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

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Re: John Karl's Celestial Navigation in the GPS Age
From: Sean C
Date: 2018 Apr 30, 16:45 -0700

Aonghus,

In my copy of 229 vol. 2, on the page for LHA 32° and Lat. 24° contrary name, the tabular values given for a declination of 18° are:

Hc: 37°40.2'

d: -47.6'

Z: 140.5°

The value of Hc must be adjusted for the 8.5 minutes of declination using the "d correction" and the interpolation tables. So, we turn to the interpolation table on the inside front cover. (The table in the front is for declination increments ranging from 0' to 31.9'. The table in the back is for increments ranging from 28' to 59.9'.) We look at the row for 8.5' of declination increments and the column for the tens part of the "d correction": 40'. There, we find a value of 5.7'. Next, we look at the nearest decimals row and units column for the rest of the correction: 7.6' and we find a value of 1.1'. We add this to the previous value of 5.7' to get a total correction of 6.8'. The "d correction" is negative, so we subtract 6.8' from the Hc of 37°40.2 to get 37°33.4'. (I'm not sure why John's value differs by 0.1', but this is trivial.)

Now we must adjust the Z value according to the rules at the top or bottom of each opening of 229. Since our latitude is South, we use the rules at the bottom of the right hand page. And since the LHA (32°) is less than 180°, we must add the Z value to 180° to get the azimuth. 180° + 140.5° = 320.5°, which we can round to 321° if we like.

If any of that was unclear, feel free to ask for clarification.

Cheers!

Sean C.

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