lengthen vs move ahead

lengthen

verb
  • To become longer. 

  • To make longer, to extend the length of. 

move ahead

verb
  • to proceed; to make or signify progress 

  • to continue without being distracted; to keep going 

  • to go on to something else 

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