mean solar day vs sunlight

mean solar day

noun
  • the average length of a solar day, used for practical timekeeping 

sunlight

noun
  • All the electromagnetic radiation given off by the Sun, especially that in the visible spectrum that bathes the Earth. 

  • Brightness, hope; a positive outlook. 

  • Synonym of sunrise. 

verb
  • To work on the side (at a secondary job) during the daytime. 

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