NavList:

Paul Hirose, you wrote:
"If a round of sights are taken at other than the planned time, the author says the solution is to adjust the assumed position east one degree per four minutes early. Nothing else need be changed in the precomputation."

Yes, this is one of the great "unsung" tricks of celestial navigation, especially with tables like the "Selected Stars" Pub.249 vol.1.

You also wrote:
"The usual nautical practice is to reduce each body in a round of sights with a different assumed longitude, selected to make its local hour angle an integer. The assumed latitudes are the same integer value. However, in the Combat Crew example the same AP is used for all three stars in the fix. I've never seen that before and have not figured out how the precomp is arranged to make that work."

What you describe here sounds an awful lot like the practice that is now "usual". With some tables and in some accounts of celestial navigation, the navigator is encouraged (or compelled) to use a different AP for each body. This leads to a "messy" plot, and it's prone to error. But with planning and judiciously timed sights, we can use one AP, which is nice (if AP's and intercepts are necessary at all). The tabular layout of "Selected Stars" makes this easy. Each row in the table is a little simulation of the sky for some value of LHA and some selected latitude. We compare what we see with that simulated sky. It's as if each row is a little "paper planetarium" or the output on paper from a sky simulation app. But if we play around with the "app" or the tables, we realize that a difference in longitude is equivalent to a difference in time at the Earth's true rate of rotation --true meaning it's the rate of rotation relative to the distant stars, the "sidereal rate" of rotation. A single degree change in LHA Aries (or longitude) is equivalent to a time shift of 3min 59.3sec. We can shift any pre-computed sight or sights by adjusting the time by nearly four minutes for every degree of longitude change in the AP. This is equivalent to dropping down one row in the "Selected Stars" table. And we can do the same with individual sights. Take a first star sight at any convenient time. Then follow up with another star sight as close to four minutes later as you can manage. Drop down one row in the table for each sight's Hc and Zn. Plot them all from the same AP, which can be conveniently placed at the center of the "compass rose" on the local plotting sheet.

A note for the computation fans: the author of the article suggests a similar trick for the Moon. Is that a good idea? How quickly would you run into trouble with the proposed adjustment? Maybe it's discussed more further in the article, but the brief reference I saw in the first few paragraphs seems rather "fishy".

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA

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