Twelve degrees below. How do I know?
 

Some UK CAA night flying regulations use the phrase,
"...not less than 5 take offs and five landings at a time when the depression of the centre of the sun was not less than 12° below the horizon."

How can I determine, on any particular day, without undertaking a course on astronomy, at what time the sun will reach that point in the evening?

Doesn't that definition coincide with what various sources publish as sunset and sunrise times?

As I understand it from Air Nav, sun appears to move across the horizon at 15 degrees per hour. So I'd take the official time for "night" (sunset plus 30mins UK) as being 7.5 degrees. So adding 1degree = 4mins meaning its probably 46-50 mins after official sunset time?

Although I thought you just had to be doing them at "night"?

Hope this helps...

If I look up the weather at EGLF here I also get sunrise and sunset.

Today the times are

Sunrise - 7:23 (the time the sun appears above the horizon)
Civil twilight 6:49 (It is bright enough to see the horizon and due normal work without lights after this time)
Nautical twilight 6:09 (The sun rises to within 12 degrees of the horizon - it is pretty dark before this and hence counts as nights for takeoff and landing)
Astronomical twilight was 5:31 (It is proper pitch dark before this)

Definitions of Astronomical events

Sunrise/Sunset is defined as the instant in the morning/evening under ideal meteorological conditions, with standard refraction of the sun's rays, when the upper edge of the sun's disk is coincident with an ideal horizon.
Beginning/Ending of civil twilight is defined as the instant in the morning/evening, when the center of the sun is at a depression angle of six degrees (6°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, the illumination is such that large objects may be seen but no detail is discernible. The brightest stars and planets can be seen and for navigation purposes at sea, the sea horizon is clearly defined.

The brightest stars and planets can be seen and for navigation purposes at sea, the sea horizon is clearly defined.

Beginning/Ending of nautical twilight is defined as the instant in the morning/evening, when the center of the sun is at a depression angle of twelve degrees (12°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, it is dark for normal practical purposes. For navigation purposes at sea, the sea horizon is not normally visible.

Beginning/Ending of astronomical twilight is defined as the instant in the morning/evening, when the center of the sun is at a depression angle of eighteen degrees (18°) below an ideal horizon. At this time the illumination due to scattered light from the sun is less than that from starlight and other natural light sources in the sky.

I think that works out to be sunset + 48 mins (averaged). But his wil depend on time of year etc.etc. easiest way is to use the tables provided in the aip

I seem to recall that darkness for aviation purposes is defined as 30minutes after official sunset. At least, that's what I trained for when I did my night rating and that's the time we went off blocks to start the exercises.

Aviation 'Night' has several context with different times (and each authority has their own way of specifying it). However, in a nut shell night is important for

1 - when must you have a night qualification/rating (generally civil twilight to civil twilight - roughly 30-45 minutes before/past sunrise/sunset - and specifically 30 minutes in the UK)

2 - when does a take off or landing count for night currency (generally before or after nautical twilight - or about an hour before/after sunrise/sunset )

3 - when must navigation lights be displayed (before civil twilight, 30 after sunset/30 before sunrise in the UK)

Many handheld GPS's will tell you the local sunrise sunset times. I'm certain that you could not be found wrong for flying with reference to this information.

When they say "horizon", is that the one in front of you while sailing across the ocean, or the one beside you while you take of in a mountain valley. I've had to look up at a 40 degree angle to see the horizon while flying in the mountains.

And the proof.. Aside from your radio call to ATC, or transponder return recorded along with a time, I think that the authorities would have hard time proving your takeoff time with precision equal to that of the positon of the sun as described.

Funny that they'll define sunset with such precision, and yet not define "icing" at all (well in Canada anyway)!

Pilot DAR

Thanks DenhamFlyer, I thought that there would be tables somewhere but I had failed to find them. The biggest delay I found to what I now know is the end of 'Civil Twilight' I was 1 hour and ten minutes after sunset at SCATSTA in late July.

Sunset + 30 minutes.

Most self respecting GPS will have the time of sunset if you punch in the airfield name.

All the other definitions are irrelevant.

If you're flying from a manned field (which is more than likely as you'll need lights), then the Tower will give you the time of sunset and by definition time of official night for that evening.

Cusco

... In the UK ... Except for night landing currency.