observance vs observatory

observance

noun
  • That which is to be observed. 

  • The practice of complying with a law, custom, command or rule. 

  • Observation or the act of watching. 

  • A rule governing a religious order, especially in the Roman Catholic church. 

  • The custom of celebrating a holiday or similar occasion. 

  • Reverence; homage. 

observatory

noun
  • A place where stars, planets and other celestial bodies are observed, usually through a telescope; also place for observing meteorological or other natural phenomena. 

  • A lookout (vantage point with a view of the surrounding area) 

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