# NavList:

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

**Re: Cannot dispense with the assumed position at sea**

**From:**Bill Noyce

**Date:**2004 Feb 20, 08:57 -0500

My attitude is that an AP is simply a convenient place from which to compute an Hc, and plot an intercept. If using certain tables, there are constraints on how it's chosen; if you're plotting on an actual chart there may be other constraints you'd like to follow as well. But I disagree with the following, if I understand it correctly: > A celestial EP will only be as good as the AP. The more one has to > guess at > the AP, then the less confident one can be about estimating where the > vessel is on the celestial LOP. One interpretation is that you reduce a single sight, draw a LOP, and then drop a perpendicular to it from some previously-chosen position (perhaps called your AP?). In that case, I agree that, while the LOP might be pretty good, your position along it has all the uncertainty of the previous position. On the other hand, if your observations and sight reductions are good, then your true position is much more likely to be somewhere near the LOP than far from it. Another interpretation would be that a round of sights leading to a celestial FIX is "only as good as" the AP(s) chosen for reducing them. This I strongly disagree with. If your celestial fix comes out very far from your AP, then use the new fix (or positions near it) as a new AP, and do the calculations over again, getting new Hc's, Z's, and intercepts. Really this is just correcting for the fact that we use straight lines to approximate circles of position -- if we plotted the circles, or otherwise corrected our LOP's to better approximate circles, this wouldn't be needed. But simply doing the whole thing over is "simpler" because it uses familiar operations, and also serves as a useful check. The only place this can fall down is if you make just two observations, and at least one of them is very high, or the azimuths are nearly the same or nearly 180 degrees apart. Then the two circles of position may intersect in two points on the globe that are fairly near each other. A badly-chosen AP can lead you to the wrong one of those intersections. A third observation at a quite different azimuth will resolve the ambiguity. -- Bill