The Twilight Zone

A few days ago I wrote about how the sun is setting earlier and earlier this month, and how we are losing so much sunlight in the month of August (See here). In that article, I also alluded to the fact that on the plus side, there would be more hours of darkness in which to observe. But how to astronomers measure the darkness? When does it get truly dark after the sun goes down?

We measure The Twilight Zone.