Why do airplanes fly in an "arch" path?

Question

When airplanes fly over water they fly in a sort of arch path, why don't they just fly in a straight line?

Does it have something to do with the Earth's shape or rotation?

Answer 1

It has to do with the distortions of some maps.

Look at the direct flight path on a globe.  

Answer 2

They do fly in a straight line, but the line is only straight in three dimensions, on the surface of a sphere such as the Earth.  When this straight path is projected onto a flat map of the Earth, it often looks curved.  It's just an illusion created by trying to show a round Earth on a flat map.

Routes that are exactly north-south usually appear as a straight line, and so do routes that are exactly east-west along the Equator.  Everything else will look curved, with east-west travel near the poles having the most curved appearance of all.

Answer 3

That arc is known as the "great circle" route.  You're exactly right - it does have to do with the shape of the Earth.  It's actually the shortest path and it is a straight line, but it appears to be a longer arc on a flat map.  If you use a globe and string to connect any two points, you will see the shortest distance is an arc - the same path you see on a trans-oceanic flight track.

Edit: The modern way for visualizing the great circle route is to use Google Earth.  Use the ruler tool between two points.  If you rotate to globe to look at the line straight on, you'll see that it is indeed a straight line.  If you rotate the globe so it is centered on the equator with north up like most maps, you'll see the arc.

Answer 4

If the flew in a straight line, they'd have to go underwater.

Think about two cities on opposite sides of the Atlantic. Visualize a globe in your head. A straight line would go under the ocean and through the outer core of the Earth. Planes can't do that ... yet!

Answer 5

the earth is round [most people say]  if its flying at 40,000 feet it really is going in an arch...but pilot thinks hes going straight cause hes staying at 40,000 ft

Answer 6

Most flat maps are Mercator projections, on which Great Circle routes appear as curved lines that arch toward the north in the northern hemisphere and toward the south in the southern hemisphere.

To visualize a great circle route, get a length of string and stretch it across a globe from departure point to destination.  Note where the string passes on its route.  To do a scientific job of it, note the latitude at which the string crosses each parallel of longitude.  Now plot these points on a Mercator projection, and you will see the "arch" shape you describe.

alternatively, you can go to the library and ask the librarian to help you find information on great circle routes and map making.

Answer 7

It is called the Great Circle Route and it can be prove on a globe with no reference to latitude or longitude.

Just put the string at two points at the same latitude, distance from the equator, then notice there is slack in the string if you move it up.  (If you did this south of the equator you couldn't get the string to stay on the surface)  That means the shortest route is to fly in a northern, or southern if you're below, arch.

It makes a little more since when you realize the shortest distance between Tokyo and New York is over the pole.