Answer
See diagram. The horizontal arrows indicate the velocity of the Earth's surface north and south of the equator.
The greatest tangential speed is at the equator because it is farthest from the Earth's rotational axis.
When a cannonball is launched north or south, its east-west velocity component is the tangential speed of the Earth at the launch point. It is similar to throwing a ball straight out the window of a moving train; it will have a component of velocity along the train tracks, equal to the train's speed.
If the cannonball is aimed toward the equator, the target (i.e., the equator) is moving east faster than the launch point, and so the cannonball will land west (i.e., "behind") the point it was directly aimed at.
The name for this is the Coriolis effect.
