See how the points equally distant from the top and bottom are rotating in the same direction and in the same speed? That only works if you’re standing outside of the sphere. If you’re standing on the bottom half’s surface, as you would be in the case of a planet, everything flips over. People in Canada and New Zealand both look “up” to see the sky. Up is a relative direction. However, those people all agree on what the directions North, South, East, and West are. These items do not change regardless of where you’re standing. North is always toward the “north pole."
To measure the Coriolis Effect, you don’t work from the top of the Earth to the bottom. Instead, it’s measured from the North Pole in the Northern Hemisphere, and the South Pole in the Southern Hemisphere. This means that while both halves of the globe are rotating in the same direction, the Coriolis Effect acts in one direction in the northern hemisphere and the other in the southern hemisphere, as illustrated in the accompanying figure. To make things even more confusing, the Coriolis Effect doesn’t act in a uniform manner.
There are two things that determine how much the Coriolis Effect alters an object’s course: how fast the object is moving, and its latitude. The closer you get to the equator, the less the item deflects, and in fact directly on the equator you don’t have this effect at all. (This is one of the many reasons that space launch points are typically close to the equator.) But of course we’re not talking about the space shuttle here. We’re talking about weather and weather systems. It still matters.
Let’s say I make five paper airplanes and stand five different places on a planet, each on the same longitude line, as you see in the image.
Now, let’s ignore the fact that I shouldn’t be able to do this for the moment--I mean, just how far can a mere mortal throw a paper airplane? But if I aim to throw the plane along the lines that I’ve drawn in black the X on the other side, throwing each due East at the same speed, all but 1 will miss its X. Planes 1 and 5 will look like they’ll miss their mark the worst to anyone out in space, because they are farthest out from the equator. If you’re watching from on the planet everything looks just fine.
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