# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Great Circle Course via calculator & HO 208
From: Bill B
Date: 2016 Oct 5, 01:33 -0400

```On 10/4/2016 11:38 PM, David Fleming wrote:
> In any event the path connecting ponts 1 degree of longitude at 41N are
> not significantly different fron a great circle path.

Seriously? The small circle at parallel 41 is "not significantly
different fron a great circle path?"

"A great circle, also known as an orthodrome or Riemannian circle, of a
sphere is the intersection of the sphere and a plane that passes through
the center point of the sphere. This partial case of a circle of a
sphere is opposed to a small circle, the intersection of the sphere and
a plane that does not pass through the center. Any diameter of any great
circle coincides with a diameter of the sphere, and therefore all great
circles have the same circumference as each other, and have the same
center as the sphere. A great circle is the largest circle that can be
drawn on any given sphere. Every circle in Euclidean 3-space is a great
circle of exactly one sphere." --Wikipedia

"A great circle is the largest possible circle that can be drawn around
a sphere. All spheres have great circles. If you cut a sphere at one of
its great circles, you'd cut it exactly in half. A great circle has the
same circumference, or outer boundary, and the same center point as its
sphere. The geometry of spheres is useful for mapping the Earth and
other planets. The Earth is not a perfect sphere, but it maintains the
general shape. All the meridians on Earth are great circles. Meridians,
including the prime meridian, are the north-south lines we use to help
describe exactly where we are on the Earth. All these lines of longitude
meet at the poles, cutting the Earth neatly in half. The Equator is
another of the Earth's great circles. If you were to cut into the Earth
right on its Equator, you'd have two equal halves: the Northern and
Southern Hemispheres. The Equator is the only east-west line that is a
great circle. All other parallels (lines of latitude) get smaller as you
get near the poles. Great circles can be found on spheres as big as
planets and as small as oranges. If you cut an orange exactly in half,
the line you cut is the orange's great circle. And until you eat one or
both halves, you have two equal hemispheres of the same orange. Great
circles are also useful in planning routes. The shortest path between
two points on the surface of a sphere is always a segment of a great
circle. Plotting great circles comes in very handy for airplane pilots
trying to fly the shortest distance between two points. For example, if
you flew from Atlanta, Georgia, to Athens, Greece, you could fly roughly
along the path of one of Earth's great circles, which would be the
shortest distance between those two points. When planning routes,
however, pilots have to take other factors into account, such as air
currents and weather. Great circles are just general paths to
follow."--National Geopraphic

```
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