"In the most high and palmy state of Rome, / A little ere the mightiest Julius fell, / The graves stood tenantless and the sheeted dead / Did squeak and gibber in the Roman streets / As stars with trains of fire and dews of blood, / Disasters in the sun, and the moist star / Upon whose influence Neptune’s empire stands / Was sick almost to doomsday with eclipse." (Act I, scene i).
After seeing the ghost of King Hamlet, and relating the battle between the king and Fortinbras, Horatio compares this event to the killing of Julius Caesar. Surrounding the assassination of Caesar were many supernatural and unexplainable occurrences, including seeing ghosts. Since they are seeing the ghost of Hamlet, Horatio is speculating that perhaps this is an omen prefacing turmoil within the nation of Denmark.