How does the earth's rotation affect the path of a bullet?

Dear Cecil

Your article “How does the earth’s rotation affect the path of a bullet?” contains a statement which is factually wrong.

You say: “the current world record for a confirmed kill in combat is 2,430 meters, or roughly 1.5 miles.”

That record - 2,430 metres, or 7,972 feet - was set by a Canadian in 2002.

However, your article obviously wasn’t researched properly. The current record is actually held by a British soldier, who in May 2010 - more than a month before your article was published - shot and killed 2 Taliban soldiers with a sniper rifle from a distance of 8,120 feet or 2,475 metres.

For help in getting your facts straight on this one, please see: The Times & The Sunday Times :slight_smile:

Those rifles are made of metals with interesting properties if they respond to heat by contracting. Where can I find some of this stuff?

The article was correct when written in April but sat in the queue for a while. Naturally we’ll update it. Thanks for calling this to our attention.

I arranged for this outstanding column to be posted on the Virginian Citizens Defense League website, and it’s elicited not one but THREE comments which may be of interest. These are online in the 7/21/10 of their VA-ALERT update, at vcdl DOT org.
– RoyBoy

  1. VA-ALERT reader: firearm myths

VA-ALERT reader John Pepper emailed me this in response to a previous alert item, “How does the earth’s rotation affect the path of a bullet?”

Hey Philip, number three has a common error. Part of last sentence in paraghaph 3 of story number 3 about Earths rotation effects,"- no doubt the best shooters do it instinctively." , is dead wrong. Look up the word instinctively and you will get why it is wrong. Almost everyone is born with instincts which enhance with age a natural course. That is up to a point like my age were it has been going in reverse.

Best shooters do almost nothing instinctively in relation to use of firearms. Yeah I know someone used it in pulp gun magazine articles. The someone is wrong.

There is nothing natural or instinctive about shooting and in fact the best shooters work hard to overcome natural or instinctive tendencies. The best shooters under observation may seem to be doing all manipulations, handling and firing so smoothly and well that an impression of instinctive and natural may so impress the observer. Not so. Much long hard work and repeated drills have brought them up to that effective competent level. Yes they do reach a point that what they do is almost on seemingly automatic reaction and manipulation without thinking, but not quite as ingrained practice muscle memory still has a connection to the brain.

Having spent the greater portion of life conditioning and training shooters, right after initial safety lecture, demonstration and student participation safety testing and before handling and firing I tell them," To become good at shooting you have to go against natural tendencies and learn to do things that are not natural." Now after this has been brought up will have to add instinctive after natural to my spiel.

Firearms use has much mythology interwoven in tales with word use misnomers accepted by the unknowing. So much for so long that many accept them as facts.

My dear friend and stickler for word meanings Jeff Cooper spent much energy informing people about misnomers, misinterpretations, B.S. statements,etc One he repeatedly brought up as to wrongly used in relation to firearms use was instinctive.

Let me give a very specific example that is so common and so widely used it would give me a headache if I was susceptible to headaches. Fortunately I am not.

Many not thinking logically call their firearm a weapon. For its primary design purpose it is not a weapon, but rather a launcher for the true weapon, the bullet or projectiles it launches. Yes for a field expedient secondary purpose it can be used as a weapon such as a rifle vertical or horizontal butt stroke, butt smash, muzzle slash or muzzle thrust or poke, but this use is rare and not primary. The bullet or projectiles are what delivers the killing or disabling blow so truly and logically are the weapon or weapons. A pistol or revolver has primary design purpose as the launcher tool of bullet weapons yet useable as a bludgeon - - - - - - knowing that it may later be inoperative after you have bashed someone with it. Never bashed anyone with a handgun, but have used the rifle butt stroke to good effect. Like I said rare when compared to the number of rifle rounds of ammunition I have fired.

Older military rifles with intentional design features intended for using with butt strokes, slashes etc, could be some what accepted overall as weapons due to the built in robust design feature of it possibly being used for secondary purpose. Try it with the various current AR plastic designs and end up with pieces in hand and some on the ground.

Getting across to new at it shooters that the weapon is the bullet which I do, shortens their learning curve, their concentration more immediately on where the bullet goes and not on the tool that launched them. Concentration over all else.

The best shooters while concentrating at moment of final trigger squeeze can have a wasp or bee stinging them on exposed flesh or other painful distractions and ignore it until the squeeze is completed. Sometimes for a multiple shots.

It is certain that Cecil’s intentions were good, but suggest he study instinctive in relation to the use of arms.

The Navy SEAL was just enhancing his claims a bit, the known effect for practical purposes of little value except for the very few who may shoot at extremely long ranges. Perhaps one out of ten thousand or more riflemen ever called on for such expertise. During my 1,000 meter practice shooting never factored the known rotation of the Earth effect into hit calculations although if say the very next shot had to be fired in a totally different direction I may have. Never know as the range was limited to only one direction of fire, any other direction a hazard to inhabitants.

Since I am on this corrective rant which may end up being read by others and Cecil ( please forward to him) will mention wind effect on rifle bullets wind being touched on in this story. Again about people frequently claiming something they think is correct, but isn’t.

"The cross wind pushes the bullet off its flight path. - - - - - No it doesn’t.

What happens is when a cross wind is present the bullet vectors into the wind causing it in its flight path to maintain a slight angle toward the direction the wind is coming from. The air flow around the bullet is then at a higher velocity on the down wind side of the spinning bullet having greater distance to go than on the up wind side. Since as velocity increases pressure decreases the bullet moves in direction of less pressure. If one draws a diagram of bullet vectored into wind in relation to a line between rifle muzzle and target showing this slight vectoring into up wind then draws lines representing air flow around bullet it can be seen the down wind side air flow distance is greater than the up wind side. Turbulence at flat base of bullet is also slightly greater at down wind side adding to lower pressure effect. If one said the bullet is pressured from up wind instead of pushed from upwind it would be a better way of putting it. - - - - – - - - - Alas this is probably a nit pick cry in the wilderness of firearms mythology.

  1. VA-ALERT reader: more on the Coriolis effect

Charles Ashinoff emailed me this:

Hey Phillip,

Regarding the Coriolis effect, Cecil’s statement in paragraph #5 “More important things can go wrong, and besides, assuming your target is standing, what’s a couple inches up or down?” is only half true. If shooting straight north from the equator, the bullet will impact left of target and opposite if shooting straight south. The comment quoted above only relates to due east or west trajectories. Sure, all of this is meaningless (as well as completely baffling) to the casual shooter, but to a long range shooter, 800 + yards, it can be as significant as a 10 mph wind.

Anyone with an iPod Touch or iPhone can download some great apps from iTunes that calculate bullet trajectory for many different loads. Some of these apps take Coriolis into full account requiring you to enter your lat/long and trajectory compass heading. It is amazing stuff!

Charles Ashinoff

  1. VA-ALERT reader: German rail guns did have to compensate


The big rail guns the Germans used to shell paris did have to compensate:

“The Paris gun was used to shell Paris at a range of 120 km (75 miles). The distance was so far that the Coriolis effect – the rotation of the Earth – was substantial enough to affect trajectory calculations. The gun was fired at an azimuth of 232 degrees (west-southwest) from Crepy-en Laon, which was at a latitude of 49.5 degrees North. The gunners had to account for the fact that the projectiles landed to the right of where they would have hit if there
were no Coriolis effect.”

I think there is one at Aberdeen proving grounds north of Baltimore. As much as I detest Maryland and its infantile laws I may just have to disarm and go up there…

Ric Bergstrom

Welcome to the Straight Dope Message Board, PeachSchnapps, we’re glad to have you here (especially when you bring such an outstanding update!) and we hope you’ll look around and join in some of the other discussions.

When someone starts a thread, we like them to provide a link to the column under discussion. Saves searching time, helps keep everyone on the same page. No biggie, I’ve edited your post to make the column title into a link. You’ll know for next time, and, as I say, welcome!

Actually, you might could benefit from a little help here yourself. According to your cited article, the British soldier made the two shots plus one to disable the rifle back in November of 2009. The “May 2010” you refer to, the 2nd actually, is the date the article was published. All that happened in the interim was that Sniper Harrison was involved in at least two more skirmishes, broke both arms, recuperated and returned to action.

Thanks for the update and cite; one extraordinary display of remarkable skill under pressure.

Some replies to comments posted by royboy:

comments from VA-ALERT reader John Pepper:

Please provide a cite that “weapon” does not include “projectile launcher”. Dictionary searches include guns as weapons. Such as this one:

Or how about a bow and arrow:

It is all well and good to differentiate the contacting element from the launching element, but the word “weapon” is not that refined.

Let me restate what I think he is saying: the air velocity vector against the bullet is a combination of the vector of the bullet through still air and the vector of the wind. This makes the resultant vector for the air against the bullet to be slightly off center from the angle of the bullet.

Then he starts talking about fluid flow and how the air flows around the bullet, and seems to be likening it to the discussions of how a wing works and the argument between “Bernoulli vs Newton” (which we have hashed to death on other threads).

I will grant that his description seems accurate (once I waded through it), until we get to the sentence I underlined. Pressure and push are the same thing. Low pressure does not pull, which he seems to be suggesting with his wording that the air pushing the bullet off course is an inaccurate description. Low pressure is less push. You have a small push (low pressure) on one side and a large push (high pressure) on the other, causing a pressure difference, i.e. resultant push in one direction. It is not inaccurate to say the wind pushes the bullet off course, though the physics of that push is perhaps more complicated than that statement suggests.

comment by Charles Ashinoff:

If you read more closely, you will see that Cecil addressed this in paragraph 2 and 4


Paragraph 5 was only talking about vertical deflection.

Did I miss it or has nobody bothered to point out the fact that the Coriolis effect is AN ILLUSION.

The illusion is that you shoot a projectile on a straight line to the target and it appears to veer off at an angle. But the TRUTH is that the target does not lie in a straight line where you’re aiming (because you and the target are both moving along curved lines). So when the projectile follows the truly straight path (straight in the sense that it does not move left or right, yeah I know it moves down from gravity, don’t distract me here) we have the illusion that it’s curving away.

Projectiles do NOT veer off a straight path away from their targets. The targets themselves are veering off while the projectile continues straight.

It’s not an illusion; whether the bullet curves or not depends on your reference frame.
Powers &8^]

“Illusion” is the wrong word - it suggests a purely visual effect that doesn’t actually occur. But the bullet really does end up in a different place than the shooter aims.

It is an actual effect, and is treated as an acceleration in the dynamics equations. The “reality” of why the bullet ends up in a different place is more complicated, but it’s like arguing over centrifugal force. It’s a name for an effect caused by the choice of reference frame.