Lecture 12 — Opinions of others on the Milky Way.

Let us now explain the origin, cause, and nature of the milky way. And here too let us begin by discussing the statements of others on the subject.

Of the so-called Pythagoreans some say that this is the path of one of the stars that fell from heaven at the time of Phaethon's downfall. Others say that the sun used once to move in this circle and that this region was scorched or met with some other affection of this kind, because of the sun and its motion.

But it is absurd not to see that if this were the reason the circle of the Zodiac ought to be affected in the same way, and indeed more so than that of the milky way, since not the sun only but all the planets move in it. We can see the whole of this circle (half of it being visible at any time of the night), but it shows no signs of any such affection except where a part of it touches the circle of the milky way.

Anaxagoras, Democritus, and their schools say that the milky way is the light of certain stars. For, they say, when the sun passes below the earth some of the stars are hidden from it. Now the light of those on which the sun shines is invisible, being obscured by the of the sun. But the milky way is the peculiar light of those stars which are shaded by the earth from the sun's rays.

This, too, is obviously impossible. The milky way is always unchanged and among the same constellations (for it is clearly a greatest circle), whereas, since the sun does not remain in the same place, what is hidden from it differs at different times. Consequently with the change of the sun's position the milky way ought to change its position too: but we find that this does not happen.

Besides, if astronomical demonstrations are correct and the size of the sun is greater than that of the earth and the distance of the stars from the earth many times greater than that of the sun (just as the sun is further from the earth than the moon), then the cone made by the rays of the sun would terminate at no great distance from the earth, and the shadow of the earth (what we call night) would not reach the stars. On the contrary, the sun shines on all the stars and the earth screens none of them.

There is a third theory about the milky way. Some say that it is a reflection of our sight to the sun, just as they say that the comet is.

But this too is impossible. For if the eye and the mirror and the whole of the object were severally at rest, then the same part of the image would appear at the same point in the mirror. But if the mirror and the object move, keeping the same distance from the eye which is at rest, but at different rates of speed and so not always at the same interval from one another, then it is impossible for the same image always to appear in the same part of the mirror. Now the constellations included in the circle of the milky way move; and so does the sun, the object to which our sight is reflected; but we stand still. And the distance of those two from us is constant and uniform, but their distance from one another varies. For the Dolphin sometimes rises at midnight, sometimes in the morning. But in each case the same parts of the milky way are found near it. But if it were a reflection and not a genuine affection of these this ought not to be the case.

Again, we can see the milky way reflected at night in water and similar mirrors. But under these circumstances it is impossible for our sight to be reflected to the sun.

These considerations show that the milky way is not the path of one of the planets, nor the light of imperceptible stars, nor a reflection. And those are the chief theories handed down by others hitherto.

77. After determining concerning falling stars and comets, the Philosopher here determines concerning the milky circle.

First, he states his intention and says that we must talk now of the milky circle: the how and why of its appearance and what that milky brightness is. But we shall adhere to that order whereby we first discuss what others have said.

second at of the so-called (345a13), he executes his proposition.

First he presents the opinions of others;

second, his own opinion, at let us recall (345b31).

The first is divided into three parts according to the three opinions he cited;

the second opinion begins at Anaxagoras, Democritus, and their schools (345a25);

the third at **there is a third ** (345b9).

78. Regarding the first he does two things.

First, he presents the opinion at and says that some of the philosophers called "Pythagoreans" held that the milky circle is a certain path. But their opinions differed: for some asserted that it was the path of a certain star that passed through this part of the heaven after abandoning its own course when the heavens once went off the path—an event which the fables declare happened under Phaethon. Others say that the sun once travelled that path. Consequently, by the motion of the sun or of a star that area of the heaven was, so to speak, scorched, or affected in such a way that a patch of whiteness now appears there.

79. second at but it is absurd (345a18), he rejects this opinion and says that it is an inconsistency for those who posited this opinion that they did not at the same time see that, if the journey of the sun or a star were the cause of this brightness in this region of the heaven, such a disposition has much more reason for appearing in the circle of the Zodiac than in the milky circle—for not only the sun, but all the wandering stars move through the Zodiac. Now the whole zodiacal circle is visible to us, at different times, because at night half of it always appears above the earth (for the earth has the status of a point compared to the sphere of the fixed stars: hence earth's largeness does not conceal anything in the Zodiac from us); but although the entire Zodiac is visible to us, no such disposition appears in it, save in the region where it is joined with the milky circle.

80. Then when he says, Anaxagoras, Democritus, and their schools (345a25), he presents the second opinion.

First, he recites it and says that the followers of Anaxagoras and Democritus declared that the milky brightness which appears in the heaven is the light of certain stars. For when the sun is borne below the earth, they said, the earth's shadow reaches as far as the sphere of the fixed stars and covers some of them so that they do not receive the sun's rays; yet it does not cover all of them, because, due to the smallness of the earth, its shadow does not cover the entire heaven but only a small area. For they said that the brightness of the stars faced by the sun does not appear, because it is prevented from appearing by the sun's rays reaching them; consequently, no milky brightness surrounds them. But of the stars which the sun's rays do not reach, because the earth prevents this, their own light appears, which they identify as the milky brightness.

81. second at this, too, is obviously (345a33), he rejects this opinion for two reasons.

In presenting the first of them he says that what it claims is impossible. For the milky brightness always appears in the same stars—the milky circle being seen to be one of the largest circles, which divide the sphere in half. But because the sun does not always remain in the same place in the heaven, other and other stars must always be being obscured from the sun's rays by the earth's shadow—since the shadow's motion must be imagined as opposite to the sun's course. Therefore, if the cause of the milky brightness' visibility were the earth's shadow blotting out the stars, then as the sun moved, the milky brightness would also have to shift. But this is not seen to happen, because it always appears in the same place and in the same stars, as was said. Consequently, the aforesaid theory is false.

82. He gives the second reason at besides, if astronomical demonstrations are correct (345a36), and says that astronomical arguments and considerations prove that the sun is larger than earth and that the fixed stars are much farther from the earth than the sun is, just as the sun is farther from us than the moon is. Now when a shining body is larger than the opaque body whose interposition produces a shadow, the shadow does not increase indefinitely, rather it ascends in pyramid form to a cone of a certain quantity which is proportionately less as the shining body is less distant from the opaque one, and the more the former's size exceeds the latter's. Hence it is plain that the cone of the earth's shadow is not projected very far with respect to the rays coming from the sun; neither does the earth's shadow, which we call "night," reach as far as the fixed stars. What has to happen is that the sun has a view of all the fixed stars and that the earth blankets none of them. The reason why it blankets the moon and eclipses it is that the moon is lower than the sun, as has been said. Consequently, it is plain that the theory under discussion presupposed something false.

83. He presents the third opinion at there is a third (345b9).

And first he recites it, saying that there is a certain third theory about the milky circle. For some said that the milky brightness is due to our vision's being reflected from certain stars to the sun; as a result a brightness appeared around those stars striking our vision, so that they act as a certain mirror for the sun's brilliance, as Hippocrates also said when explaining the appearance of comets.

84. second at **but this too is impossible **(345b12), he rejects this theory for two reasons.

He sets down the first of these and says that the above opinion posits something impossible. And he lays down this proposition: if everything, i.e., the entire system, namely, the beholder, the mirror, and the object seen by means of the mirror, all remain at rest, then of necessity the same part of the emphaseos, i.e., of the form appearing, will appear on the same sign of the mirror, i.e., at the same point at which the reflection of the visual line took place. But if the mirror should move, and likewise the object seen by means of the mirror, while the beholder remains at rest, and if the two things in motion always remain at the same distance from the beholder, but with respect to one another are neither moving with equal speeds nor always at the same distance, then under these conditions it is impossible for the same appearance to occur in the same part of the mirror. For in practice it makes no difference whether the mirror and the thing seen are moving with unequal speeds, or one is in motion and the other at rest; and if this latter were the case, it is plain that the figure of the thing seen would appear now in one spot and now in another spot of the mirror, on account of the varying opposition according to position.

And I say this, if the beholder is at rest: for if the beholder were to move, and the mirror remain at rest, while the object moved, then the figure of the thing seen could appear in the same spot on the mirror—since the beholder's motion could compensate for what was lacking due to the motion of the object seen, if they were thus proportionately moved. Hence, when the beholder remains at rest but the mirror and the thing seen are moved at unequal speeds, it is necessary that the figure not appear at the same spot on the mirror.

But the stars in the milky circle that are assumed to be a mirror, are in motion; moreover, there is motion of the sun, to which a reflection of our vision is assumed to take place (the sun therefore plays the role of the thing seen); but we, the beholders, are at rest, because the earth is at rest (for the motion involved by our moving about on the earth makes no perceptible difference with respect to so great a size. Furthermore, the stars in question and the sun are equally moved in relation to us and their distance from us always remains the same. This does not mean that our distance from the sun is equal to our distance from the stars (for it has been previously said that the stars are higher than the sun), but that the sun, by its motion, does not increase or decrease its distance from us. And the same is true of a star. And by a "greater or less distance" is to be understood a distance that is significant with respect to the distance between the sun and the stars: we say this because of the smallness of the earth. But in relation to one another, the sun and the stars are not always equidistant: because Delphis, i.e., the constellation of the Dolphin, which is in the milky circle, rises sometimes in the middle of the night and sometimes at dawn; and it is plain that this constellation is farther from the sun when it appears at night than when it appears at dawn. But the parts of the milky circle remain forever in the same place—which should not be the case if it were an appearance caused by a reflexion; for this brightness would not exist in the same place, as has been shown. Hence it is plain that the aforesaid theory is false.

85. He gives the second reason at again, we can see the milky way reflected at night (345b25), and says that at night the form of the milky circle appears in water and other such mirror-like bodies. But it is inadmissible to say that in these cases vision is reflected by the water to the sun—in other words, considering the distances involved, it seems most unacceptable that there should be two reflections: one, namely, from the water to the milky circle, and another from the milky circle to the sun.

Finally, in summary he concludes that the milky circle is neither the path of any of the planets, as the first opinion held; nor the light of certain stars not regarded by the sun, as the second opinion said; nor the reverberation of our vision from the stars to the sun, as the third opinion maintained. And before his time these were the prevailing opinions about the "galaxy" [Milky Way].