What if we adhere to the literary translation? Perhaps it would spill some beans ...

сейчас = сей+час = this [very] moment
теперь = now

So, being more generic, "now" includes "this moment", but not the other way around. However, they are both interchangeable when "now" is used in a more narrow sense of "this moment".

What do you think of that? Does it make sense?