constancy vs fidelity

constancy

noun
  • The quality of being constant; steadiness or faithfulness in action, affections, purpose, etc. 

  • An unchanging quality or characteristic of a person or thing. 

fidelity

noun
  • Faithfulness to one's duties. 

  • Accuracy, or exact correspondence to some given quality or fact. 

  • Loyalty to one's spouse or partner, including abstention from cheating or extramarital affairs. 

  • The degree to which a system accurately reproduces an input. 

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