Sun's True Bearing

This forum has been developed to discuss aviation related topics.

Moderators: ahramin, sky's the limit, sepia, Sulako, lilfssister, North Shore, I WAS Birddog

Post Reply
G3
Rank 1
Rank 1
Posts: 36
Joined: Thu Feb 19, 2004 6:30 am

Sun's True Bearing

Post by G3 » Wed Jun 26, 2019 10:04 am

Does this Formula work?

Suns True Bearing= (15 × GMT (in decimals))- West Longitude(in decimals)
Thanks for the Feedback.

G3
---------- ADS -----------
  

Mapleflt
Rank 6
Rank 6
Posts: 467
Joined: Sat Jun 20, 2009 6:13 pm

Re: Sun's True Bearing

Post by Mapleflt » Wed Jun 26, 2019 10:48 am

Yes it does; set it into the astro compass, cast a shadow with the lubber line and re-cage the C14 but ALWAYS, ALWAYS do the math twice :roll:
---------- ADS -----------
  

G3
Rank 1
Rank 1
Posts: 36
Joined: Thu Feb 19, 2004 6:30 am

Re: Sun's True Bearing

Post by G3 » Wed Jun 26, 2019 4:26 pm

Mapleflt,

Thanks. :)
Its been a long time since I used one of those. Someone mentioned that formula in one of those "There I was" BS sessions that Old guys have. I had never heard of it before, neat trick.

G3
---------- ADS -----------
  

C.W.E.
Rank (9)
Rank (9)
Posts: 1190
Joined: Mon Dec 18, 2017 2:22 pm

Re: Sun's True Bearing

Post by C.W.E. » Wed Jun 26, 2019 4:40 pm

Ahhh those were the good days, brings back a lot of memories about flying in the Arctic. :D
---------- ADS -----------
  

Meatservo
Rank 10
Rank 10
Posts: 2426
Joined: Wed Mar 16, 2005 11:07 pm
Location: Negative sequencial vortex

Re: Sun's True Bearing

Post by Meatservo » Wed Jun 26, 2019 5:32 pm

It's actually sort of an approximation of the Sun's "local hour angle". What it really tells you is the angular distance in longitude between where you are and the place on Earth that the sun is directly overhead. Thus, at the equator it wouldn't work, but the farther North you are, the better it works. It works very well indeed at the latitudes in which you might need it because your compass is unreliable.

If you look at an astrocompass, you'll see that there is a way to set both your latitude AND your longitude. Getting the sun's Greenwich Hour Angle from a nautical Almanac, and then correcting this to Local Hour Angle using your current Longitude is a bit more accurate than approximating it with the "rule of thumb" you mentioned, because the almanac gives numbers corrected for seasonal variations. The almanac gives GHA and declination, and setting the astrocompass to your local latitude will align its "horizon drum" with the equator, eliminating the error due to Latitude.

BUT, the short answer is yes, that rule of thumb is correct, and works very well in Northern Canada. It works because the sun goes around the world at 15.02 degrees per hour. So you can work out how far 'round it is after noon zulu, using zulu time. From way up North, its angular distance and its geographical distance will be pretty close.

This stuff was all on the INRAT and ATPL exams when I was a student. Even though it's rather arcane ephemera now, (I can't even set the compass at work anymore, it isn't even a real compass, it's the IRU correcting to "magnetic" from a geographic database) it makes me feel a bit sad they don't teach it anymore. Being connected to the motion of the heavens through navigational knowledge felt like it was important. It felt like now that I am a pilot, they're finally dialling me into "the secrets" of how things really work, that common folk will never know.
---------- ADS -----------
  
If I'd known I was going to live this long, I'd have taken better care of myself

C.W.E.
Rank (9)
Rank (9)
Posts: 1190
Joined: Mon Dec 18, 2017 2:22 pm

Re: Sun's True Bearing

Post by C.W.E. » Wed Jun 26, 2019 5:52 pm

Another long gone means of navigating was navigating by sound.

And it actually worked.
---------- ADS -----------
  

Jet Jockey
Rank 4
Rank 4
Posts: 293
Joined: Sun Sep 22, 2013 8:42 am
Location: CYUL

Re: Sun's True Bearing

Post by Jet Jockey » Wed Jun 26, 2019 6:43 pm

By 2030 we may be navigating with a True Heading... no more magnetic compasses.
---------- ADS -----------
  

Mapleflt
Rank 6
Rank 6
Posts: 467
Joined: Sat Jun 20, 2009 6:13 pm

Re: Sun's True Bearing

Post by Mapleflt » Thu Jun 27, 2019 1:08 pm

AstroCompass.JPG
AstroCompass.JPG (1.73 MiB) Viewed 662 times
Ah the memories, navigation north of 60 before magenta
---------- ADS -----------
  

Post Reply

Return to “General Comments”