# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Compass correction using sunrise
From: Howard G
Date: 2022 Jun 28, 01:26 +0000

Hi Francois

I have only kept a casual eye on this subject without delving into the reasoning for it – what interests me is why would you do this

You wrote…

And just to give you a brief explanation of my professional navigation knowledge – I am a retired air force P3 Orion long range maritime aircraft navigator with over 4000 of flying.

I have no inherent knowledge of maritime navigation per se – however you mention the word ‘compass’ – and that needs defining.

Is this a wet magnetic  compass, a gyro compass, a GPS compass reading or other.

Let’s assume a wet compass – I have a lot of experience at ‘swinging an aircraft wet compass’ – and ‘swinging it’ means aligning at building a correction table after the aircraft has had a major refit – and this is a long process of using a highly accurate and highly aligned precision land compass – standing out in front of an aircraft at least 50 metres away – in the middle of an open piece of runway – and turning the aircraft to the 4 cardinal settings N,S,E, W and noting the difference between the precision and the aircraft compass – and building a correction table to attach to that compass and that swing.

It is also highly specific to that aircraft, that time, that location and generally is still only a correction.

Any such attempt to get some sort of true azimuth from the sunrise would by definition be a true direction calculation and would not give you a measure of the variation W that would exist between true and magnetic for that actual location in the North Atlantic.

Or am I missing something.

Howard G

From: NavList@fer3.com <NavList@fer3.com> On Behalf Of Francois Tremblay
Sent: Tuesday, 28 June 2022 11:00
Subject: [NavList] Re: Compass correction using sunrise

Hello All.

First of all I want to thanks all of you guys (David, Al) for your awesome explanations. I'm very much impressed by the quality of the discussions on this forum and the by time you take to help others. As I mentionned earlier, I've been working for decades in engineering, a field of work that has degraded a lot lately. Nowaday, science is not what it used to be. I'm glad to see people on a forum such as this one so dedicated to the science of celestial navigation. It's fairly new for me, but it is changing the way I see the sky and, I should say, the world. Stars and planets that I used to barely see up above my head are now acquaintances.

Now, regarding my problem, with all of your explanations, I come to the conclusion that the difference between my results is due to the inherent precision of each method. Nothing really wrong with the process.

Method 1. Using the Almanach's sunrise time and HO-249: Sunrise time is not what I expected it to be. If I'm not mistaken, the almanach considers the sunrise as the moment the upper limb hit the horizon, not the center of the sun. It would explain the error. Adding this time to the time it takes for the sun to climb half its diameter seems to solve the problem.

Method 2. The slight inacuracy of this method seems to be the result of rounding

Method 3. Seems to be the most accurate method.

Again, thanks for helping me with this. I will certainly sleep better tonight (unless I end up looking at the sky all night :-)

Francois

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