The sun sets when you see it set - just because you are looking at where it was, not where it is doesn't make any difference.

It is also up only when you see it - before that it is dark.

I remember an episode of Dr Who where the baddie sent a signal to set off an explosion from a long way off - so they travelled in the TARDIS in an instant, and cut the wires, or whatever, before the signal arrived - possibly from Mars.

Simple.