Bad Astronomy: Misconceptions and Misuses Revealed, from ...

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much by definition you don’t make a circle there. You just spin,
making no eastward motion at all.
As you move north from the equator, you can see that your
eastward velocity decreases. At the equator you are moving nearly
1,670 kilometers per hour (1,030 miles per hour) to the east
(40,000 kilometers in 24 hours � 1670 kph). At Sarasota, Florida,
at a latitude of about 27 degrees north, you are moving east at
1,500 kph (930 mph), and by the time you reach Wiscasset, Maine,
at 44 degrees north latitude, you are moving east at only 1,200
kph (720 mph). If you brave the chill of Barrow, Alaska, you’ll be
at latitude 71 degrees north and moving at a leisurely 550 kph
(340 mph). Finally, at the north pole, you aren’t moving east at all;
you just make your tiny circle without any eastward movement.
Let’s say you stop in Sarasota, which is a reasonable thing to do,
given the climate there compared to Barrow. Now imagine someone
on the equator due south of your position takes a baseball and
throws it directly north, right toward you. As it moves northward,
its velocity eastward increases relative to the ground. Relative to
you, that baseball is moving 1,670 kph � 1,500 kph � 170 kph
(1,030 mph � 930 mph � 100 mph) or so to the east by the time
it reaches you. Even though the fastball is aimed right at you, it
will miss you by a pretty wide margin! By the time it gets to your
latitude, it will be a long way to the east of you.
That’s why cannonballs are deflected as they travel north or
south. When they are first shot from the cannon, they have some
initial velocity to the east. But if they are fired north, they reach
their target moving faster to the east than the ground beneath them.
The cannoneer needs to aim a bit west to compensate. The reverse
is true if it is fired south; the cannonball reaches its target moving
slower than the ground, and needs to be aimed to the east to actually
hit the target.
In our baseball example above, the distances and times involved
were large, letting the Coriolis effect gather some steam.
In reality, it’s a tiny effect. Let’s say you are driving a car north at
100 kph (60 mph) in Wiscasset, Maine. The Coriolis effect deflects
you by the teeny amount of 3 millimeters (0.1 inches) per second.