Signs

Jeremiah 10:1-2 - Hear what the LORD says to you, O house of Israel. This is what the LORD says:
"Do not learn the ways of the nations
or be terrified by signs in the sky,
though the nations are terrified by them.

I wonder what signs in the sky terrified the people. Consider the various phenomena that could be called signs:

• clouds
• storms
• thunder
• lightning
• eclipses
• comets
• moon phases
• auroras
• meteors
• planets

Some (all?) of the people recognized signs in the sky. God himself says there are signs -- it's just that the people shouldn't be afraid of them. I wonder what they are signs of. Why would people be afraid after seeing them?