Do you simply want to turn on artificial lighting when the natural daylight fails?

If so, as others have already said, you just need a simple photocell - no calculations or digital processing are required.

In this coutry, the natural light level can often be low enough to require artificial lighting long before actual sunset!

Or do you specifically need to know the actual time of sunset - for religious reasons, or something like our "ligthing-up time"?

("ligthing-up time" is the law when drivers have to turn-on car lights on the road, and is related to sunrise/sunset)

Or is it really 2 separate questions:

1. How to turn lights on when it gets dark;

2. How to calculate times of sunset/sunrise (and, hence, the Islamic "AZAN" times)?