Good night

[Fuji Abound]

Yesterday the sun set at 4pm (where I am). Official night was therefore at 4.30. Now it was a very clear day but without much of a moon. When I landed at 4.28 it was still surprisingly light. I reckon by about 4.45, just about all the natural light had gone. It got me thinking why we reckon official night starts at half an hour after sunset. Is it just one of those arbitrary numbers?

[GT]

Depends on latitude, so I'm led to believe. When I flew in Florida some years ago I was told that night was one hour after sunset.

Regards, GT.

[Kolibear]

A bit of both, I suspect.

Near the equator, sunset is very short, there is not a lot of dusk as we know it.

As you pointed out Fuij, on a clear evening it can be light for 30 mins or more after sunset, but on a cloudy evening it can be dark before sunset.

So it looks as though 30mins. is about the right period of time.

[Fuji Abound]

GT - that is interesting because my experience of the tropics is as yours - sunsets are shorter and the transition to dark much quicker. That is what you would expect. Perhaps the CAA were being cautious when they came up with half an hour or perhaps the onset of night came about first for other purposes and then they thought they might as well use the same base for night ratings.

[Flyin'Dutch']

FA wrote:

Is it just one of those arbitrary numbers?

Indeedy;

CAA 30 mins and the FAA rules it to be 60 mins

FD

[Brooklands]

IIRC from when I did my night rating, there are three different night time definitions:

Civilian night time, when the sun is >=6 degrees below the horizon
Nautical night time, when the sun is >=12 degrees below the horizon
Astranomical night time when the sun is >=18 degrees below the hoizon


The 30 mins after sunset is used to define when you start logging the time as night flying, but I'm sure the above definitions are relevent in some way as well, but without my books to hand I can't say what.

One point is that at northern latitudes in summer, you may not actually get the latter two (and if you go north of the arctic circle, you won't get any of them at midsummer!)

[RichyRich]

Civilian night time, when the sun is >=6 degrees below the horizon

Without using a book, Brooklands, you can work this out.

The Earth turns 360 degrees in 24 hours (we call that one day :D ). This equates to 0.25 degrees per minute, or 4 minutes per degree depending what you're looking for.

So, if civilian night is when the sun is 6 degrees below the horizon, that means 24 minutes after the sun sunk below the horizon. Half an hour is near as dammit to that, perhaps this is the source of the half-hour rule.

GT's argument that it depends on latitude seems upside down. Surely in Florida, they'd have only a couple of minutes, if it gets darker quicker. However, back to the notes from Brooklands, perhaps they get their time from Nautical night (which'd make more sense, considering all the nautical terminology used in aeronautics).

"STOP WAFFLING RICHARD" I hear myself saying.

[Brooklands]

Richy,

It wasn't the time part I was uncertain about, but the relevance of the different 'type' of night w.r.t. night flying.

I agree with your calculation, but you have taken the earth's inclination and latitude into account - the further north you go in summer the less night you get (ie sun below horizon for less time), until you get north of the artic circle, when at midsummer you get no night at all.

[englishal]

The Earth turns 360 degrees in 24 hours

Actually, its 23hrs 56 minutes :}

[tyro]

RR: " So, if civilian night is when the sun is 6 degrees below the horizon, that means 24 minutes after the sun sunk below the horizon. Half an hour is near as dammit to that, perhaps this is the source of the half-hour rule."

Sorry, I don't buy that at all. Nothing in celestial mechanics is ever that simple. Maybe it works for you because you live on the equator, or another planet.

For some people who live in Finland, for example, the sun never reaches 6 degrees below the horizon for many weeks in summer. Come to think of it, maybe Finnish GA types like to fly around at midnight without bothering about night ratings.

[High Wing Drifter]

A bit of an interest here as I recently covered this topic in my ATPL studies but it does not actually answer the question. So reading between the lines here is my attempt at a thread stopper:

My notes say the end of evening twighlight is when the sun is 6 degs below the horiz as previously stated.

Simplistically, the duration of twighlight would be 6/(15deg/hr*cos(latitude)). Therefore as the earth is tilted 23.5 degrees then from 66.5 lat to the poles the sun either doesn't set or doesn't rise and in the tropics, the sun will move at 15deg/hr (the speed of the rotation of the earth).

I think this is a red-herring but adds up nicely: Looking at some extracts the air almanac that provides the sunset/rise and twilight times as durations. The duration of twighlight in winter at 54deg lat is 45 mins (ish) and under 30 mins (ish) in summer. It just so happens that 6/(15*cos(54))= 45mins (ish) and that 6/(15*cos(54-23))=30mins (ish). The 23 is the tilt of 54 deg lat in mid- summer. However, I think I might have expected +/- 11.75 degs lat between winter and summer rather than the full-on 23.5 in one season??

The ANO's half an hour appears to be an crude safe value for the year. The 45 mins to darkness in mid-winter may answer Fuji's question. God knows why the FAA say night is one hour after sunset though.

Now, obvisouly the velocity of the earth's surface at the tropics is quick in relation to high latitudes so hence the faster onset of darkness...I think.

Well there you have the contents of my brain. Ugly ain't it?

HWD :confused: