Reading Gaiman

I always wondered how they noticed comets,
after all, the sky is filled with stars and planets,
and I've never quite been able to tell the two apart,
see which was which.
And why did comets scare them so? ( this line, i can't quite get)

Stars fall,
Which is much worse,
But that does not make us fear that wars will come,
and fires and plagues will come,
established things be overturned
and new things come -- and new things are never comfortable things.
So I walk down to the woods, and stare up at the night.
So many stars.
But only one comet, obvious, and perfect and precise, its tail a ghost and white against the night. On seeing it, I understand at last.
And shiver, for the change that's always coming. - Excerpts from Gaiman's Nightfall