View Full Version : Flying a great circle
Flying a great circle track, how would you work out if your true course would increase or decrease? Or does your true course remain the same? (Forget about magnetic).
Checkboard
2nd Sep 2001, 10:19
A flight which follows a constant True course (a track which makes the same angle with all meridians of longitude) is called a Rhumb Line. A Great Circle, the shortest distance between two points on the Earth's surface differs from a rhumb line, unless the Rumb Line also lies along a great circle (the Equator and the meridians of longitude are the only examples). Most Rumb Lines spiral towards the pole in a shape that has the mathematical name of "loxodrome", thus rhumb lines (excluding north/south tracks, and the equator) lie on the equator side of the great circle track, and are concave to the pole.
Who your true course will vary as you fly along a Great Circle track changes depending on which hemisphere you are flying in and which direction you are travelling.
tubby one
2nd Sep 2001, 13:10
your true course will vary as a function of your change of longitude
twistedenginestarter
2nd Sep 2001, 15:37
PFM
Not entirly clear what you are asking.
Great circle is always polewards of the rhumb line. From this you can work out what will happen.
If you are in the Northern Hemisphere and are setting out westwards, the GC will be greater (in terms of Track degrees) at the start and less at the end. Thus it will be decreasing as you proceed. Going eastwards from the same starting point the GC will be increasing as you go.
Thanks people,
Thats answered my question... :)
Transition Layer
3rd Sep 2001, 11:48
PFM,
Someone posted this link a while back - http://gc.kls2.com/
It'll give you a visual picture of what people are talking about, and clearly shows how the great circle track bends towards the poles.
Cheers,
TL
Tim Zukas
7th Sep 2001, 03:41
If you want the details:
Pretend for a moment that the earth is a sphere. Along any great-circle track, the sine of the course (azimuth) times the cosine of the latitude remains constant. So if you start from latitude 45 deg N on a course of 45 deg true the constant is 0.5-- so your course will become 90 degrees at latitude 60.
If we assume the earth is a spheroid, then substitute "reduced latitude" for "latitude" in the above (and "geodesic" for "great-circle").