NavList:

Brian W, you wrote:
"This is only another tool in the box, to be used if you need it. All done, in less than a minute, lying in the bunk."

But have you actually accomplished anything here? I think you'll discover if you look at it again that you have talked yourself into something that feels like a correction to GMT, but it isn't anything of the sort. [I've got a folder full of things like that myself!]

The fundamental problem here is that if you re-work your sights, adjusted by 55 seconds of time as you suggested, you would find that all of your lines of position shift by almost exactly the same amount. You haven't closed the gap with your Moon LOP. That's not how this works! Instead, in a case like your example where the Moon is nearly in the prime vertical (and a bunch of other conditions are satisfied which we can ignore for now), the Moon LOP will shift at a rate of roughly 0.1 nautical miles relative to the other LOPs for every twelve seconds of difference in GMT. 

Let's start at a location near the equator, maybe sailing west of the Galapagos, so that we don't have to worry about the dLon scaling issues which distracted you in your example. Suppose I have a star due east and the Moon due east, too. I shoot the altitudes and correct them. Then I work up the LOPs for some specific GMT, call it T. Since both bodies are due east, I get two nice "longitude" lines of position. That is, the LOPs run north-south and nicely discriminate longitude but provide essentially no information about latitude. Suppose also that these two LOPs are separated east-west by ten miles. Thinking it might be a problem with my GMT, I work up both sights again for GMT equal to T+40 seconds. I find that both LOPs are shifted in longitude by almost exactly the same amount: 40 seconds shifts the longitudes by 10 minutes, or 10 miles in this equatorial latitude. They both move by nearly that same amount. We haven't closed the 10 mile gap between them. Looking more closely, we see that the Moon LOP shifts a bit differently. Its true position among the stars (its SHA in navigators' terms) has changed by about 0.3 minutes of arc in 40 seconds, and its LOP shifts by a similar amount (under ideal circumstances). That's how we could use the Moon's altitude to detect an error in GMT --by that very small offset.

Frank Reed