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From: Jim Thompson (no email)
Date: Thu Sep 02 2004 - 08:21:16 EDT
Bill,
I found that the key for me was to spend time learning the coordinate
systems carefully, early on in my training. I still keep going back to the
basic definitions and diagrams to remind myself of the terminology and
relationships. You might find these images and explanations helpful, maybe:
Bowditch's glossary always keeps me on track:
http://www.irbs.com/bowditch/
http://pollux.nss.nima.mil/pubs/pubs_j_apn_sections.html?rid=187
www.marineplanner.com/bowditch/bowditch.cfm
Upper branch. That half of a meridian or celestial meridian from pole to
pole which passes through a place or its zenith.
Lower branch. The half of a meridian or celestial meridian from pole to pole
which passes through the antipode or nadir of a place.
See Bowditch Figure 1524a.
You can see a body crossing the lower branch of your meridian from your
position in certain situations.
(I have a hunch that the terms "upper" and "lower" come from class-conscious
European culutures pre-1900, when everybody on the same side of something
was "upper" crust, and all the rest were "lower"?)
Coordinate Systems in CN:
http://jimthompson.net/boating/CelestialNav/CelestNotes/Coordinates.htm
(in particular the section on the Navigational Triangle near the bottom,
where there are a couple of diagrams for meridian crossings of the upper and
lower branches.)
Meridian Transit sight reduction:
http://jimthompson.net/boating/CelestialNav/CelestNotes/SightReduction.htm#M
eridianTransit
Noon Sun:
http://jimthompson.net/boating/CelestialNav/NoonSunSight.htm
Jim Thompson
www.jimthompson.net
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