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John Karl and Andres Ruiz have been puzzled by the expression "pole" of a 
great circle. I take it that Frank is referring to a direction that is along 
the axis of that circle; or alternatively, the Geographical Position of such 
a direction, on the Earth's surface, 90� distant from every point on that 
great circle. If you think of a great circle as the rim of a bicycle-wheel 
around the sky, then a line prolonging the direction of its axle would be 
the direction of its two poles. If the great circle happens to be that of 
the Equator, then its poles would be the North Pole and the South Pole. A 
meridian circle has poles that are on the Equator, 90� from where the circle 
crosses the Equator

If the circle is tilted to the equator, such as the Ecliptic circle, so will 
its poles be. If you look at a decent star atlas or star globe, you should 
find marked on it both the line showing the plane of the Ecliptic tilted to 
the Equator, and also the two "poles of the Ecliptic". Those are the points 
that stay constant in direction, in space, while the whole coordinate 
system, including the Earth's poles, and the direction of Aries, slowly 
precesses around that axis, taking about 26,000 years to do it (for my 
fellow-pedants, let me admit that some complications have been evaded here).

Diversion: indeed, that's one of the virtues of using ecliptic latitude and 
ecliptic longitude to define star positions, as did Hipparcus and Ptolemy. 
Unlike declination, the ecliptic latitude of a fixed star never changes. 
Unlike Sidereal Hour Angle (or its inverse, Right Ascension), ecliptic 
longitude of all fixed stars changes steadily at about 50 arc-seconds per 
year. End of diversion.

 It doesn't have to be a great-circle, to have such poles; any circle will. 
For example, the small circles that make up the various latitude rings, all 
share common poles, the North and South poles. Tilted circles will have 
tilted poles.

The pole of a circle is a concept that isn't used much in navigation, though 
perhaps it could be.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message ----- 
From: "Andres Ruiz" 
To: 
Sent: Thursday, March 11, 2010 7:19 AM
Subject: [NavList] GC pole


Dear John,

I think Frank is talking about the vertex of the GC, it maximum and minimum, 
like the in a GC sailing:

*         Vertex B->max: dB/dL = 0

*         Node (B = 0), cross with the equator

where B is the latitude and L the longitude

regards,

---

Andr�s Ruiz

Navigational Algorithms

https://sites.google.com/site/navigationalalgorithms/ 




PS: Recently I have a copy of your book "Celestial Navigation in the GPS 
Age", a good reading!!!



________________________________

De: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] En 
nombre de John Karl
Enviado el: mi�rcoles, 10 de marzo de 2010 19:30
Para: NavList@fer3.com
Asunto: [NavList] Re: Star - Star Observations

Frank,

To understand the Lord Ellenborough trick, It seems that I need to know what 
the Pole of a great circle is. I've never come across this term,

Also, what do the two asterisks mean (as in *pole*)?

JK

   

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