# NavList:

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

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Re: Alain Gerbault logbook
From: Lars Bergman
Date: 2019 Jan 10, 11:59 -0800

Paul,

Further to my previous post, where I was puzzled of why Gerbault calculated both COLAT + DEC and COLAT - DEC, now I believe I know why: To calculate the azimuth to be able to plot the LOP.

Using the same formula as for hour angle, but changing a few variables, it is "readily seen" that

hav(AZ) = sec(LAT) · sec(ALT) / [sec(s) · sec(90° - (s - DEC))],
where s = (COLAT + DEC + ALT) / 2,

and you get the log of the numerator = 0.1277 + log sec(10°8') + 0.1277 + log sec(17°57') = 0.2839,
s = 52°36', and log denominator = 0.1277 + log sec(52°36') + 0.1277 + log sec(44°47') = 0.6208,
using the same strange table as before, resulting in 9.6631. This gives azimuth as N85°W, and the direction of the LOP N5°E / S5°W.

You could thus use the same table and the same set-up for calculating both hour angle and azimuth.

Another thing I noticed, is that RAMS of 32m1s indicates a GMT of 31 March 3h something, civil time, so it seems Gerbault is still using the old style of astronomical days, in 1925, the year that NA switched to civil days. But it must have been pitch dark when sighting this evening.

Lars

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