Great circle navigation in the southern hemisphere

Hey everyone,
I am just casual fan of all things maritime, but I also have a side hobby debunking flat earth videos on YouTube.

As great circle navigation is powerful evidence that we live on a globe, I like to point this out to flat earthers. Great circle routes in the southern hemisphere are particularly difficult to reconcile with the proposed flat earth model.
Anyhow, as part of a case study I’m working on, I’m tracking a ship on Marine Traffic, a bulker called Panormos that is currently sailing from Port Kembla, Australia to Caldera, Chile.
The predicted great circle path based on my own reckoning would havr the ship go near the 60th parallel south before bending back north. However, it only went down to about latitude 43 or 44.
Would this be to avoid ice and treacherous conditions? Or are these more favorable currents further north? Just wondering why the ship is taking a longer route than is available.

She will be on a composite great circle. Google it for more info but basically you use it to gain some of the advantages of a GC but avoid bad weather, ice etc.


Good luck convincing flat earthers.


You can never win. But it is a fun intellectual exercise to unpack all of the errors and misrepresentations of their claims. Since they don’t accept any evidence from our space programs (those being a hoax), you have to look to many other real world things that have been known for centuries. Navigation is a great place to start.


It is quite normal to limit how far south you go due to weather this time of year.

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Great circle routes benefit airplanes the most. Not much of a savings on ships, all things considered.

Why is that?

Airplanes can fly over continents, above bad weather regions and icebergs, and even over the North Pole.

For ships, only the very long transoceanic E<>W routes in the mid-latitudes are interesting (e.g. North America to Europe or East Asia).
Near equatorial W<>E routes or N<>S routes have only small advantages, if ever.

You’re taking a savings of maybe a couple hundred miles which is great, however it may be more beneficial to take the longer route to avoid weather, currents etc.

An airplane isn’t really effected much and the fuel burn is tremendously greater.

Having ECDIS has decreased the advantages of using a rhumb line. Without the requirement to plot on a paper chart changing from RL/GC or back is just a matter of one click on a pull-down menu. For the watch-standers there is little difference as they are just following the track.

Easy access to the weather routing program combined with the ECDIS has added a lot of flexibility to how we route transoceanic.


Yes indeed, even for the shortest routes.
The processor needs about the same time to calculate a rhumbline or a great circle.
The permanent bearing of the rhumbline is just in theory; it is updated as often as a GC course, to correct for wind, wave and current drift, compass errors or steering precision.

However, here the question from a non-navigator was about disproving the ‘flat earth theory’ with great circle sailing. I hope that he understood my basic explanation.

What I do not know:
Does ECDIS calculate a GC on a mean spherical earth, or a Geodesic on an ellipsoid earth?

Yes, you’re right. My post was irrelevant really as far as the topic here. Also not really even correct as we still use rhumb lines on short legs same as always.

As far as the GC an ellipsoid is used.

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Thank you everyone for your feedback. I am going to assume that the reasons for not maximizing the great circle route for Panormos was due to weather and currents. In the coming weeks, I hope to put together a YouTube video comparing the Panormos track on a globe versus a flat earth map. On the flat earth map (an azimuthal equidistant projection centered around the north pole), the course would go north and follow the coast of North America before arcing south down to Chile. I think we can all agree that this would be VERY long route, haha!

Also, it would appear that most of you here do not believe that the earth is a flat plane. This contradicts several commenters on flat earth channels who assure me that it’s an open secret in maritime circles that the earth is flat, and that no curvature is accounted for in navigation. Not that I believed them, but it was hard for me to get good information for real mariners. Glad I discovered this forum! Thank you.

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Bwahahahahahaha! :rofl::rofl::rofl::rofl::rofl:

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Fancy that. A group of crackpots versus a group of mariners who studied spherical trigonometry don’t see eye to eye.

I don’t really need to do the math to know that the difference between a great circle sailing and a rhumbline for a 1500 mile stretch will cut substantial time off of my ETA on the order of 8 to 10 hours. This means nothing to a flat earther since, at least from what I saw on that Netflix doc, they mainly get off on feeling their smarter than everyone else, most of all the scientists. It’s a little disturbing to watch in practice.

I can say I have yet to meet a flat earth ship’s officer. Maybe in the engine room…:wink:

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On NY to London, the difference between the two routes is only 123 NM. So a 1500 NM stretch would only save you half of that, or 62 NM. You’d have to be travelling pretty slowly shave off 8-10 hours. Unless your route was high-latitude.

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Thanks. I forgot to put the “+” after “1500”. Short distance sailings are not advantageous for great circle. My point was that the flat earth theory doesn’t hold up to speed, time, distance calculations between the two sailings. We live on an oblate spheroid and that is a fact.

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Flat earth Twitter is the best Twitter. Apparently there is a giant wall around us. You cant see it when flying. Also, try explaining height of eye to them. That’s good for several hours of hitting your head against a wall.

From my own experience of a few hundred transatlantic trips, I would say that GC or RL are not your top considerations when planning your ocean passage, typically between Lands End and your destination on the east coast, or the other way around. Your top considerations would be the weather you are likely to encounter, particularly the predominant direction of swell and wind on that specific trip. In summer, you would also consider that your GC courses could take you into ice zone, as you can see from the International Ice Patrol charts and iceberg limits. You would want to keep out of trouble by just following RL over GC. As regards weather, encountering head-swell for a couple of days would not only wipe out all the savings in distance that you hoped to make from your GC plan, you could also lose even more time if you have to reduce your speed to alleviate hull-stresses from pitching and pounding. As Captain, I would worry more about these factors than geometrical difference between GC and RL distances. GC tracks take you into higher latitudes and you are more likely to encounter dense patches of fog, which would also weigh on my decision of taking GC route. A prudent Captain would want to slow down to a safe speed in dense fog.
Please don’t get me wrong. I am not disputing anyone’s logic in favor of cutting the distance by following GC route over RL. As Captain, I should make every effort to save the distance and fuel. But I also need to be pragmatic about the ‘real’ impact of my decisions on the performance of the voyage in terms of making my ETA and so forth. For the nautical students who may be reading these posts, it may be a good idea to consider composite GC over a long stretch of ocean passage such as South or North Pacific. I would admit that my knowledge could be outdated as I have switched my profession a long time ago.

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Increased forecast accuracy and having weather routing software on the captain’s desk and wheelhouse has changed the game.

The framing of GC vs RL seems a bit crude to me. It doesn’t seem like two problems anymore, that of saving time and fuel vs weather, finding the optimum route seems like one problem. Once the route that optimizes the applicable parameters is found converting that route into a usable track is simplest part of the problem.

Also the ECDIS makes it quicker and easier to change the route when the situation changes.

Of course all the usual caveats apply, tools shouldn’t be used without understanding, objects in the mirror appear closer and so forth.