Help with the different twilight definitions
 

It is either somewhere in the cobwebs of the mind or in one of the reference books groaning the office shelves, but could someone please remind me of not only the definition/difference among Astronomical twilight, Nautical twilight, and Civil twilight...but also, and equally important, why we have those distinctions?

I was brought up short when a non-pilot friend asked me about these and I had to admit that I could not remember. Yes, I know tucking into Wikipedia would be a good first cut at it but I also know that tech forum members will undoubtedly bring some keen insight (ha ha) to the answer.

This is what I learned back in the day.

The Different Types of Twilight, Dawn and Dusk

6, 12 and 18 degrees below the horizon.

Civil is darkish you can get about with just enough light to not run into big things

Nautical you have enough stars to navgate by and it's dark on ground.

Astronomical is fully dark sky..

Why? Because it's the way it is?

.. what keeps my interest steadily in PPRuNe is that set of interesting little snippets that arise so regularly .. all rather amazing.

Good info--thank you.

compressor stall,
Nautical twilight allows the marine navigator to see both the stars he wants to shoot and the horizon at the same time (necessary when using a marine sextant). At and before civil twilight, you can see the horizon but not the stars you might need. After astronomical twilight, the horizon is no longer visible.