Earth's Rotation and Flight
The Coriolis effect describes how the Earth's rotation appears to deflect a projectile's path over long flight times. It is a frame-of-reference effect rather than a physical force pushing on the projectile: while the projectile flies in a straight inertial path, the rotating Earth carries the target slightly aside beneath it, so the point of impact shifts relative to where the shooter aimed.
Why Long Flight Times Matter
Because the Earth turns slowly, roughly one revolution per day, the ground moves only a tiny amount during a short flight. The effect is therefore negligible at ordinary distances and becomes measurable only at very long range, where the projectile stays airborne for a second or more and small angular shifts have time to accumulate into inches of displacement.
The Horizontal Component
The horizontal part of the Coriolis effect depends on latitude and the direction of fire. In the Northern Hemisphere a projectile appears to deflect to the right of its path, and in the Southern Hemisphere to the left, regardless of which way the shooter faces. The magnitude grows toward the poles and shrinks toward the equator, where the horizontal component vanishes.
The Vertical Component (Eötvös Effect)
There is also a vertical component tied to firing east or west. Shooting toward the east, in the direction of Earth's rotation, makes the projectile strike slightly high, while shooting west makes it strike slightly low. This vertical piece, sometimes called the Eötvös effect, depends on latitude and the east-west component of the shot, and it is independent of the horizontal deflection.
A Predictable Correction
Because the Coriolis effect depends only on latitude, direction of fire, and time of flight, it is fully deterministic. Ballistic solvers compute it from the shooter's coordinates and heading, so it is applied as a fixed calculated correction rather than something read from wind flags or estimated on the fly.
Worked Example
Picture a shot at 1,000 yards with a time of flight near 1.5 seconds, fired at a mid-northern latitude. The horizontal Coriolis deflection might amount to only a few inches to the right, smaller than typical wind drift at that distance. A separate east-versus-west shot of the same length would show a small vertical shift, high toward the east and low toward the west, on a similar scale of a few inches.
A Common Misconception
Coriolis is often imagined as a strong force that dominates long-range shooting. In practice it is one of the smallest corrections, usually less than the error in a wind call, and it only rises to relevance at extreme range after drop and wind are already handled precisely. It is also not the same as spin drift, which comes from the projectile's own spin and always deflects the same direction for a given barrel, independent of latitude or heading.