declination vs declivity

declination

noun
  • At a given point, the angle between the line connecting this point with the geographical center of the earth and the equatorial plane. 

  • At a given point, the angle between magnetic north and true north. 

  • A refusal. 

declivity

noun
  • The downward slope of a hill; the downward slope of a curve. 

  • A downward bend in a path. 

  • An inward curve of the exoskeleton of an insect, such as between body segments; a segment of an insect's body where the exoskeleton curves inward. 

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