month vs sidereal day

month

noun
  • A period of 30 days, 31 days, or some alternation thereof. 

  • A period into which a year is divided, historically based on the phases of the moon. 

sidereal day

noun
  • One day of sidereal time; the time in which Earth rotates once around its axis, relative to the point of the vernal equinox (about 23 hours, 56 minutes, 4 seconds). 

  • The rotational period of Earth; the time in which Earth rotates around its axis 360 degrees. 

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