Do Snipers Compensate for the Earth’s Rotation? What the Coriolis effect means for hitting your target—and driving your car

Slug Signorino

I recently read an article about a Navy SEAL sniper. The author lists possible variables that go into determining a shot, one of which is the rotation of the earth. How exactly does this affect a bullet in flight? Also, for my nonsniper purposes, does it affect my gas mileage? —Jason, Sacramento

The article I’m guessing you saw, entitled “The Way of the Sniper,” appeared in Men’s Journal, November 30, 2009. Written by Rick Telander, it tells the story of Navy sniper Scott Tyler. Telander writes:

“Each rifle a sniper uses has unique characteristics that are compounded by the ammunition and many, many exterior factors. There is wind. There is humidity. There is the spin of the Earth. There is even the fact that as a rifle is fired, its barrel heats up, the metal contracts, and the bullets are propelled faster.”

Reading this, your columnist didn’t doubt the rotation of the earth affects a bullet in flight. That’s because of the Coriolis effect: Any object moving horizontally on or near the earth’s surface is deflected slightly off course due to the spinning of the planet. The Coriolis effect has a big effect on phenomena like hurricanes and other weather systems, a small effect on small objects. But if the small object is a precisely aimed rifle bullet, and that bullet travels far enough, it’s not something you can completely ignore.

The question in my mind was: Wow, if at all, did a shooter account for the Coriolis effect when aiming? Your wind, your humidity, and for that matter your temperature and barometric pressure—these are all dynamic conditions that, to varying degrees, a marksman will factor into each shot. However, it’s hard to imagine a sniper on the field of battle thinking, “Damn, I better get the latest data on the rotation of the earth.”

Una agreed this was unlikely and began inquiring about what shooters actually do. She couldn’t reach Telander or a military sniper but did talk things over with a couple of hard-core target shooters at her local rifle range and online. Based on that plus her own calculations, she determined as follows:

1. Range is critical. At 100 yards, typical of what a police sharpshooter might encounter, most environmental factors, including the Coriolis effect, are negligible. But military snipers generally are much farther away, typically 400 yards and up—the current world record for a confirmed kill in combat is 2,430 meters, or roughly 1.5 miles.

2. At 1,000 yards the Coriolis deflection is small but not necessarily trivial. Una computed that at the latitude of Sacramento, a bullet traveling 1,000 yards would be deflected about three inches to the right. In addition, because gravity pulls the bullet down as it flies, you’d have to aim higher or lower depending on the degree to which you were facing east or west. If you were firing due east, you’d have to aim six inches lower, since the earth is rotating toward you, meaning your target would be slightly closer by the time the bullet arrived. If you were firing due west, you’d have to aim six inches higher.

3. Amateur long-range shooters can improve their aim using laser range finders and scopes with bullet-drop compensators; they’ll also consult cheat sheets of bullet and rifle performance and their own log of prior results, called a DOPE (“Data on Personal Equipment”) book. Military snipers may not always have access to such stuff in combat. But let’s take it as given that, one way or another, you can adjust for obvious environmental factors in the field—no doubt the best shooters do it instinctively.

4. Horizontal deflection caused by the Coriolis effect is more esoteric but in theory easy to adjust for, since it’s a function of your distance from the equator. When possible, any shooter, whether professional or amateur, makes a few test shots on arriving at a new location and tweaks his or her sights accordingly. Mostly this is to correct for maladjustments due to jostling in transit and such, but it also compensates for the Coriolis effect.

5. As we’ve seen, vertical deflection depends on, and can vary considerably according to, what direction you’re shooting. Nonetheless, none of the amateur shooters we heard from worried much about it, and my guess is military snipers don’t, either. More important things can go wrong, and besides, assuming your target is standing, what’s a couple inches up or down?

Turning now to your wimpy civilian concerns: Don’t sweat the Coriolis effect on your gas mileage. In Sacramento, the rotation of the earth causes your car to drift about 16 feet to the right per mile. That may be an issue if you’re barreling down a narrow two-lane, but correcting for it costs you less than a hundredth of a mile per gallon. —Cecil Adams

Is there something you need to get straight? Take it up with Cecil at

Our Readers Say

May want to double check that:

A couple things. First, the Coriolis effect will only alter the bullet's final elevation if the marksman is firing directly along the latitudinal line of his current location (both firing point and target are located at X degrees, Y minutes, Z seconds, exactly). Secondly, the hypothetical bullet fired in Sacramento will only be affected to the right, as stated above, if the marksman if firing in southerly direction. If the marksman fires northerly, the bullet will be affected left.

Some other factors that affect a bullet's trajectory are ground temperature, air temperature, the rifle barrel's distance from the ground given the previous two variables, the shooter's overall elevation, pressure changes (firing from or into cover), difference in elevation between rifle barrel and target, target's velocity, shooter's velocity, bullet's velocity (the most common for an amateur shooter to adjust for), as well as those listed in the article.

Happy shooting!
You made the statement "if the target is standing, what's a few inches up or down?" Honestly, it makes an enormous difference. If you are aiming center mass, yes, you will probably still hit your target, but if you are too low, you will only serve as to wound a target. In high stress situations, this can be a costly mistake. If your target is one of high value, such as an enemy officer, a killshot is crucial and mistakes cannot be afforded.
You state that: "In Sacramento, the rotation of the earth causes your car to drift about 16 feet to the right per mile." Wouldn't the fact that your car is attached to the ground with rubber tires stop that effect of mobbing you to the right? Your car may experience slight pressure to be moved to the right but the tires hold it attached to the earth. If the car was hovering it would really move it to the right but the car in reality is nailed down to the SURFACE of the earth. Maybe that slight pressure to attempt to move the car to the right or left really wears out the tires faster?
DOPE stands for Data on Previous Engagement in the US Military and Intelligence Community.

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