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.

These show the irregularity of the spheroid DEFGH.

• Towards the poles, its shape is flatter

This makes the tails of Comets appear straighter and wider.

• In the bend between the poles and the ecliptic, this makes it appear more curved
• Deviating from the opposite Sun and according to the length of this bend, it appears 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. The fixed stars emit light from themselves.

This would cause their coma, if there is any, to scattered and be very brief.

Stars do not appear circumscribed by a uniform line. Instead, they are surrounded by wandering rays from all sides.

This also causes their twinkling.

Jupiter and Saturn have very clear atmospheres.

• This causes short comae sometimes to appear from around their dark sides.

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 been seen as comets by the Egyptians sometimes.

He says he saw a comet from one of the stars which are in the thigh of the Dog. This would have been:

• a very oblique refraction in the air, or
• a defect of his eyes

This is because he says that it was less conspicuous when he was focusing his eyes on it than when he was looking away.