# NavList:

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

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Re: Jack Aubrey's fixing of longitude
From: Albert Rodok
Date: 2011 Jun 24, 04:10 -0700

Hi Frank,
I enjoy very much this kind of brain storming. You guys are forcing me to put these old neurons of mine to work… Long time ago that I used to check our chronometers- just for the fun of it – by taking lunars. The very hard way – using trigonometric and logarithm tables – as neither a calculator nor lunar distance data were available on board.

Regarding your remarks about using the azimuth: Yes, determining the precise moonset azimuth can be difficult, although it all depends on the atmospheric conditions. I have some times seen the moon going down clear, and in the tropics this is usually a fast event. As the azimuth is also sensitive to declination, the accuracy of the method depends on the actual "behavior" of the moon. Continuing with the brain storming: you could set the theodolite to a certain altitude (in order to avoid refraction problems), let's say 5°, and measure the azimuth when the moon reaches this altitude…

As you see, I am trying to "rescue" the call "two seven four" for which the List has no explanation…

At the moment I tend to another interpretation of this literary fantasy. As no lunar data for planets was available, I think that the occultation of Venus can also be ruled out, as an occultation (please correct me) is a special case of lunar (distance = 0). Nor was it a normal lunar, as the team was waiting for something to happen and a lunar can be taken any time.

To sum up: they waited for the occultation of a star. And again, here comes the "azimuth man"…

By measuring the exact azimuth you can reduce the local hour (angle), as now you have the precise declination and the local latitude. The Nautical Almanac gives you the Greenwich time for this event.

You could of course use an artificial horizon and take a shoot of the moon, which however might become problematic if she is already very low (we never trusted altitudes of less than 15°).

Albert

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