Is the follow quote a 1) metaphor or simile or an examples 2) synechdoche or metonymy
"Sun seems to have set" (on an idea)
"a figure of speech that replaces the name of a thing with the name of something else with which it is closely associated. We can come across examples of metonymy both from literature and in everyday life." (Source)
So I thought the sun replaces a day (sun and daytime are closely related), and so it is an example of metonymy.
Yet my book says its a metaphor. I can somewhat see it is metaphorical in that it replaces the time span of an idea with the time span of a day, but thought metonymy is a better fit.
Context of quote
The greatest recent event – that ‘God is dead’, that the belief in the Christian god has become unbelievable – is already beginning to cast its first shadows over Europe.For the few at least, whose eyes – the suspicion in whose eyes is strong and subtle enough for this spectacle, some sun seems to have set and some ancient and profound trust has been turned into doubt; to them our old world must appear daily more like evening, more mistrustful, stranger, ‘older’. But in the main one may say: The event itself is far too great, too remote from the multitude’s capacity for comprehension even for the tidings of it to be thought of having arrived as yet. Much less may one suppose that many people know as yet what this event really means – and how much must collapse now that this faith has been undermined because it was built upon this faith, propped up by it, grown in it; for example, the whole of our European morality. This long plenitude and sequence of breakdown, destruction, ruin, and cataclysm that is now impending – who could guess enough of it today to be compelled to play the teacher and advance proclaimer of this monstrous logic of terror, the prophet of a gloom and an eclipse of the sun whose like has probably never yet occurred on earth? - Friedrich Nietzsche