Following the long range shooting series? Here’s the next installment of external ballistics theory: the Coriolis Effect. On Bullet Trajectory Part 1 and part 2, I talked about Coriolis Effect as a variable which affect the bullet flight both on the horizontal and the vertical plane of the trajectory. But what exactly is Coriolis Effect?

When talking about ballistics, the Coriolis Effect refers to the deflection on the trajectory of the bullet generated by the spinning motion of the Earth. Its effect is negligible at medium distances, but becomes important around 1000yds and beyond, especially because it can add to other minimal errors and keep you off target.

Coriolis effect affects everything not firmly attached to the Earth’s surface. It affects fluids, like air and water, as well as floating and flying objects like ships, airplanes and… bullets.

Despite being associated with Coriolis, the phenomenon that actually affects the vertical component of the trajectory is called Eötvös Effect. The rotation of the Earth generates a centrifugal force, the same that pushes you to the side when you make a sharp turn with your car. This force acts perpendicular to the Earth rotatory axis, adding or subtracting to the gravity force. When an object flies eastward, in the same direction of Earth’s rotation, centrifugal force acts opposite of gravity, pushing it away from the Earth’s surface. If the object flies westward, in the opposite direction of the Earth rotation, centrifugal force pushes the object toward the ground concurrently to gravity force. Thus, bullets fired to the east always fly a little higher, and, conversely, bullets fired to the west always travel somewhat low.

The amount of drop change is in function of:

Latitude – The linear velocity of a point on the Earth’s surface, and thus the amount of centrifugal force, is maximum at the equator and decreases going toward the poles, where it is null.

Shooting direction, or azimuth – The amount of drop change is highest when shooting east or west, and as the trajectory angles northor south,  the amount of drop change decreases, becoming null, as the angle points toward either pole.

Muzzle velocity – The amount of centrifugal force is determined by the speed of the flying object.