speculator
Speculator (noun) means a person who watches and thinks deeply about something; an observer.
Etymology
From Latin speculātor (“spy, explorer, investigator”), from speculor (“to watch, to observe”) + -tor (“-er: forming agent nouns”), from specula (“watchtower”), from speciō (“to watch, to observe”), q.v. In some senses, an agent noun formed within English from speculate. Doublet of spectator.