NavList:

Huub, you wrote:
"Lon = GHA - HA = 105.5-108 = -2.5˚ = 2˚30' W
Can anyone enlighten me where I make the mistake? (the 2.5˚ looks correct, but why minus ? A coincidence, or my misinterpretation?)"

What is HA? Although by etymology it is "hour angle", remember that what you have calculated there is no more and no less than the difference in longitude between you (your vessel) and the subStar point of the body in question (its longitude is called GHA). You know that the body (or rather its subStar point) is at a longitude of 105.5°. And that's a West longitude because GHA is always a West longitude. Your vessel has to be 108° away from that longitude. If you travel 108° in a westerly direction from 105.5°W, you'll end up in the Pacific. If you travel 108° eastbound from 105.5°W, where do you land? Of course, you go right past the prime meridian and continue for 2.5° more. 

So -2.5° is an East longitude then? Yes, if we live with the standard positive definition of GHA as a West longitude. The problem is that the decision to make east longitude positive is an arbitrary one, and it's only relatively recently that East became positive. You're better off in almost all cases in navigation just thinking it through. This applies to dead reckoning cases, too.

Latitude is no problem: northern latitudes (and declinations, too, since they are identical to latitudes) are positive, and positive changes in latitude are northbound changes. That's been established for centuries. North is "up". North is positive. No worries. But with longitude you should always spend a moment to think through the signs. The HA is an unsigned difference in longitude. Add or subtract it from the body's longitude to get to your vessel's position.

Frank Reed