# NavList:

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

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Re: Zenith Fixes
From: David Pike
Date: 2019 Mar 22, 09:23 -0700

Andres you wrote:

Dave,
Away 1 nm???
I do not look in detail your plots, but it seems to me that scale for latitude is not taken into account. Am I wrong?
dx = dy * COS(latitude)

It’s from the ‘What do you do if your required to fly off the top (or bottom) of your Mercator plotting chart?’ trick.  It was a favourite question at the end of the RAF Basic Navigator Course.  Extend your meridians.  Draw a line from one meridian to the next at an angle equal to your next latitude.  Scribe an arc from where the line crosses the far meridian up to your first meridian.  That’s the distance to your next line of latitude.  It’s neat, plausible, and probably wrong, but it served RAF examiners well for over 40 years, and it’s close enough (y = x sec lat).

When after spending ages drawing my chart, I realised that even at 19S latitude I was still going to have to allow for convergence, I just did the arc scribing trick and found that 19S was 63/60th of the distance which I’d previously used, so I  marked a new 19S.  Then I could only use my original latitude scale to set my compass to draw the Sun arcs if I multiplied the radii by 63/60 (it’s called ‘Daves dodgy algebra’), so that’s what I did.  In truth I should have laid off my hypotenuse line (bottom left) using an angle of 19 degrees not 20, but initially I was just playing around, and when it appeared to be working, I couldn’t be bothered to redraw it.

I note your software gave a fix position for 12.00, but your sextant observations were at 11.12 and 11.21.  I couldn’t understand that.  I would have thought your fix was at 11.21, and your position at 12.00 had to be a DR position.  DaveP

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