Motion on a Globe

    Before looking at a rotating globe, we should take a moment to consider how movement works when you're forced to stay on a sphere, rather than being free to fly off in a straight line (in other words, our assumption of Newton's First Law breaks, because there's an unbalanced force acting on things). Whether you're contained between two spherical shells or held down by gravity, there's going to be a force pointed directly at the center of the sphere. Now, this force won't change the speed of anything, but it will bend the direction of motion into a circle. Thus, a "straight line" on a globe is really what's called a "great circle", a circle with its center at the center of the globe.
    Important note: The discussion on this page assumes that the only force of consequence on an object is one directed towards the center of the sphere. We're ignoring everything else for the moment. In terms of movement on the Earth itself, this means that only things that are out of contact with the ground and unaffected by air resistance will really follow a great circle. And even air itself is affected by other forces, so things do get kinda complicated in any real life application.

    For the sake of this page, we're going to ignore rotation, and divide motion into North/South and East/West. (If you have a problem with defining "North" on a non-spinning globe, this page may help.)
    North and South are easy. If you head due North or due South, you're already following a great circle...longitude lines (see the globes below) are circles with their centers at the center of the Earth. However, the East/West lines of latitude are not generally great circles. The equator is the only latitude line that has its center at the center of the Earth, all the others are centered on the axis of Earth's rotation, but not at the actual center of the planet.

Great Circle in the Northern Hemisphere.
    Say you're on the U.S./Canada border. If you look due East or due West, you'll be looking along the blue line marked out in the picture. If you were to follow a map and head due East or West, you'd also follow that line of latitude. But if you were to fire a (really fast) bullet East or West, it would follow a section of the great circle marked out by the red rubber band.
    It would not continue to go due East or due West. Even if the Earth wasn't spinning at all, an Eastbound shot would curve to the right, and a Westbound shot would curve to the left, because unless you're at the equator, neither East nor West is defined as a great circle. Any Coriolis effect has to be considered as modifying this're not getting a "straight line" in any case.
    However, if you shot due North or due South on this non-spinning globe, the shot wouldn't deflect from that path, since it's a great circle already. You'd also get an undeflected shot to the East or West at the equator.
    Any direction other than one of the four cardinal directions (North, South, East, West) would require breaking the motion into North/South and East/West pieces, then applying any drift as needed. So a shot to the Northwest would curve to the left because of the Westward part.

Great Circle in the Southern Hemisphere.
    Same sort of thing, now in the Southern Hemisphere. Someone looking East or West in the Indian Ocean would be looking along the blue line, and sailing with the aid of navigation systems could follow that latitude, always facing East or West. But anything just sent flying East would be expected to curve to the left, and anything sent flying West would be expected to curve to the right.

    Now, keep in mind, this is just the effect of being on a sphere. Even without spinning, we already have some weirdness to contend with. Adding in spin will just make it violate our intuitive assumptions even more.

Back to the Main Coriolis force Page.