What unnatural happenings does Horatio equate with the ghost's appearance?
During the first scene of Shakespeare's Hamlet, Horatio compares the spectral appearance of the ghost with the supernatural occurrences following Caesar's death: he references the reanimation of "the sheeted dead" (115), "stars with trains of fire" (117) and heavens that were "sick almost to doomsday with eclipse" (120).
This reference is significant, as Caesar's death historically marked the beginning of a civil war and, ultimately, the end of the Roman Republic (a representative government) and the Roman Empire (a state ruled by a single man). As such, these apocalyptic signs would have heralded a period of cataclysmic social change, one which must have seemed like the end of the world to its witnesses. Thus, Horatio's references display a brilliant use of foreshadowing; with the sighting of the ghost, Horatio suggests to the audience that they're in for a dark and dramatic spectacle no less significant than the collapse of the Roman Republic.
Horatio compares the ghost's appearance to that of the great days of the Roman empire when Julius Caesar was killed. Horatio recounts that the graves were emptied and the dead walked the streets of Rome shrieking. There was a meteor shower, described as stars with fiery tails, and there were signs threatening the life of the sun and the moon was almost eclipsed, which was also equated with the end of the world. He believes this ghost is just the beginning of these things that will happen to them and that perhaps this ghost is just the pretense to another battle between Hamlet and young Fortinbras, perhaps even the end of the world, and all these "unnatural" happenings.