OK, I'm a geek, but... (civil twilight)

[flyingcheesehead]

Many of you know that I like facts, figures, calculations, and even interpreting regulations...

So I decided to try, for once, to accurately log some night flight time. Perfectly by the book(s)... And I think I'm going to give up and go back to the "eh, that seems about right" method.

For the purpose of this calculation, the flight was on 3/30/2018, departing KUES at 1905 local/0005Z (sunset: 19:14) and arriving KSPW at 21:19 local/0219Z (sunset: 19:43). Doing the figuring on a flight that departed before sunset but arrived well after any definition of "night" had taken effect seemed like a good idea at the time.

14 CFR 61.51 does not specifically define "night" in terms of logging time, thus we revert to 14 CFR 1.1 which states:

Night means the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the Air Almanac, converted to local time.

"Twilight" is an interesting concept in itself. There's three levels: Civil Twilight, Nautical Twilight, and Astronomical Twilight. Civil Twilight is when the sun has set (dropped below the horizon) and there are no shadows, but there is enough light to see terrestrial objects without additional light - Officially, when the center of the sun is less than 6º below the horizon. Nautical Twilight starts after Civil Twilight and is the period when objects can't be seen, but the horizon can, when the center of the sun is 6º-12º below the horizon. Finally, Astronomical Twilight is the period when the horizon can no longer be seen, but there is still some light in the sky, and the center of the sun is 12º-18º below the horizon.

Now, the Air Almanac publishes tables that supposedly tell us when this happens. Here's the one for the period in question:

Screen Shot 2019-05-02 at 12.45.25 AM.png


Hokay, I think somehow I actually know less than when I started. So off I went for more research. GHA = Greenwich Hour Angle. I think I'm supposed to calculate the LHA (Local Hour Angle) and then compare the two or something... And I'm assuming Dec. means declination, but I haven't decided yet if I'm supposed to care.

Has anyone ever done this stuff? Looks like it could be almost a complete course in celestial navigation...

 

[PeterNSteinmetz]

because once I can't identify objects on the ground directly below me without lights... That's the end of evening civil twilight.

Observed this once on a landing on a moonless evening. Was watching things on the ground and once civil twilight ended, there was a rather abrupt transition to not being able to see things on the ground. The FAA definition actually made some sense in this case.