Μετάφραση - Τουρκικά-Αγγλικά - kuyruklu yıldızlar vardır; yetmiÅŸ yılda bir...

Hi rollingmaster,

I'd use "Earth" instead of "world"

"Either one sees once in a lifetime or not..." is a bit clumsy.
What about:
"You see one only once in a lifetime or you don't..."

What do you think?

Hi lilian,

I'm agreeable.

You know comets...> There are (some) comets

Hi lilian,

I would translate like at merdogan's suggestion but I thought that "you know comets" is more meaningful in English.

If merdogan's suggestion is appropriate, I agree with him.