Willy - It's a little hard to visualize but ... the Earth rotates from West to East. When you're shooting toward the West, the bullet is no longer coupled to the earth and what is happening is that the West target is moving toward the bullet while the bullet moves toward the target (exact closing speed depends on latitude ... the effect is greatest at the equator). As a result, the bullet hits the target sooner than if the Earth wasn't rotating and therefore it doesn't have as much time to drop as far ... hence the up error. When shooting at an East target, the bullet takes longer to hit it because the target is moving away from the incoming bullet. The bullet has more time to drop ... hence the down error.
(sorry about the extra post but I once again got bit by the comment character limit)
(sorry about the extra post but I once again got bit by the comment character limit)
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