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

flyingcheesehead

flyingcheesehead

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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. :no:

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

14 CFR 1.1 said:
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.
Click to expand...

"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...
 
flyingcheesehead said:
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. :no:

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



"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:

View attachment 73837

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...
Click to expand...

I'll never see a sunset quite the same again
 
Until the FAA comes out to every airport to watch aircraft and then compare against a logbook entry or makes some type of logging machine that is installed in every aircraft, I'm not racking my brain on this for splitting minutes or seconds on the twilight times.... :)

I just use Weathermeister to tell me the times and then go from there for logging. ;)

Screen Shot 2019-05-02 at 4.27.22 PM.png

Cheers,
Brian
 
flight2000 said:
Until the FAA comes out to every airport to watch aircraft and then compare against a logbook entry or makes some type of logging machine that is installed in every aircraft, I'm not racking my brain on this for splitting minutes or seconds on the twilight times.... :)

I just use Weathermeister to tell me the times and then go from there for logging. ;)

View attachment 73838
Click to expand...

Aha! A tool! :)

I used to have an iOS app for figuring this kind of stuff out, but the developer never updated it, and eventually some of the API calls he was using became unsupported and it quit working... Maybe around the iOS 11 update or so. I still haven't found a good replacement.
 
luvflyin said:
I'll never see a sunset quite the same again
Click to expand...

Well, I'll still enjoy the sunset. But I'll also keep my eye out on the ground afterwards, because once I can't identify objects on the ground directly below me without lights... That's the end of evening civil twilight.
 
flyingcheesehead said:
Aha! A tool! :)

I used to have an iOS app for figuring this kind of stuff out, but the developer never updated it, and eventually some of the API calls he was using became unsupported and it quit working... Maybe around the iOS 11 update or so. I still haven't found a good replacement.
Click to expand...

I tried all of my other WX websites to see if they had the twilight hours listed (looking for free sites since Weathermeister is subscription based) and I only found sunrise/sunset data, no twilight times. Strange, but I'm glad someone does it.

Brian
 
Timeanddate.com gives the start and end times of civil twilight in local time.
 
i use the Rise&Set app. it does all the work of calculating civil twilight for me based on my location. KISS.
 
You warm my heart. I actually changed that regulation. It's one of the two times my name has made it into the Federal Register (the other is the FAA noting but ignoring my comment on a proposed AD).
 
flyingcheesehead said:
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.
Click to expand...

You forgot about the Twilight Zone, which begins at the point that you can see terrestrial objects but cannot see the FAA guy lurking near the FBO...



-Skip
 
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Log three takeoffs and three landings every 90 and a flight review every two years and call it good.
 
57769C79-AD60-4F41-B964-696462330C91.png You are using the wrong page, try page A133.
Take the Lat and Long of the airport that you are trying to calculate twilight for.
KSPW is 43°10’N 095°12’W
First you interpolate for Latitude using the values in the table.
45°N is 1854
40°N is 1849
So 43°N would be 1852
Then you have to correct for your Longitude, the times in the table are for the center Longitude in the time zone. In this case that would be 90°W. Each degree that your Longitude differs from that is 4 minutes of time so since you are approximately 5.25 degrees west that would be 21 minutes. Since you are west of the center of the time zone twilight is later and you add the 21 minutes.
1852+21=1913
Then because of daylight savings time you have to add an hour and civil twilight at KSPW is 2013.
 
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I fly to Billy Mitchell Airport on the Outer Banks of NC. It is on NPS land, and take-offs and landings are prohibited 30 minutes after sunset and 30 minutes before sunrise. I carry the USNO sunrise/sunset chart (linked above) with me as the airport is a popular place for rangers to hang out, and they will issue you a ticket if you land late. Sometimes those friday evening headwinds can be a challenge.
 
The Aeroweather app also computes and displays beginning and end of civil twilight (as well as sunrise and sunset).

The auto logging function in Garmin Pilot will divide your time into day and night. The one time I did what the OP did, take off in day and land in night, it looked like it split the time correctly (I.e. to within +/- .05 hr) based on my own post-flight calculations.
 
flyingcheesehead said:
Well, I'll still enjoy the sunset. But I'll also keep my eye out on the ground afterwards, because once I can't identify objects on the ground directly below me without lights... That's the end of evening civil twilight.
Click to expand...
What do you do when there is a bright moon? ;)
 
You must be really hard up for night hours to go thru all that BS. Add 30 minutes to sunset or subtract 30 from sunrise and call it a day.
 
Just round up..... no matter what, round up... in 1/2 hr increments.
 
Wheels said:
View attachment 73840 You are using the wrong page, try page A133.
Take the Lat and Long of the airport that you are trying to calculate twilight for.
KSPW is 43°10’N 095°12’W
First you interpolate for Latitude using the values in the table.
45°N is 1854
40°N is 1849
So 43°N would be 1852
Then you have to correct for your Longitude, the times in the table are for the center Longitude in the time zone. In this case that would be 90°W. Each degree that your Longitude differs from that is 4 minutes of time so since you are approximately 5.25 degrees west that would be 21 minutes. Since you are west of the center of the time zone twilight is later and you add the 21 minutes.
1852+21=1913
Then because of daylight savings time you have to add an hour and civil twilight at KSPW is 2013.
Click to expand...

Note to self: Try not to give up prior to page 874 in the future. :eek:

I finally found the instructions for the twilight tables... 121 pages before those tables start. This is why people hate the government.

And I wonder if the FAA expects us to correct for the "height of the observer" if we're in flight... Probably, considering they give those heights up to 60,000 feet and Everest is only 29,028 feet.

I love accuracy in numbers, but this is crazy. I'm going back to guessing.
 
Timbeck2 said:
Can we please get an in-depth discussion about night and day? I'm still confused. :confused:
Click to expand...
Here ya go:
By Al Bowlly, someone who made over 1000 records in the 1930s. I bet few here ever heard of him (I hadn't not to long ago). Song written by Cole Porter.
 
Geekdom for the win! Challenge Accepted.

KUES Lat/Long: 43°02' 27" N / 88°14' 13" W
KUES Lat/Long in Decimal: 43.04083 N / 88.23694W
Average Declination of sun on 3/30/2018: 3°48' 49" or 3.81375

Sunrise

Find the hour of sunrise (GHA of Sunrise):
360 - ( arccos ( -Tan ( Declination) x Tan ( Lat)) + Long = GHA
360 - ( arccos ( -Tan ( 3.81375 ) x Tan ( 43.04083 ) ) + 88.23694 = 354.6679

The integral hour (hour without minutes) closest but still less than 354.6679 is 11:00z with a GHA of 343° 52' 48" or 343.88 at a declination of 3° 50' 18" or 3.8383.

We could use the chart to find sunrise is between 11:40 and 11:50z and interpolate to find sunrise as 11:43:08.59 or we can plug the GHA of the integral hour and declination into the next formula to find minutes:

Find the minutes after the integral hour (GHA) of sunrise:
( 360 - arccos ( -tan (Declination) x tan (lat)) + long - GHA of the Integral Hour) / 15
( 360 - arccos ( -tan(3.8383) x tan(43.04083) ) + 88.23694 - 343.88 ) / 15 = 0.717655 hours or 43:03.55 which is 5.04 seconds different from our interpolated value.

We could throw the integral hour out in front of the entire equation to arrive at 11.717655z as sunrise but most of us dont read time in decimal form and would want to know the minutes which first requires subtracting the 11... Still the second equation would look like this:
Integral Hour + ( 360 - arccos ( -tan (dec) x tan (lat) ) + long - GHA of the integral hour ) / 15 = Sunrise

The time of sunrise on March 30, 2018 was at approximately 11:43:03.55z which is then converted to local time -5 hours so 06:43:03.55.
Time and Date says it was 06:38. They do use slightly different coordinates of 43°01' N / 88°14' W but when calculated with these coordinates, the time is only slightly more than a tenth of a second earlier, not 5 minutes.

You could also TLAR it for minutes by not plugging the exact declination in. That gives you the result of equation 1 (where we found the integral hour) - the almanac hour value divided by 15.

(354.6679 - 343.88) / 15 = 0.7192 hours

Converted to minutes and seconds comes out to 11:43:09.12 that gives you a value is only half a second (0.53) off our interpolated value instead of 5 seconds off.


Sunset
Very similar to sunrise, you just dont have a starting value of 360.

Find the hour of sunset (GHA of Sunset):
( arccos ( -Tan ( Declination) x Tan ( Lat)) + Long = GHA
( arccos ( -Tan ( 3.81375 ) x Tan ( 43.04083 ) ) + 88.23694 = 181.8060

The integral hour (hour without minutes) closest but still less than 181.8060 is 00:00z with a GHA of 178° 55' 18" or 178.9216 at a declination of 4° 02' 54" or 4.0483.

We could use the chart to find sunset is between 00:10 and 00:20z and interpolate to find sunset as 00:11:32.25 or we can plug the GHA of the integral hour and declination into the next formula to find minutes:

Find the minutes after the integral hour (GHA) of sunset:
arccos ( -tan (Declination) x tan (lat)) + long - GHA of the Integral Hour) / 15
arccos ( -tan(4.0483) x tan(43.04083) ) + 88.23694 - 178.9216 ) / 15 = 0.2070 hours or 12:25.17 which is 52.92 seconds different from our interpolated value.

Again, we could throw the integral hour out in front of the entire equation to arrive at 00.2070z as sunset but most of us dont read time in decimal form and would want to know the minutes which first requires subtracting the hour (when the hour is greater than 0)... Still the second equation would look like this:
Integral Hour + ( arccos ( -tan (dec) x tan (lat) ) + long - GHA of the integral hour ) / 15 = Sunset

The time of sunset on March 30, 2018 was at approximately 00:12:25.17z which is then converted to local time -5 hours so 19:12:25.17.
Time and Date says it was 19:16. They do use slightly different coordinates of 43°01' N / 88°14' W but when calculated with these coordinates, the time is only slightly more than a tenth of a second earlier, not 4 minutes later.

Again, you could also TLAR it for minutes by not plugging the exact declination in. That gives you the result of sunset equation 1 (where we found the integral hour) - the almanac hour value divided by 15.

(181.8060 - 178.9216) / 15 = 0.1923 hours

Converted to minutes and seconds comes out to 00:11:32.28 that gives you a value is nearly the same as (only 0.03 off) our interpolated value instead of 52 seconds off.

Solar Noon can also be found by finding the point at which GHA is equal to your meridian which in this case would be about 17:57z.

You should be able to calculate the various twilights times but I dont recall exactly how; its not quite as simple as adding/subtracting 6/12/18 degrees to the calculated sunrise/sunset GHA (actually this would be 5.17/11.17/17.17 as sunrise/sunset actually occurs when the center of the sun is about 0.833 degrees below the horizon)


You get into some even more interesting calculations when you start considering altitude... Altitudes aren't particularly difficult, they're linear and work out to extending the given phase of sunrise/sunset by 1 minute per every 5000' ft but the math behind it is a little more involved and it invalidates your tables/charts.

Speed is another interesting factor particularly when you are traveling fast in an east or west direction (doubt a typical piston would qualify as fast enough to make a difference though) and at higher latitudes as you can speed up or slow down the onset of day/night.

You also get into some interesting discussions when you start factoring in terrain. Death Valley for example is below sea level and is abutted to the West by 13,000' peaks. On the ground, you certainly have "night" conditions and sunset due to the relative horizon well before the sun has officially set according to the astronomical horizon. Just like altitude, this isnt necessarily that much more difficult to work out but it does involve more math and it does invalidate your charts/tables.
 
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flyingcheesehead said:
because once I can't identify objects on the ground directly below me without lights... That's the end of evening civil twilight.
Click to expand...

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.
 
OK, but can I log simulated twilight if I wear dark sunglasses?
 
flyingcheesehead said:
Nicely done!

Now where'd ya learn all that?
Click to expand...

Celestial Navigation for Sailing.

That's why the Air Almanac also publishes GHA and Declination of Aries/Venus/Jupiter and other navigable planets and stars as you'd use the stars during the twilight hours (particularly nautical twilight since you still have a horizon but its dark enough to also see the brighter of the stars) to find your location. Altitude again messes the apparent angles and declinations but it is possible to celestial navigate in a plane you just have to correct for altitude.


TL;dr: Lots of stuff about finding your location with nothing more than an almanac and a sextant or quadrant.

If I dont know where I am but I have a sextant or quadrant, an accurate date/time and an almanac, I can get reasonably close to my location by working the problem in reverse.

Without a sextant/quadrant you can still guesstimate roughly based on the angles but It'll be rough (or you can make a rough quadrant pretty quickly/easily; it'll still be rough but it'll be more accurate than trying to visualize the angles involved).
Without an accurate date/time, you can use the North Star which is always at a declination of 89 degrees which makes the North Star great for determining latitude in the northern hemisphere but not so great for finding longitude or latitudes near or south of the equator or finding the date time. Alternatively you can use the sun and solar noon with a rough estimate of date or latitude to determine the solar date/time and latitude. If you track the sun long enough or at the right time of year (i.e. days leading up to and after the solstices [winter/summer]) or cross check it with the north star, you can find the solar date/time and latitude without any estimate as the declination varies between from +23.5 degrees (summer) and -23.5 degrees (winter) with a 0 degree declination on the equinoxes (spring/summer)...

If it were the first day of spring/fall and I were "lost" at KUES, the sun should reach its azimuth at a relative angle of 43.04083 degrees, on the summer solstice it should be 66.54083 degrees and on the winter solstice it should be 19.54083 degrees. I should see the angle continually grow or shrink leading up to the solstice, stop changing on the solstice before going back in the opposite direction after the solstice. If I didn't want to wait upwards of 6 months for the next solstice, I could work out my latitude from the north star and then measure the sun, subtract my north star calculated latitude to find the declination and repeat it the following day to determine which solstice we are moving towards which then would allow me to accurately predict/determine the exact solar date/time of noon on the 3rd day.

Note that I said "solar date/time." Time is relative and without a fixed reference point, I can only determine the solar date/time for my exact position which does not necessarily coordinate with noon on the local clock (daylight savings for example, changes in earth's speed of rotation, larger/smaller timezones such as China)

Finding longitude is where LHA's and almanacs really come into play. The only reason Solar Noon works so well for determining latitude is because we have a fixed point (north star) and a predictably moving point (sun) to use as reference points. Since the earth is spinning and is canted on its axis we do not have a point at a fixed point to use as reference for calculating longitude. About the closest we can get is solar noon which occurs as the sun crosses the meridian you are at but without the fixed reference point it is impossible to tell whether our solar noon is at the prime meridian or somewhere else on the planet, in other words solar noon looks the same for every point of longitude along the specified line of latitude no matter where you are so you need a static reference. LHA is nothing more than the angle the celestial object makes with the horizon at your local meridian; where it can get complicated/confusing is in converting it to GHA

Since we're dealing with 90-degree points, the angles are always complementary so if you know your longitude, you can just shortcut most of the process by adding (western longitudes) or subtracting (eastern longitudes) your longitude from the GHA. It puts everything into terms of GHA position simplifying most of the math though you need to account for the position of the celestial body east of you (sunrise equation of 360 - rest of the equation) vs west of you (sunset equation).

I'm pretty sure I recall there being a way to triangulate longitude without the almanac using multiple navigation stars and different times but I dont remember it. There's also something you're supposed to do with the error correction table using the moons altitude to find time corrections to apply but I dont remember it... I probably did a bad job explaining all of this (particularly LHA), I've forgotten quite a bit of it and my memory of what I do remember isn't really clear enough to explain it more clearly... I'm not even sure everything here that I do remember is correct. I've never really had to use it either, most of my sailing voyages are inland or coastal so I use coastal navigation (aka pilotage and DR) and even the few trips I've taken far enough off shore that coastal navigation wasn't possible... well GPS makes knowing where you are a whole lot more accurate, faster and possible to obtain without a steady visual (helpful on a pitching vessel) of the horizon or sky and without having to have knowledge of your local navigable stars. Sextants and Quadrants on boats are quickly becoming like E6B's and paper charts in airplanes; there as a backup to our other tools but usually collecting dust and certainly not getting enough use to be proficient on in the event you REALLY needed it.
 
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