The bullet flies in a straight line through the air. The Earth, and the target standing on it, rotate underneath. Over a second or two of flight the target has moved slightly, and the bullet lands somewhere the target no longer is. That is the whole of it.
Two separate components
It is easier to keep straight if you treat them as two different effects, because they depend on different things.
| Component | Depends on | Direction |
|---|---|---|
| Horizontal | Latitude and time of flight | Right in the northern hemisphere, left in the southern |
| Vertical (Eötvös) | Shooting azimuth and latitude | Shoot east: high. Shoot west: low. North/south: nothing |
The horizontal deflection is the one most people mean by 'Coriolis'. It does not depend on which way you are facing — north, south, east or west, a shot in the northern hemisphere drifts right. It is strongest at the poles and zero at the equator.
The vertical component is the opposite: it does not care about hemisphere, only about whether you are shooting with the Earth's rotation or against it. Shooting east adds to the bullet's speed relative to the rotating frame and it lands high; shooting west, low. It vanishes for due north and due south shots and it is largest at the equator.
How big is it really?
Both components scale with time of flight, so they follow the same pattern as every other small correction: invisible up close, then suddenly on the scoresheet.
- Under 500 m — a few millimetres to a centimetre or so. Far below your group size. Ignore it.
- 600–800 m — on the order of a couple of centimetres. Still smaller than a typical wind-call error.
- 1,000 m and beyond — several centimetres, and growing. Now comparable to other corrections you already apply.
- Extreme range, 1,500 m+ — a genuine part of the solution, and no longer optional.
The honest framing: if you are missing at 600 m, Coriolis is not why. Fix your muzzle velocity, your scope height and your wind call first — each of those is worth more than this correction by a wide margin.
What the solver needs from you
- LatitudeA degree or two of precision is plenty. The app can take it from GPS, or you can type it once for your usual range and forget it.
- Shooting azimuthOnly needed for the vertical component. The app's compass reads it, or you can take it from a map.
- HemisphereCarried by the sign of the latitude. A southern-hemisphere shooter entering a positive latitude gets the horizontal correction on the wrong side.
- Nothing elseTime of flight comes out of the trajectory the solver already computed.
Because a fixed range has a fixed latitude and usually a fixed firing direction, this is a one-time setup rather than a per-shot input. Set it, leave it on, and stop thinking about it.
Coriolis and spin drift are not the same thing
They are frequently confused because both push a northern-hemisphere, right-hand-twist rifle to the right. The mechanisms are unrelated: spin drift comes from the bullet's own rotation and follows the rifling; Coriolis comes from the planet's rotation and follows your latitude.
Switch barrels to a left-hand twist and spin drift reverses while Coriolis does not. Fly to the southern hemisphere and Coriolis reverses while spin drift does not. A solver applies them separately for exactly this reason.
The pragmatic position
Leave it enabled. It costs nothing, it is correct, and it means your long-range data does not carry a hidden constant that you would otherwise absorb into your wind calls. Just do not let it distract you from the inputs that are moving your impacts by ten times as much.
Frequently asked
Does Coriolis matter at 1,000 yards? +
It is a few centimetres — real, measurable, and smaller than most shooters' wind-call error at that distance. Worth including because it is free, not worth agonising over.
Which way does it push in the southern hemisphere? +
Left. The horizontal component reverses across the equator. Enter a negative latitude and the solver handles it.
Do I need a compass for it? +
Only for the vertical component, which depends on whether you are shooting east or west. The horizontal component needs latitude alone.