Interpolation in Pub.229

[Rumata]

Gentlemen,

In a pretty routine calculation I faced a problem which forced me to redo that calc three times. Let me explain myself:

Three sighting of Sun and three sightings of Moon. Regular fix. Supposed to be.  Except one nuance: The lat is a whole number  of degrees and 30 minutes. No problem.  The question is what App.Lat. to select: 30 minutes less or 30 minutes greater to get a whole number for the entry into Pub.229.  

Thank you

It makes no difference, really, as far as I can tell.  LOP is LOP and you need two or more to get a fix.  With a body observation to the south I would probably choose 30 minutes greater.  I try to make my AP such that ZN lays closest to my DR.  In other words I always try to place my AP such that my DR is between the body and the AP choice and as close as I can get it.  Where Im at (East Longitude) with a morning shot I try to use a smaller AP Long; with an afternoon shot I try to use a larger AP long. Another way to see it is that I almost always try to make HC less than HO when that ZN favors the DR.... if that makes any sense for you... :)  However, it doesnt always work out that way.

Just estimating the body's azimuth (ZN) at the time of observation can make AP choices a bit easier.  On the work sheet, lay a pencil down in the sun's general direction from the DR and you can easily judge which AP ZN choice will lay closest to your DR.  If estimating the sun's azimuth is too difficult, as is sometimes the case, then take a compass reading of your shadow and take the reciprocal after adjusting for variation.

joe

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Joe, thank you. But as you know, taking Ap too liberally will eventually make  d ( Hc-Hs) too big for a reasonable LOP (because Pub.229 requires Lat. input)  It was exactly what has happened in my case. Your explanation makes sense. Always taking Ap as close to your DR Lat.  as possible. But when it, DR Lat., is something like N39-30, then I think interpolation is needed for Hc, and Z  corresponding to 39-30, but not to 39-00 or 40-00. It was what I got.

When I tried  ( in different case) change Assumed Lat for calculating LHA for a less difference with DR Lat, ( let' s say GHA is 324-07.1. And DR Lat is let's say 05-05.6E.  Then the Lat. assumed is 5-52.9, to get whole number LHA.  Then the difference between Dr Lat and Assumed Lat. is hell too big.) results were not always satisfactory.

Anyway, you made the point.

Let me clarify one thing...  I didnt mean to imply that AP should be as close to the DR as possible - I meant to say that I like to choose an AP whose ZN lays as close to DR as possible..  One thing is not the other.

In your example you said the assumed lat is 5-52.9.  This is wrong because your assumed lat should be a whole number closest to DR - in your case you mean Longitude i think...  Anyway you can choose AP Long 4-52.9 instead to get you closer to DR if THAT ZN also comes closer to DR.  It appears in your example that you are talking about a late morning shot.... I would use the closest lat whole number north and the 4-52.9 for AP Long; this would put the DR between the AP and the body. (Im not sure what your Lat is)  Then your LHA would be 329.  Im thinking that unless your DR is near DEC your ZN will be something like 160 or 170.

Dont be afraid of big intercept numbers.  I recently had an intercept of 52.? that put me and the ZN within a mile or two of my exact Position using a Gunter quadrant variant.

good luck
joe

Joe, I think I didn't express myself clear enough. In my previous posts by a typo I put Lat, where it should be Long. Yes, you are right, for the assumed Lat. one needs to take the whole number. Any Publications requires the whole number for Lat. But, when you calculate LHA, which is in my case  LHA=GHA+Long., ( long.. is E and I need to get LHA as a whole number. So, when I tried to use 4 deg Long. instead of 5 deg. I got LOP so far away from two other Sun shots, one can't reach it with a  six feet pole. And, also, you're right, that I kinda afraid of big intercepts. When I got an intercept close to almost one degree-I think I did wrong.

Well, practice makes us sane. ;>

Thank you