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Author
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Topic: Starfinder to determine GC initial bearing etc
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Nireland
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posted December 19, 2023 02:39 PM
Following the interesting item on U tube. There seems to be a limit on this practice. Where a trip, (say New York to Hong Kong by air) ends at a point on the other side of the world, that is diametrically opposite the 0°Aries departure line -- then it seems to me that a lower latitude destination like HK at some 22°N can not feature under the blue disc (any of them !). Then all opportunity to raise the required great circle measurements is lost.
Does this interesting method truly have such geographical constraints.
From: causewaykayak
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Nireland
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posted December 19, 2023 02:53 PM
As I am in an Internet hole on the Irish coast, perhaps anyone with a view or able to give instruction on this apparent lacuna might email me directly at [email protected]
The card or limited access i was granted to make this post expires in hours so I might well not be able to recover answers from this forum.
Thank You . John, on the Giants Causeway Ireland.
From: causewaykayak
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David Burch
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posted December 19, 2023 09:46 PM
The initial examples were for distances less than 5400 nmi (90º) so they were easy. The blue templates cover 90º. We can do longer distances but it is a bit more work.
For example, going (by airline) from NYC (40.7, -74.0) to Hong Kong ( 22.2, 114.3). The Lon difference is 171.7 headed west, or 188.3 headed east, essentially on opposite sides of the earth, meaning GC route is up over the pole.
Start by setting up as explained leaving from NYC, and then draw in the meridian at the Lon difference and use the red template to mark the lat of HK, as shown.
Then place the 45N template centered on NYC and draw a short border where you run out of blue disk as shown, and mark the the approximate heading intervals in the directon of HK. Then move the template to the HK side as if you were gong HK to NY, and from this you can then figure either direction and the distance.
The distance will be about 90º on the NYC side and about 26º or 27º on the HK side for a total distance of 116 x 60 = 6960 to 117 x60 = 7020 and the initial heading will be about 350 from NYC or 007 from HK.
The actual answer is 7002 at 351.
From: Starpath, Seattle, WA
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David Burch
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posted December 19, 2023 09:54 PM
Here is a closer view of the action area.
This is showing a straight line for the headings, but when you do it and follow the curves is is closer to 350.
From: Starpath, Seattle, WA
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Nireland
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posted December 20, 2023 03:31 PM
Many Thanks for this supremely prompt reply.
I have been way too timid in my approach.
Of course, the symmetrical curve of the Great Circle! Now I could kick myself for not approaching this problem from both ends.
This whole business is delightful in it's elegance. Thank you for bringing a new dimension to us.
John Ruston
Ballycastle
County Antrim .
From: causewaykayak
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David Burch
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posted December 20, 2023 05:15 PM
By the way, this device from 1921 (2102-A) is probably the only such thing still in use today... although what we are using now (2102-D) is more like the 1939 version.
From: Starpath, Seattle, WA
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