equatorial vs tropic

equatorial

adj
  • of, near, or relating to the equator 

  • perpendicular to the plane of a ring 

noun
  • A kind of telescope mounted so as to have two axes of motion at right angles to each other, one of them parallel to the axis of the Earth, and each carrying a graduated circle, one for measuring declination, and the other right ascension, or the hour angle, so that the telescope may be directed, even in the daytime, to any star or other object whose right ascension and declination are known. 

tropic

adj
  • Of, or relating to the tropics; tropical. 

  • Having the quality of indirectly inducing a biological or chemical change in a system or substrate. 

  • Pertaining to, involving, or of the nature of a trope or tropes. 

  • Hot and humid. 

noun
  • Either of the two parallels of latitude 23°27′ north and south of the equator; the farthest points at which the sun can be directly overhead; the boundaries of the torrid zone or tropics. 

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