Lecture 9 — The opinions of others concerning comets.

Let us go on to explain the nature of comets and the 'milky way', after a preliminary discussion of the views of others.

Anaxagoras and Democritus declare that comets are a conjunction of the planets approaching one another and so appearing to touch one another.

Some of the Italians called Pythagoreans say that the comet is one of the planets, but that it appears at great intervals of time and only rises a little above the horizon. This is the case with Mercury too; because it only rises a little above the horizon it often fails to be seen and consequently appears at great intervals of time.

A view like theirs was also expressed by Hippocrates of Chios and his pupil Aeschylus. Only they say that the tail does not belong to the comet iself, but is occasionally assumed by it on its course in certain situations, when our sight is reflected to the sun from the moisture attracted by the comet. It appears at greater intervals than the other stars because it is slowest to get clear of the sun and has been left behind by the sun to the extent of the whole of its circle before it reappears at the same point. It gets clear of the sun both towards the north and towards the south. In the space between the tropics it does not draw water to itself because that region is dried up by the sun on its course. When it moves towards the south it has no lack of the necessary moisture, but because the segment of its circle which is above the horizon is small, and that below it many times as large, it is impossible for the sun to be reflected to our sight, either when it approaches the southern tropic, or at the summer solstice. Hence in these regions it does not develop a tail at all. But when it is visible in the north it assumes a tail because the arc above the horizon is large and that below it small. For under these circumstances there is nothing to prevent our vision from being reflected to the sun.

53. After determining concerning falling stars and the like, the Philosopher now determines about comets.

First, he states his intention and says that we must speak now of comets and the milky circle, observing with respect to each the following order:

first, we shall present the ***doubts (***342b25), i.e., the objections to what others have said,

and then we shall state what we think.

second at Anaxagoras (342b27), he pursues his proposition in the order stated.

First, therefore, he presents what others thought of comets;

second, he determines about them according to his own opinion, at we consider a satisfactory explanation (344a5).

The first is divided into two parts:

in the first he presents the opinions;

in the second he disproves them, at these views (343a20).

The first is divided into three parts according to the three opinions he presents.

54. First therefore, he gives the opinion of Anaxagoras and Democritus who said that comets are symphases (342b27), i.e., co-appearances of [planets] the wandering stars. These are five in number (namely, Saturn, Jupiter, Mars, Venus and Mercury), some of which, as they approach one another seem to touch, and there seems to be one star, and "flowing hair" appears [the coma, hence "comet"], due to the increase of light.

55. He presents the second opinion at some of the Italians (342b29), which was that of certain Pythagoreans living in Italy who said that a comet is one of the wandering stars [planets], but that the phantasy, i.e., vision, of it occurs only after a long lapse of time, because it exceeds, i.e., departs from the sun only slightly—as is the case with the star Mercury, which, because it only slightly digresses from, i.e., moves away from, the sun, frequently does not appear, appearing only after a long time, having for a long time not appeared.

56. He presents the third opinion at** a view like theirs** (342b35), which was that of certain followers of Hippocrates, and of Aeschylus, his disciple. This opinion is similar to the second in supposing that a comet is one of the wandering stars; it differs in that the second opinion held the wandering star had a tail [coma] of itself, whereas this third opinion holds that it does not have a tail of itself, but, since it is wandering, sometimes acquires a tail by its position. According to this opinion a certain moisture is attracted by the star, and, since they assume that vision occurs by a beholder emitting visual rays, they posited that a visual ray reaches that moisture attracted by the star and is then reflected toward the sun. In this way the attracted vapor acts as a certain fiery mirror for the sun (for things are visible in a mirror because of reflection); and they say that it is thus that the tail is formed.

57. Then he assigns the cause regulating the time of its appearance and says that a comet star appears at greater intervals than the other stars, i.e., is rendered invisible longer than the other stars, because time-wise it is very slow in getting clear of the sun, i.e., only when it has completed its entire cycle. He calls this being left behind: for the wandering stars are said to be left behind with respect to the first motion, either because they are moved in a contrary direction and thus seem to retreat by their own motion, or because, as some say, they are moved more slowly than the first heaven which in its diurnal motion revolves everything else. So they said that a comet star is left behind by the sun to the extent of the whole of its orbit; and therefore, when it returns to the point where it first began to recede, it appears once more and remains in view until it again gets in conjunction with the sun. They also said that this star moves away from the sun not only according to longitude, but also according to latitude, declining to the north and south winds, i.e., to the north and to the south.

58. He also assigns the cause regarding the place of this star's appearance. And he says that this star does not appear in between the two tropics, namely, of Cancer and Capricorn; for the sun travels through that portion of the heaven and consumes the moisture, so that in that portion of the heaven this star cannot attract any water. But when it shifts to the south, receding from the sun's course, it finds there an abundance of moisture, because it had not been consumed by the sun. But because of the obliquity of the horizon, for us who live in the north the part of its parallel circle which is above the earth is small, whereas the part below is larger; consequently, the sun which, at night, when comets are visible, is under the earth, is so far from the moisture attracted by the star that a man's vision cannot be reflected from the moisture to the sun—whether the sun is near the tropic, namely, that of Capricorn, or whether it is in the summer turnings, i.e., in the summer tropic, which is that of Cancer. For no matter where the sun is under the earth, its distance from the contracted vapor is too great for reflection, either from the circle or from the latitude of the zodiac. But when that star is left behind by the sun toward the boreal [north wind], i.e., the north, then it can acquire a tail—because there is there much moisture, and the circumference of its circle above the horizon is large there, whereas the part below is small. Consequently it is easy for man's reflected vision to reach the sun.