noun (n)

• a person who becomes aware (of things or events) through the senses(noun.person)

  Synonym:
  beholder, perceiver, percipient
  

  source: wordnet30
• an expert who observes and comments on something(noun.person)

  Synonym:
  commentator
  

  source: wordnet30
• One who observes, or pays attention to, anything; especially, one engaged in, or trained to habits of, close and exact observation; as, an astronomical observer.(noun)

  source: webster1913