A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2021 Jun 28, 11:16 -0700
David Masters you wrote: I am new to celestial navigation and this forum.
Welcome to NavList. As no US commentator has picked this up, let a Brit have a crack at it. This looks like a circular slide rule which uses the difference in latitude and longitude between two points to find the course and distance between them. It appears you can only find the rhumb line (Mercator) solution.
It looks like you find the rhumb line distance first and then the course. The distance side appears to work like this. Northings = 60chlat. Eastings = 60chlong x cosmeanlat.
Distance = square root (northings squared + eastings squared).
In logs, which is what slide rules are all about, log distance = ½ (2xlog(60chlat) + 2xlog(60chlong x cosmeanlat, or log distance = log(60chlat) + log (60chlong x cosmeanlat). It’s just a question of the manufacturer drawing the scales correctly.
Once you’ve found the distance it looks like they’re saying something along the lines of cosine course = base/hypotenuse = chlat/distance. It might not be exactly that, because unfortunately, it appears the course scale can’t handle all combinations of chlat and distance in one go. Using Chlat and Distance gives courses from 45-90 degrees. If you end up in the small segment marked ‘warning’, you must use equivalent chlat and distance from the distance side to give you a course between 0 and 45 degrees. I’ve no doubt someone else will be able to explain how it does this.
You don’t need a user manual, because the instructions are actually written on the computer. If you have one, that’s fine. If you haven’t, you need to find photographs clear enough to read what’s written. Google 'USAAF A-5 Computer' (that’s what worked best for me) and you’ll find several photographs, some clearer than others. Find a close-up one and you’ll be able to read the instructions. DaveP.