For example, if I were in Tangier, Morocco, and wanted to fly to Whangarei, New Zealand (the antipode on the globe) – wouldn’t it be about the same time to go up instead of across?
ETA: Thanks so much for the detailed explanations!
For those who are wondering why I picked Tangier/Whangarei, it was just a hypothetical! The-Minmus-Derp explained it perfectly: *Whangarei and Tangier airports are antipodes to the point that the runways OVERLAP in that way – if you stand on the right part if the Tangier runway, you are exactly opposite a part of the Whangarei runway, making it the farthest possible flight.*
In: 2270
When traveling long distances across the planet, what is the best?
If you were to slice the Earth into 2D circles, you can either do them through the center of the Earth (like longitude) or mostly missing the center of the Earth (like latitude.) We call the circles whose center is the same as the center of the Earth “great circles.” As it turns out, the absolute *shortest* distance between two points on a sphere is along a great circle. So to find the shortest route between two cities, you would draw a line from each to the center of the Earth, and then draw an arc between the two.
Of course, shortest is not always the fastest. Air movement also plays a role, and high-altitude air currents like jet streams can make air travel faster or slower. So even though you have found your great circle “shortest” route, you can make that route faster by using jet streams as an express lane, even if it means deviating outside your great circle a bit.
So now you’ve found the *fastest* route. But fast is no good if your plane crashes. You need to be fairly near to airports in case of emergencies, and try not to fly over any areas that are particularly dangerous. For this reason, you might modify your “fast” route to make the flight safer with just a little lost efficiency.
Next, you need to consider whether your flight is *legal.* Countries often designate “no fly zones” where civilian traffic is not allowed, and you probably want to stay away from military bases and countries unfriendly to yours who might not guarantee your plane’s safety (or even threaten to shoot it down.) So again, you need to modify your route to ensure it is legal.
Lastly, you need to consider traffic. Even though the skies are big, there are other planes there, and many are using the same routes. You need to make sure the route you are flying is clear so there’s little risk of running into another aircraft while taking your flight.
And this is what leads to the *best* routes: mostly based on great circles, but adjusted to account for things like air currents, safety, legality, and coordination with other flights.
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