{"id":5601,"date":"2020-11-23T02:14:35","date_gmt":"2020-11-23T02:14:35","guid":{"rendered":"https:\/\/www.wideners.com\/blog\/?p=5601"},"modified":"2020-11-23T19:38:06","modified_gmt":"2020-11-23T19:38:06","slug":"what-is-the-coriolis-effect","status":"publish","type":"post","link":"https:\/\/www.wideners.com\/blog\/what-is-the-coriolis-effect\/","title":{"rendered":"Long-Distance Shooting: What Is The Coriolis Effect?"},"content":{"rendered":"

By Guy J. Sagi<\/a><\/span><\/p>\n

Sleek flying bullets, improved cartridge design and firearms held to strict CNC tolerance contribute to any new long-distance shooting records, but there is more involved than just the latest and greatest. To connect regularly at 1,500 yards and beyond marksmen must adjust for a variety of factors largely immune to cutting-edge technology. Gravity leads the list, but another heavyweight lurks in the firing solution\u2014Coriolis effect.<\/span><\/p>\n

What Is The Coriolis Effect?<\/h2>\n
\"a

Despite urban legend, the Coriolis effect has little to do with the direction your toilet flushes.<\/p><\/div>\n

Coriolis effect is a product of the Earth\u2019s rotation and, in the case of firearms, moves the target away from the original point of aim when the bullet finally arrives. Total deviation is determined by time of flight, shot direction (azimuth), how fast the planet is moving at both the firing point and target and more. It\u2019s dictated by a lesser-known Coriolis force that, thankfully, includes inertia as a key component. Together they create some simple rules of thumb worth remembering when you push the distance.\u00a0\u00a0\u00a0<\/span><\/p>\n

The previously obscure precision-marksmanship component didn\u2019t get the respect it deserved until Mark Wahlberg delivered a pair lines of the movie \u201cShooter.\u201d \u201cThere\u2019s 6- to 10-second flight time so you have to shoot at where the target\u2019s going to be,\u201d he said while explaining an assassination plot would require a criminal intimately familiar with the factor. \u201cEven the Coriolis effect, the spin of the Earth, comes into play.\u201d<\/span><\/p>\n

The term went mainstream overnight and watering-hole experts soon began lengthy lectures on its everyday impact. It\u2019s often incorrectly cited as forcing toilets to flush in one direction in the Northern Hemisphere, for example, and the opposite way in the south. Despite scientific evidence to the contrary\u2014and fully functioning commodes on the equator\u2014the urban myth persists.<\/span><\/p>\n

Storm Theory<\/h4>\n

The claim\u2019s longevity is partly explained by the rotation of major storms. North of the equator hurricanes and cyclones turn counterclockwise. To the south they spin the opposite direction. It is indeed Coriolis effect at work with the weather, but there\u2019s a big difference between them and tap water.<\/span><\/p>\n

Clouds and typhoons are airborne, free of the earthly requirement to rotate. Meanwhile, the landscape below continues to do so at a scalding pace of roughly 1,000 miles per hour at the equator, slower toward the poles. Bullets on their way to a target are also in flight, albeit temporary. Yet, a target that appears stationary to a shooter continues to move while waiting for the projectile to arrive.<\/span><\/p>\n

Northern VS Southern Hemisphere<\/strong><\/h3>\n

Calculating the compensation for Coriolis is not exactly a firing-line friendly. It hinges on the earth\u2019s speed of rotation at the shooting and target positions. It reaches its maximum at the equator and lowers to a theoretical zero as latitude increases. The direction of the shot is critical and with some trigonometry and bullet flight time, you get the precise information required to know where the target will be by the time your projectile arrives.<\/span><\/p>\n

For those who insist,<\/span> here\u2019s a paper<\/span><\/a>\u00a0published by the Naval Surface Weapons Center, Dahlgren Laboratory, in 1975. Sharpen a couple No. 2 pencils and warm up the calculator before you dive in, though.<\/span><\/p>\n

Thankfully, Sir Isaac Newton saves the day once again. His first law of motion does not make the math and measurements less time consuming, but it applies and supports the first, easy-to-remember Coriolis rule of thumb.<\/span><\/p>\n

All shots taken in the Northern Hemisphere, other than those directly east and west, deflect to the right. Those south of the equator veer left.<\/p><\/blockquote>\n

Here\u2019s why. Newton\u2019s first law of motion states any object in motion tends to stay in motion and one at rest tends to stay at rest. So, the bullet on a .308 Win. cartridge at the equator is moving east at 1,000 miles per hour even while waiting in a rifle\u2019s chamber. Hit the trigger and send it due north at a target and it leaves the muzzle at the expected velocity but also maintains that 1,000 mph motion to the east after leaving the barrel. The target, closer to the north pole, isn\u2019t rotating as fast. So instead of connecting in the bullseye the bullet veers right.<\/span><\/p>\n

Understanding Shot Displacement<\/h4>\n
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Can the distance and direction you’re shooting potentially affect your accuracy with the Coriolis effect?<\/p><\/div>\n

Shooting south in the Northern Hemisphere means the firing line, rifle, marksman and bullet are moving slower than the target. By the time that bullet arrives at the target it has moved from the point of aim and the bullet again hits to the right.<\/span><\/p>\n

The opposite is true for the same reasons in the Southern Hemisphere. Shot displacement there, due to Coriolis Effect, will all be to the left.<\/span><\/p>\n

Of course, there aren\u2019t too many situations or ranges that provide due north or south orientations. Those calculations are where the trigonometry comes in. The azimuth (compass bearing toward the target) must factor into the equation for an accurate firing solution.<\/span><\/p>\n

Just like those late-night commercials, \u201cBut wait, there\u2019s more!\u201d<\/span><\/p>\n

Coriolis Effect: Shooting East To West<\/b><\/h3>\n

What if you\u2019re shooting due east or west? Coriolis still applies even though you and the target are rotating at the same speed. The second rule of thumb is also simple to remember.<\/span><\/p>\n

All shots, regardless of hemisphere, will impact lower than expected when sent west and high when traveling east.<\/p><\/blockquote>\n

The Earth is a sphere. So, targets to the west are rotating toward you and rising slightly on the horizon (atop the planet\u2019s circumference). The bullet, in theory, continues toward your point of aim, which is no longer there because it\u2019s moved up.<\/span><\/p>\n

Anything to the east rotates down, in relative terms. The concept is easy to grasp, but difficult to embrace on a firing line when steel at 1,500 yards seems to be unmoving. It is, though, and so are you.\u00a0<\/span><\/p>\n

In The Beginning<\/h4>\n

The Coriolis effect begins with the bullet\u2019s flight, literally point blank, although deviation is microscopic up close. So at what point should a shooter begin correction for Coriolis? That\u2019s determined by skill, firearm and load accuracy and a host of other variables that must be weighed individually.\u00a0<\/span><\/p>\n

A <\/span>research report<\/span><\/a> issued by Michael Courtney and Amy Courtney in 2012 helps clarify the decision. In it the pair, who were attending the Air Force Academy at the time, provided Coriolis corrections for 168-grain Barnes TTSX bullets launched from a .30-06 Sprg<\/a> at 2,800 fps due west at a latitude of 45 degrees north. At 500 yards the bullet struck 1\/2 inch low. At 1,000 they found deflection was between 3 to 4 inches. Precision shooters at that distance and beyond adjust, but it\u2019s up to you if it\u2019s really beneficial. There’s a difference when the pursuit is hunting or ringing 3-foot-wide steel bison targets in unpredictable wind.\u00a0\u00a0\u00a0<\/span><\/p>\n

Even without running the numbers, though, the rules of thumb provide a solid foundation when it comes to addressing the challenges of really stretching the distance. The precise firing solution, however, gets even more complex when you combine the two and then weigh other variables including the difference in elevation.<\/span><\/p>\n

Advanced Coriolis Effect Calculator<\/b><\/h2>\n

A variety of ballistics apps for smartphones and handheld units remedy the headache.<\/span> Wideners<\/span> has provided a convenient Coriolis calculator you can use for free here. Our suggestion is to plug in your load\u2019s information, preferably with the velocity produced by your barrel length. Then print out our mini DOPE sheet<\/a> to stuff in your range bag or tape to your stock.<\/span><\/p>\n