constantly vs periodically

constantly

adv
  • Recurring regularly. 

  • In an unchangeable or invariable manner; in every case. 

  • In a constant manner; occurring continuously; persistently. 

periodically

adv
  • intermittently or recurrently 

  • In a manner that is regular or periodic. 

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