# NavList:

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

**Re: Great Circle Course via calculator & HO 208**

**From:**Gary LaPook

**Date:**2016 Oct 3, 02:56 -0700

I guess nobody noticed. I posted a little experiment for non-navigators so that they could use Google Earth to get a sense of the difference between a rhumb lilne and a great circle course.

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Here is the answer to my little experiment. The great circle course is only 5,644.18 NM long

while the rhumb line between the two points is 7,591.45 so you would save almost 2,000 miles

using the great circle. This is obviously an extreme case that I chose to illustrate this point. The

initial great circle course leaving Milwaukee is almost straight north, 005̊° T and the final course

is almost straight south, 175̊° T. On the way the course changes through all the intermediate

directions, 6° then 7 then 8°.....then 90°.....then 120°....then, finally 175.°

The rhumb line never changed, being 90.000000000000000000̊°.

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Loking at this again I figured that if the starting point and the destination were on opposite mericians then you would need only one waypoint for the great circle, 90 north, and in that case the course would be a great circle and a rhumb line. Since the latitudes of the departure and destination is the sam, 43 degrees north, the the distance, if the initial course was set as straight north up to the north pole and then straight south to the destination the distance would be 2820 nm to the pole and then an additional 2820 nm from the pole back down to 43 degrees north, a total of 5640. (90-43 = 47 degrees; 47 degrees x 60 nm/degree = 2820 nm; 2820 nm x 2 = 5640 nm.) Anybody see the problem? The routing to the pole and then to 43 degrees north is not a great circle because the difference in longitudes was only 173 degrees so the Google Earth great cirsle distance should be less than 5640 nm but ts is given as 5644.18 nm, 4.18 nm longer than flying rhumb line courses to the pole and then to the destination. If we do the clasical computation ourselves we find that the great circle distance between those points is 5626.26 nm so the Google Earth distance is 17.92 nm longer than our standard calculation shows.

Does this mean that Google Earth uses a more sophisticated algorith for it's computation than the one used by navigators for centuries?

Frank, help me on this.

gl

g