NavList:

   

Reply

I have looked at Jim Thompson's new diagrams and the
2002 thread posts by Paul Hirose and others.  Below I
have summarized what I think I'm reading.  Can you let
me know if I have indeed understood?

Due to the oblateness of Earth, the plumb line differs
from the actual center point of Earth by a
navigationally significant amount, up to 12 minutes of
arc.

But this fact does not effect the practice of
celestial navigation because the lat-lon grid itself
and the Almanac's GP data are based on the plumb line.

The local plumb line may differ by additional amounts
as well, called deflections, due to surface anomalies,
but not by a navigationally significant amount.

-Jeff S.


--- Jim Thompson  wrote:
> Jeff, While waiting for Frank's reply, I wrote this
> as a direct result of
> his post (and others), seeking the same "why", I
> think:
> "Geodesy: The Earth is neither flat nor spherical,
> and the center of the
> earth is not where you thought it was"
>
http://jimthompson.net/boating/CelestialNav/CelestNotes/Coordinates.htm#Geod
> esy
>
> Jim Thompson
> jim2@jimthompson.net
> www.jimthompson.net
> --------------------
> Outgoing email scanned by Norton Antivirus
>
> > -----Original Message-----
> > From: Navigation Mailing List
> > [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On
> Behalf Of Fried Squash
> > Sent: Wednesday, February 09, 2005 10:36 AM
> > To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> > Subject: Re: Plumb-line horizon vs. geocentric
> horizon
> >
> >
> > --- Frank Reed  wrote:
> > > I don't know if this was mentioned this time
> around,
> > > but it's worth
> > > repeating. Local horizontal coordinates, as
> already
> > > discussed, are defined by  the
> > > plumb-line. The zenith is opposite the direction
> of
> > > local gravity and  therefore
> > > the horizon of the coordinate system is
> essentially
> > > parallel with the  sea
> > > horizon. How much difference would it make if we
> did
> > > it differently? If we
> > > defined the zenith to be opposite the direction
> to
> > > the Earth's exact center,  where
> > > would the horizon be? If you work it out, the
> > > difference is just about
> > > (12')*sin(2*Lat). At 45 degrees latitude, the
> > > direction to the zenith would be
> > > fully 12 minutes of arc shifted if we defined
> the
> > > coordinates in terms of the
> > > Earth's center. That's a substantial difference
> from
> > > the point of view of
> > > sextant observations.




__________________________________
Do you Yahoo!?
Yahoo! Mail - now with 250MB free storage. Learn more.
http://info.mail.yahoo.com/mail_250

   

Reply