ball lightning vs meteoroid

ball lightning

noun
  • A short-lived glowing ball sometimes observed to float in the air and thought to consist of ionized gas associated with thunderstorms. 

meteoroid

noun
  • A relatively small (sand- to boulder-sized) fragment of debris in a star system that produces a meteor when it hits the atmosphere 

How often have the words ball lightning and meteoroid occurred in a corpus of books? (source: Google Ngram Viewer )