If a cannonball were fired from Arizona to Montana it would curve toward the right because of the Coriolis effect. If it started out going due north it would end up northeast of where it started (blue arrow). If you were trying to hit a target due north of you then you would have to point your cannon toward the northwest (yellow line).
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Map courtesy of the Perry-Castaneda Library at the University of Texas
The Coriolis Effect
Let's pretend that you're on the top of a mountain in
Arizona and you have a cannon. Let's make it a game; the object of the game is to to fire a cannonball and hit a flag pole that's on the top of
another mountain straight north of you in Montana, almost a thousand
miles (1600 km) away. Which way should we point our cannon?
You might think this is easy - if the flag pole is north we should
point our cannon north, right? This is a good guess but it won't really work. Why? Because the Earth is spinning. The Earth's rotation would make it look like your cannonball was curving to the right so you'd miss the flag pole! This is called the Coriolis effect.
So what should you do? If you point your cannon a little bit toward the northwest, then when it curves to the right it will head right for the flag pole!
The Coriolis effect makes things look like they're curving toward the right in the northern hemisphere and toward the left in the southern hemisphere. This doesn't just work with cannonballs, it also works on winds and ocean currents. The Coriolis effect is what makes hurricanes spin around. It's also important on other planets, in stars, and in space.
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