hour vs solar day

hour

noun
  • A time period of sixty minutes; one twenty-fourth of a day. 

  • A season, moment, or time. 

  • The time. 

  • Used after a two-digit hour and a two-digit minute to indicate time. 

  • The set times of prayer, the canonical hours, the offices or services prescribed for these, or a book containing them. 

  • A distance that can be traveled in one hour. 

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 hour and solar day occurred in a corpus of books? (source: Google Ngram Viewer )