What unnatural happenings does Horatio equate with the ghost's appearance?

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.