days vs solar day

days

noun
  • A particular time or period of vague extent. 

  • Life. 

adv
  • During the day. 

solar day

noun
  • one day of solar time, the time between successive noons; the time in which a planet such as Earth apparently rotates once around its axis, relative to the sun (on Earth about 24 hours as an average - the length of solar day is not constant throughout the year) 

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