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» Online Classroom   » Inland and Coastal Navigation   » Public Discussion of Inland and Coastal Navigation   » Great Circle & the TI-89 Titanium

Author Topic: Great Circle & the TI-89 Titanium

 - posted November 13, 2009 01:03 PM      Profile for chetco           Edit/Delete Post 
Why is it when you compute a Great Circle using the TI-89 Titanium it runs a course that heads south? Especially in a Great Circle course from Cape Town to Western Australia. What do you do to have it compute a "north" great circle?

Thank you.

From: oregon
David Burch

 - posted November 13, 2009 01:16 PM      Profile for David Burch           Edit/Delete Post 
It should compute in the direction you ask, as long as it is less than 180° distant.

the starting point is always the "DR position" and the destination is "Destination."

if this does not work, then please list the exact values you care about and how you entered them.

... unless of course i am missing something, which is not beyond me!

From: Starpath, Seattle, WA

 - posted November 13, 2009 05:45 PM      Profile for HHEW           Edit/Delete Post 
There may be a question of definition here.

The StarPilot program you're using calculates the shortest course between - to take your example - Cape Town and western Australia. By definition, the shortest course between two points on a sphere is the great-circle course.

For the longer course to the north, you'd use the Rhumb Line program - (1) on the **Route Sailings** menu.

One way visualize great-circle courses in the southern hemisphere is to to take two points on a globe on the same southern parallel. If you try to connect the two points by stretching a string to the north of them, you are trying to take a course across a fatter part of the globe. That course is longer. The string won't lie that way.

On the other hand, as you stretch the string between the two points it will automatically arc to the south. That's because you're stretching it across a part of the globe that is slimmer than the part across which the latitude parallel runs. That bowed course is the great-circle course.


 - posted November 14, 2009 07:04 AM      Profile for chetco           Edit/Delete Post 
Thank you, both. Gentlemen, I don't have a challenge understanding the Y's and WHATS of the GREAT CIRCLE, question came out of my wondering "what if I wanted to go north of the "rhumline" as compared to south. The calculator did exactly what is was supposed to. I was just wondering if there is a way a person could make the calculator compute a GREAT CIRCLE course on the other side of the RHUMLINE. I do I remember to include the (-) sign for South Lat, and W Lon.

Thank you,

From: oregon
David Burch

 - posted November 14, 2009 09:50 AM      Profile for David Burch           Edit/Delete Post 
pls tell me the lat-lon of your desired start point and desired end point. --david
From: Starpath, Seattle, WA

 - posted November 14, 2009 02:02 PM      Profile for chetco           Edit/Delete Post 
Cape Town, 33°55.11S, 18°26.33E, but remember you've got to get south of the Agulhus Banks. So I puy a waypoint in, 37°00S, 20°00E. Then to Geraldton, West Australia, 28°76S, 114°58E.

Dep -33°55.11, 18°26.33E
Waypoint -37°00, 20°00E
Dest -28°76, 114°58E

From: oregon
David Burch

 - posted November 14, 2009 07:20 PM      Profile for David Burch           Edit/Delete Post 
Here is what i get for about your coordinates. it is small, so use ctrl+ to increase font... or copy and paste to notepad.

the route should always point from the departure to the destination. this gives each leg after choosing 5° lon intervals.

StarPilot PC Saturday, November 14, 2009
Global Settings
DR Lat/Lon
Dest Lat/Lon
Times in GMT-00
Watch err=00:00:00
Max sights= 60
Temp(F)= 50
Pres(mb)= 1010
Mag Var= 000.0°
IC= 000°00.0'
HE(ft)= 10
Date= 11/14/2009
Max/Min/Max Mag
Document Untitled Page 1
Dist(nm)/Course(T)/Len(nm)/Next Lat/Lon
4675.52 117.0° 82.9 -36°49.9'/020°00.0'
4592.91 116.0° 261.69 -38°38.1'/025°00.0'
4332.16 113.0° 249.88 -40°09.0'/030°00.0'
4083.18 109.8° 239.96 -41°23.7'/035°00.0'
3844.09 106.5° 231.88 -42°23.0'/040°00.0'
3613.05 103.2° 225.57 -43°07.9'/045°00.0'
3388.29 099.8° 220.97 -43°38.9'/050°00.0'
3168.12 096.4° 218.02 -43°56.5'/055°00.0'
2950.9 092.9° 216.68 -44°00.9'/060°00.0'
2735 089.4° 216.94 -43°52.2'/065°00.0'
2518.85 086.0° 218.8 -43°30.3'/070°00.0'
2300.84 082.5° 222.28 -42°54.9'/075°00.0'
2079.37 079.1° 227.43 -42°05.4'/080°00.0'
1852.76 075.7° 234.31 -41°01.2'/085°00.0'
1619.3 072.4° 242.98 -39°41.4'/090°00.0'
1377.2 069.1° 253.51 -38°04.9'/095°00.0'
1124.61 066.0° 265.93 -36°10.8'/100°00.0'
859.63 063.0° 280.25 -33°57.7'/105°00.0'
580.39 060.1° 296.36 -31°24.4'/110°00.0'
285.09 057.4° 286.1 -28°46.0'/114°34.0'

From: Starpath, Seattle, WA
David Burch

 - posted November 14, 2009 07:31 PM      Profile for David Burch           Edit/Delete Post 
Actually.... with apologies, i just read carefully what you are asking for... the answer in NO it does not do that because there is no navigational significance to that route.

we have two types of routes to work with. the straight line rhumbline route on a mercator chart where the true heading remains the same at all times, and the shortest distance GC route.

Or in another light.... it might seem that since there is a CG route on one side of the RL that is shorter than the RL, then there might be a mirror image of it on the other side, which is also shorter than the RL, and that is simply not the case. It would be longer than both of them, with no tactical significance.

From: Starpath, Seattle, WA

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