Albert Barnes Commentary Jeremiah 10:2

"thus saith Jehovah, Learn not the way of the nations, and be not dismayed at the signs of heaven; for the nations are dismayed at them." — Jeremiah 10:2 (ASV)

Signs of heaven - Extraordinary appearances, such as eclipses, comets, and the like, which seemed to the pagan to portend national calamities. To attribute importance to them is to walk in pagan ways.