Why The Tails Of Comets Are Not Straight

137. How beams also appear

If the eye is towards Point S, it will be prevented by the rays of the Sun from seeing the Comet itself.

It will see only a part of its coma, like a fiery beam.

This will appear either in the evening or in the morning, according to whether the eye is closer to Point 4 or Point 2.

If the eye is in the middle Point 5, the comet might appear one in the morning and the other in the evening.

138. Why the tails of Comets, not always directly opposite the Sun, nor always straight, appear

This coma or tail is sometimes:

• straight
• curved
• in a straight line, which passes through the centers of the Comet and the Sun
• deviating somewhat from it
• wider, narrower, or even brighter

These depend on the lateral rays converging towards the eye.

For all these things follow from the irregularity of the spheroid DEFGH: for towards the poles, where its shape is flatter, the tails of Comets must appear more straight and wide; in the bend between the poles and the ecliptic, more curved, and deviating from the opposite Sun; and according to the length of this bend, brighter, and narrower.

All the causes explained here have been observed so far about Comets in reality.

139. Why do such comae not appear around Fixed Stars or around the higher planets Jupiter and Saturn?

This is because:

1. They are not usually seen in Comets.

This is because their apparent diameter is not greater than that of fixed stars, because then these secondary rays do not have enough strength to move the eyes.

2. As for the fixed stars, because since they do not borrow light from the Sun, but emit it from themselves, their coma, if there is any, must be scattered here and there, and must be very brief.

Indeed it seems to be such around them: for they do not appear circumscribed by a uniform line, but surrounded by wandering rays from all sides; and perhaps not incorrectly we will also refer their twinkling (of which, however, there may be several other causes) to this.

But as for Jupiter and Saturn, where the air is very clear, sometimes even short comae, extending towards the part away from the Sun may be seen around them.

I have read something like this somewhere, though I do not recall the author.

Aristotle says in his Meteorology, book 1, chapter 6 on the fixed stars, that they have also been seen as comets by the Egyptians sometimes.

But what he reports about a comet, which was seen by him from one of the stars which are in the thigh of the Dog, either by a very oblique refraction in the air, or rather by a defect of his eyes, for he adds that it was less conspicuous when he was focusing his eyes on it than when he was looking away.