Why The Tails Of Comets Are Not Straight

138. Why the tails of comets are sometimes a little curved, not always straight, nor directly opposite the Sun.

The tail or hair of comets does not always appear completely straight [going from the comet to the sun in a line].

Instead, it:

• deviates or bends from it somewhat.
• does not always appear equally broad
  • Sometimes it appears narrower
  • Sometimes it appears brighter when the rays that come from its sides converge toward the eye

All of these are caused by the the sphere DEFGH not being regular.

Its shape is flatter towards the poles [minor axis of ellipse] than elsewhere.

• This is why the tails of the Comets are straighter and broader at the poles.

But when the tails extend obliquely across between the poles and the ecliptic, [major axis of ellipse] they are curved and bend a little from the line that passes through the centers of the Sun and the comet.

Finally, when they extend farther in those regions, they are brighter and narrower.

139. Why the Fixed Stars and Planets such as Jupiter and Saturn Do Not have with Such Tails.

This is because:

1. The planets are smaller than some comets from our perspective.

Small comets do not show a tail when when their apparent diameter is no greater than that of the Fixed Stars.

Their small size prevents the reflection of more light.

2. The stars do not reflect light.

If any tail appeared around them, it should be equally scattered on all sides.

Consequently, it would be also very short, just like the Comets named Roses.

But we do see such a tail around them.

Their shape is not bounded by any uniform line.

They are seen surrounded by rays on all sides.

Perhaps this is also why their light twinkles, although other reasons can also be given for this.

Jupiter and Saturn sometimes appear with such a tail in countries where the air is very clear and pure.

I remember reading somewhere that this was observed in the past although I do not recall the author.

Aristotle says in the book 1 of Meteorology, chapter 6, that the Egyptians sometimes saw such tails around the Stars.

I believe these are Planets rather than the Fixed Stars.

He says he saw a coma around one of the Stars in the thigh of the Dog (Sirius).

This must have happened due to some extraordinary refraction that took place in the air, or rather due to some indisposition in his own eyes.

For he adds that this tail appeared less clearly the more fixedly he gazed on it.