost the same picture for ease of comparison.

Here, Ptolemy is describing the bright band running from the lower right towards the upper left that is defined on either side by the dark dust lanes.

After that the advance belt turns aside to the east, in the shape of a segment of a circle, defining the advance edge of the milk by the star on the right knee of Ophiuchus46, and the rear edge by the star on the same [right] shin46, while the most advanced of the stars at the end of the same [right] leg47 touches the same [rear] edge. Subsequently the western rim is defined by the star under the right elbow of Ophiuchus46, and the eastern rim by the more advanced of the two stars in the same [right] hand46. From that point, there is a considerable gap of open space, in which the two stars on the tail of Serpens48 next to the star in the tip [of the tail]11. The whole of the section of this belt which we have [just] finished describing consists of an extremely fine and almost aery substance, except for the area enclosing the three joins [of Scorpius – in the previous section], which is somewhat more concentrated.

Here, I’ve clearly zoomed out a bit as the region Ptolemy refers to as we return to the center of the milky way is quite long.

In this section, Ptolemy is describing the portion of the center of the milky way to the north of the central band, leaving the southern portion out. While he describes several stars as being in “open space” we can understand today that the milky way is simply being disrupted by a large dust lane running from the lower left to the upper right.

After the gap the milk again makes a fresh beginning at the four stars to the rear of the right shoulder of Ophiuchus49. The eastern rim of this belt is defined (being just grazed) by the lone brilliant star under the tail of Aquila12, while the opposite rim is defined by the star which is some distance to the north of the four just mentioned50. From there on this belt, besides being rarefied, is also contracted into a narrow space in the area which is in advance of the star on the beak of Cygnus14, so as to produce the appearance of a gap. However, the remainder of it, from the star in the beak up to the star in the breast of Cygnus14, is wider and considerably denser. The star in the neck of Cygnus14 lies in the middle of the dense section. A rarefied section branches off to the north from the star in the breast as far as the star in the shoulder of the right wing14 and the two stars close together in the right foot15. From this point, as we said, occurs a clear gap to the other belt, [a gap] stretching from the above-mentioned stars in the Cygnus up to the bright star in the rump14.

Ptolemy opens this section by describing the section to the upper right, which is separated by another upwelling of the dust lane between ζ Aql and β Cyg.

  1. γ Cru.
  2. β Cen.
  3. ζ Lup.
  4. ζ Ara.
  5. β and γ Ara.
  6. θ, ι, and κ Sco.
  7. M7.
  8. η Sgr.
  9. λ and μ Sgr.
  10. Please note that I’ve included two stars in the picture that aren’t discussed here. This is because they will get discussed later when we revisit this region.
  11. θ Ser.
  12. λ Aql.
  13. γ Sge.
  14. λ Cyg.
  15. ν and ξ Cyg.
  16. ε Cep.
  17. ι Cas.
  18. BSC $1314$.
  19. α Per.
  20. ψ and δ Per.
  21. b, λ, $48$, and μ Per.
  22. $53$ Per.
  23. $58$ Per.
  24. α Aur.
  25. ν and θ Aur.
  26. $14$ Aur.
  27. $1$ Gem.
  28. χ² Ori.
  29. $72$ and $69$ Ori.
  30. Toomer identifies this star as κ Aur but I don’t believe there is any support for this as Ptolemy describes that star as “The bright star in advance of the advance knee [of Gemini]”. Rather, I believe this star should be given as θ Aur, “The star on the right wrist [of Auriga]”, which is what I have used here.
  31. ξ Gem.
  32. η, μ, ν, and γ Gem.
  33. This is not a case Stellarium incorrectly representing the brightness of the milky way in the region. Consider this picture by Alan Dyer. Here, Auriga is to the right and Gemini is in the bottom left with the milky way running top right to bottom left.
  34. θ CMa.
  35. μ, γ, and ι CMa.
  36. $19$ Mon.
  37. m pup.
  38. BSC 2948.
  39. $1$ and $3$ Pup.
  40. ζ Pup.
  41. δ Vel.
  42. BSC 3535.
  43. β Pyx.
  44. γ Vel and χ Car.
  45. τ Sco
  46. η Oph.
  47. $36$ Oph.
  48. ζ and η Ser.
  49. $66$, $67$, $68$, and $70$ Oph.
  50. $72$ Oph.