# NavList:

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

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Re: Borrowed Bygrave
From: Gary LaPook
Date: 2016 Jan 15, 09:49 +0000
If you read the rules for special cases in these photos of the Bygrave you will find that CPT Bygrave tells us how to compute the altitude when the declination is less than the lowest number on the scales. He tells us that  to find the altitude in this case that you interchange the declination and the assumed latitude since the astronomical triangle works from both ends, the altitudes, as calculated from either end (GP or AP,) must be the same.

BUT, he doesn't tell us how to find the azimuth!

I had to invent a method for this situation and it is on my website.
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When declination is less than one degree you can't begin the computation the normal way to find "W" because you have to start the process with declination on the cotangent scale and this scale doesn't extend below 1º. So in this case you just skip the computation of "W" and simply set "W" equal to declination. Using this method you arrive at an azimuth that is not exact but is a close approximation and in the worst case I have found the azimuth is still within 0.9º of the true azimuth but most are much closer. If the declination is less than one degree and the latitude is also less than one degree, follow this procedure and also assume a latitude equal to one degree. After you have computed the Az you then follow the same procedure discussed above for azimuths exceeding 85º by interchanging the latitude and declination and then computing Hc which will produce an exact value of Hc.

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gl

To: garylapook@pacbell.net
Sent: Thursday, January 14, 2016 6:47 PM
Subject: [NavList] Re: Borrowed Bygrave

i took these photos at the Science Museum in London.
gl

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