covertness vs occultation

covertness

noun
  • The characteristic of being covert. 

occultation

noun
  • The state of being occult (“hidden, undetected”). 

  • An astronomical event that occurs when one celestial object is hidden by another celestial object that passes between it and the observer when the nearer object appears larger and completely hides the more distant object. 

  • The disappearance of the Twelfth Imam, or Mahdi, who is believed alive and present in this world, but hidden until his reappearance at the end of time. 

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