explain vs illuminate

explain

verb
  • To make something plain or intelligible. 

  • To make plain, manifest, or intelligible; to clear of obscurity; to illustrate the meaning of. 

  • To give a valid excuse for past behavior. 

illuminate

verb
  • To clarify or make something understandable. 

  • To be exposed to light. 

  • To shine light on something. 

  • To glow; to light up. 

  • To decorate the page of a manuscript book with ornamental designs. 

  • To decorate something with lights. 

  • To make spectacular. 

  • To direct a radar beam toward. 

noun
  • Someone thought to have an unusual degree of enlightenment. 

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