Sand and Stars

Ptolemy’s Milky Way

Engraving of Ptolemy being guided by the muse Urania, from Margarita Philosophica by Gregor Reisch (1508)

15 Apr 2018

Little to me is more intoxicating than lying on my back on a moonless night in the Kalahari, gazing naked eye into the Milky Way – in this unadulterated dark sky it is a dazzling river of starlight arching across the sky, so brilliant the land seems to be showered with a dew of falling starlight. 

Claudius Ptolemy (c100-c170) captured this intoxication of visual observing with his epigram with which his famous Almagest begins in some translations, “I know that I am mortal by nature, and ephemeral; but when I trace at my pleasure the windings to and fro of the heavenly bodies I no longer touch the earth with my feet: I stand in the presence of Zeus himself and take my fill of ambrosia, food of the gods.”

Ptolemy’s Almagest is the greatest surviving astronomical work from antiquity. Written sometime around 150 AD, its published name was Mathêmatikê Syntaxis, meaning ‘mathematical systematic treatise’. It was preserved, like most of classical Greek science, in Arabic manuscripts, whence came its familiar name – ‘Almagest’ being the Arabic word ‘Al’ (‘the’) with a corruption of the Greek word ‘megistê’ meaning ‘greatest’. – thus Almagest means ‘The Greatest’. The book became known in Europe through the Latin translation from Arabic by Gerard of Cremona in 1175.

From the translation from the Arabic by Gerard of Cremona, 1200. Image credit State Library Victoria.

I have a copy of G.J. Toomer’s English translation of Ptolemy’s classic treatise (Princeton University Press; 1998). To be frank, trying to read it cover-to-cover is very heavy going, but dipping into it is a fascinating foray into Ptolemy’s search for an understanding of the universe. A lot of ink has been shed (fairly and unfairly) over Ptolemy’s work but his description in Book VIII of the Milky Way is not only one of the finest naked eye descriptions, it exemplifies the human understanding of the Milky Way – an irregular band of glowing light of unknown nature – that persisted until Galileo pointed the first telescope at it and wrote, “To whatever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view, of which very many appear rather large and very conspicuous but the multitude of small ones is truly unfathomable.”


Taking a voyage around the Milky Way with Claudius Ptolemy

The Galactic Center shines over the ESO 3.6-metre telescope at La Silla Observatory. Image credit G. Hüdepohl (

It is a superb observing experience to trace Ptolemy’s naked eye description of the Milky Way across the sky…

“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.

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

This 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 leg is a little farther south than the line [bounding] the milk lo the north, and so are the star on the left front knee and the star under the right hind hock. But the star in the left hind lower leg lies in the middle of the milk, and the stars on the hock of the same leg and on the right front hock are to the north of its southern rim, by about 2° (where the great circle is 360°). It is slightly denser in the region near the hind legs.

Next in order, the northern rim of the milk is about 1° from the star on the rump of Lupus, and the southern rim encloses the star on the burning-apparatus of Ara, but just grazes the northernmost of the two stars close together in the brazier and the southernmost of the two stars in the base, while the star in the northern part of the brazier and the one in the middle of the brazier lie right in the milk. These sections are rather less dense.

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

The following section is a little less dense. It extends along [the constellation] Aquila, maintaining about the same width throughout. The star on the tip of the tail of the snake [Serpens] 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 one is 1° from the rear rim, and the northern one, 2° [from it]. The rearmost of the [two] stars in the right shoulder of Aquila touches the same rim, while the more advanced one is cut off inside it, as is also the more advanced, bright star of those in the left wing. Furthermore, the bright star on the place between the shoulders and the two stars 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 arrowhead lies one degree from the eastern rim, while the star on the notch lies two degrees from the western rim. The section round Aquila is slightly denser, and the remainder slightly less dense.

Etienne Leopold Trouvelot
Etienne Leopold Trouvelot’s superb 1881 sketch combines a Milky Way skyscape with the positions of bright stars and an ocean horizon, and is centred on the divided area from Cygnus to Sagittarius

Next the milk extends towards Cygnus. Its north-western rim is defined in a reentrant angle by the star in the southern shoulder of Cygnus, the star under it in the same [southern] wing, and the two stars on the southern leg. Its south-eastern rim is defined by the star in the tip of the southern wing-feathers, and encloses the two stars under the same wing outside the constellation, which are about 2° from it [the rim]. 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 Cygnus 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 tiara 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.

Next the milk encloses the whole of Cassiopeia except for the star in the end of the leg. The southern rim is defined by the star in the head of Cassiopeia, and the northern rim by the star in the foot of the throne and the star in the lower leg of Cassiopeia. The other stars [of Cassiopeia] and all those round about this [constellation] lie in the milk. The areas 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].

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 Perseus, and its southern edge, which is very dense, by the bright star on his right side and by the two rearmost stars of the three to the south of that [bright star]. Enclosed in it also are the nebulous mass on the hilt, the star in the head, the star in the right shoulder and the star on the right elbow. The quadrilateral in the right and also the star on the same [right] calf lie in the midst of the milk, while the star in the right heel is also inside it, a little distance from the southern border.

Next the belt goes through Auriga, displaying a slightly thinner consistency. The star on the left shoulder, called Capella, and the two stars on the right forearm 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] defines the south-western edge. The star over the right foot lies half a degree within the same edge, and the two stars close together on the left forearm, called Haedi, lie in the middle of the belt.

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 3 stars on a straight line under the right foot of Auriga, by the rearmost star of the two in the staff of Orion and by the northernmost [two] of the four stars on his [Orion’s] hand; the brilliant star under the right hand of Auriga and the star in the rear foot of the rear twin are approximately 1° inside the rear edge, while the stars in the other feet lie in the midst of the milk.

Thence 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 ears 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, while the lone star over the head of Canis Major, outside it and at some distance, is about inside the eastern rim. The consistency in this whole region is somewhat thinner.

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 poop. The star in the middle of the little shield, the two stars close together under it, the bright star at the beginning of the deck near the steering-oar and the midmost of the three stars in the keel are just short of touching the same [western] edge. The northernmost of the three stars in the mast-holder  defines the eastern rim, while the bright star in the stern-ornament is 1° within the same [eastern] edge, and the bright star under the rearmost little shield in the deck is the same amount, outside the same [eastern] edge. The southernmost of the two brilliant stars in the middle of the mast touches the same edge, and the two bright stars at the point where the keel is cut off are about 2° 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.

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 body, but leaves the rearmost star of the three in the body 1° outside its western rim. The star in the fourth joint lies in the open space between the two belts, about the same distance from each, a little more than I°.

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 Ophiuchus, and the rear edge by the star on the same [right] shin, while the most advanced of the stars at the end of the same [right] leg touches that same [rear] edge. Subsequently the western rim isdefined by the star under the right elbow of Ophiuchus, and the eastern rim by the more advanced of the two stars in the same [right] hand.

From that point on there is a considerable gap of open space, in which lie the two stars on the tail of Serpens next to the star in the tip [of the tail]. The whole of the section of this belt which we have [just] finished describing consists of an extremely line and almost aery substance, except for the area enclosing the three joints [of Scorpius], which is somewhat more concentrated.

After the gap the milk again makes a fresh beginning at the four stars to the rear of the right shoulder of Ophiuchus. The eastern rim of this belt is defined (being just grazed) by the lone brilliant star under the tail of Aquila, while the opposite rim is defined by the star which is some distance to the north of the four just mentioned. 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 in the beak of Cygnus, 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 Cygnus, is wider and considerably denser. The star in the neck of Cygnus 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 wing and the two stars close together in the right foot. From this point, as we said, occurs a clear gap to the other belt, [a gap] stretching from the above-mentioned stars in Cygnus up to the bright star in the rump.”

What a beautiful description to guide one around the Milky Way.

Copyright © Susan Young 2018