Why are Great Circle routes the shortest flight path? Singapore Airlines' Singapore to New York flight explained

Passengers aboard Singapore Airlines flight SQ24 are often surprised to find themselves flying over the ice-covered forests of northern Alaska. Doesn't make sense, right? Singapore is about as close to the equator as you can get.

On a map, a straight line to New York would cross the Pacific Ocean, passing just north of Hawaii, cross mainland US at Los Angeles and on to New York City. But that's not the route SQ24 travels.

The track is north-east across the Pacific, passing just east of Japan, over the Aleutian Islands, Alaska and Canada before dipping down to New York City. At its furthest point from the equator, SQ24 will touch 65 degrees north latitude, close to Fairbanks, Alaska, and about one degree short of the Arctic Circle. That's because the aircraft is following a great circle route, the shortest distance between two points on the Earth's surface.

Confusion arises because we're used to seeing the world in two dimensions, on a map, but the Earth is a sphere. A straight line connecting two points on a sphere becomes a curved line when represented on a map. Therefore the shortest distance between Singapore and New York on a map is a curved line rather than a straight one.

Another way of looking at it, SQ24 takes about 18½ hours to fly from Singapore to New York City. If you were to fly from Singapore along a straight line to NYC on a map, an 18½ hour flight would only get you about as far as Denver, Colorado, and a long way short of NYC.

There are plenty more examples of great circle routes that don't seem to make sense. QF63 for example, the non-stop flight between Sydney and Johannesburg, travels deep into the Southern Ocean. In about mid-flight the aircraft passes close to the Kerguelen Islands, at 49 degrees south.

Similarly, flights between Australia and South America dip toward Antarctica. If you were to fly the shortest route from Perth to Santiago or Buenos Aires, you'd cross Antarctica, and it's a great shame that no airlines fly that route regularly (though Qantas did as a one-off repatriation flight from Buenos Aires last year - see below).

See also: Longest flight in Qantas' history flies over Antarctica en route to Australia

See also: Hour by hour: My epic trip on board the world's longest flight