RALPH: According to the ghost, an accurate description of Purgatory would frighten Hamlet's soul, turn his blood cold, make his hair stick up straight like a porcupine's quills...

SARAH: ...And make his eyes pop out from their sockets — as the ghost puts it, "start from their spheres". To start means to jump in fright, like a startled horse.

RALPH: Notice the metaphor of stars coming out of their spheres. In Europe before Galileo and Newton, stars were commonly believed to be fixed on a gigantic sphere, which rotated around the earth.

SARAH: The ghost is therefore implying that to tell Hamlet details about Purgatory would not only be terrifying, but also against the natural order, as if stars were to fall out of their orbit.