Sunday, February 18, 2007

Going in (Great) Circles:

On a spheroid like the Earth, it is a well known fact that the shortest path between two points lies on a Great Circle. What is not so well known is that this same Great Circle also defines the longest distance between those two points. The longest distance is simply the arc going the other way.

So, my question was: "what is the longest great circle route one could fly on Earth and always be over a) land, and b) water?"

Thanks to Google Earth and its Ruler/Path tool, I found the following candidates:

Over Land: Beach near Dong Hoi, Vietnam to Dakar, Senegal - 12,900 km

Over Water: Tellicherry, India to Iliamna Bay, Alaska - 29,000 km

You will note that for the overland route, I did not count small lakes as being over water. The key to this route is threading the arms of the Red Sea and the southern Mediterranean beach near Rafa. If you slightly bend the "rules" and ignore the arms of the Red Sea, you can get a much longer route, Magadan, Russia to the Northern Namibian coastline (Hoarusib Mündung/Delta) of 14,620 km. This route is constrained by the Caspian and Black Seas.

For the overwater route, I did not count very small islands as being over land. Getting around Antarctica is the key to these routes, and threading various narrows. This is where you will have to use the 'path' tool rather than the 'line' because these distances are farther around than the antipodal point. An early candidate of mine was Pulau Dramai, Papua New Guinea to Seminole Shores, Florida - 24, 460 km.

Have at it folks!



At 6:47 PM, Anonymous Anonymous said...

I discovered this site last Wednesday night (12 March 2008). I was intrigued to learn of the Tellicherry to Iliamna Bay route. I plotted it out on a great circle program that I wrote recently. From 11°27'N / 75°19'E to 59°37'N / 153°35'W is indeed 29,000 kilometers the long way around. The only place I found iffy along the circle was near the tip of the Antarctic peninsula, but that's no big deal.

If my programming is correct, from 24°00'N / 64°45'E (near Karachi, Pakistan), on an initial heading of 210.6 degrees true, there is smooth sailing all the way to 60°00'N / 166°00'E (near Kavacha, Siberia). That is a distance of 31,700 kilometers. You might have to do a little wiggling to get through the Comoros and South Shetland islands, but that's within the rules. Besides, from 24°00'N your starting point can be anywhere between 62°30'E and 67°15'E and still clear both the Mozambique Channel and the Drake Passage. That gives plenty leeway for any necessary adjustment.

The length difference between the two routes is 2,700 kilometers which is about nine percent. I consider that significant enough to report it here. I would be happy if others would verify the route or disprove it. If my math is faulty, I would rather learn about it here than on some reef in a storm at night.


P.S. I hope your browser displays the degree symbol correctly. On some machines it shows up as a weird character or isn't displayed at all.


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