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From: Jim Thompson (no email)
Date: Wed Aug 04 2004 - 17:14:11 EDT
Jim Thompson
www.jimthompson.net
Outgoing mail scanned by Norton Antivirus
-----------------------------------------
> -----Original Message-----
> From: Navigation Mailing List
> [mailto:]On Behalf Of Robert Gainer
>
> Chuck said,
> >The more traditional way of determining longitude was
> >to use a time sight at the time the sun crosses the
> >Prime Vertical (i.e., the time at which the sun is due
> >east or due west of you). This procedure is described
> >in Bowditch and elsewhere. It requires that you know
> >your latitude, which you can get from a noon sight or
> >from an observation of Polaris.
>
> Chuck,
> I don’t understand how that will work. The magnetic variation and the
> latitude must be problems in that method. If you are at 23 degrees north
> latitude or greater the sun is never due east or west. If you do not know
> the magnetite variation with some degree of accuracy wont that have a very
> large effect on the method? Is this practical at all?
> All the best,
> Robert Gainer
Robert, I really got stuck on that too when I was learning this stuff last
year. The key is to realize that the Prime Vertical Circle is not in the
terrestrial coordinate system. Let's see if my explanation stands the test
of this posting:
See the text and diagrams at
http://jimthompson.net/boating/CelestialNav/CelestNotes/Coordinates.htm#Hori
zonCoordinates
for a discussion of the Horizon Coordinate System, and
http://jimthompson.net/boating/CelestialNav/CelestNotes/Coordinates.htm#Link
ed
for an explanation of how the various coordinate systems are linked through
the PRINCIPAL vertical circle (not PRIME).
The intersection of the horizon coordinate system's PRINCIPAL vertical
circle with the celestial (and so terrestrial) equator defines the north
cardinal point in the horizon coordinate system. If you then go 90 degrees
clockwise from that point around the horizon, you reach the horizon
coordinate system's West cardinal point. See also Figure 1527a in Bowditch.
The vertical circle in the horizon coordinate system that goes through that
point is the PRIME vertical circle.
Now here's the tricky part, requiring a lot of reflection until it comes
clear in the mind: When a body is on the PRIME vertical circle relative to
your position, it is by definition also exactly west of you in the
terrestrial coordinate system. Bowditch' definition of the Prime Vertical
Circle includes this statement: "The intersections of the prime vertical
circle with the horizon define the east and west points of the horizon.".
For the practical application referred to by Chuck, see Figure 3 on
http://jimthompson.net/boating/CelestialNav/NoonSunSight.htm
titled, "Figure 3. Sun sight when the sun is on its prime vertical".
Jim
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