solar apex vs zenith

solar apex

noun
  • The point to which the Sun appears to be moving with respect to the local stars 

zenith

noun
  • The highest point in the sky reached by a celestial body. 

  • The point in the sky vertically above a given position or observer; the point in the celestial sphere opposite the nadir. 

  • Highest point or state; peak. 

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