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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
 chetco posted November 13, 2009 01:03 PM                   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                   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
 HHEW posted November 13, 2009 05:45 PM                   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.
 chetco posted November 14, 2009 07:04 AM                   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                   pls tell me the lat-lon of your desired start point and desired end point. --david From: Starpath, Seattle, WA
 chetco posted November 14, 2009 02:02 PM                   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°26.33EWaypoint -37°00, 20°00EDest -28°76, 114°58E From: oregon
 David Burch posted November 14, 2009 07:20 PM                   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.code:`StarPilot PC Saturday, November 14, 2009Global SettingsDR Lat/Lon-36°13.0'018°28.0'Dest Lat/Lon-28°46.0'114°34.0'DR Mode OFFDIP Mode NORMALTimes in GMT-00Watch err=00:00:00Max sights= 60Limb LOWERTemp(F)= 50Pres(mb)= 1010Mag Var= 000.0°IC= 000°00.0'HE(ft)= 10Date= 11/14/2009Max/Min/Max Mag085°00.0'010°00.0'2Document Untitled Page 1Dist(nm)/Course(T)/Len(nm)/Next Lat/Lon4675.52 117.0° 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                   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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