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

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

This is a discussion on How the earth's rotation affect the path of a bullet within the General Firearm Discussion forums, part of the Related Topics category; http://www.straightdope.com/columns/read/2944/how-does-the-earths-rotation-affect-the-path-of-a-bullet How does the earth's rotation affect the path of a bullet? June 25, 2010 Dear Cecil: I recently read an article about a Navy ...

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Thread: How the earth's rotation affect the path of a bullet

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    Thumbs up How the earth's rotation affect the path of a bullet

    http://www.straightdope.com/columns/read/2944/how-does-the-earths-rotation-affect-the-path-of-a-bullet

    How does the earth's rotation affect the path of a bullet?
    June 25, 2010
    Dear Cecil:

    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

    Cecil replies:

    Took a while to get to the bottom of this. But of course we did.

    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, discussed here in the past: any object moving horizontally on or near the earth’s surface is deflected slightly off course due to the spinning of the planet beneath it. 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: how, 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 want to factor into each shot. However, it’s hard to imagine a sniper on the field of battle drawing a bead and 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 did. 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 obviously 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, 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 rotates from west to east and your target would have dropped away from you slightly 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 rangefinders 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

    References
    Bearman, Gerry, ed. Ocean Circulation Oxford: Pergammon Press, 1989.

    Coughlin, Gunnery Sgt. Jack; Kuhlman, Capt/ Casey; Davis, Donald. Shooter: The Autobiography of the Top-Ranked Marine Sniper New York: St. Martin’s Press, 2005.

    Field Artillery, Volume 6, Ballistics and Ammunition B-GL-306-006/FP-001, Issued on the authority of the Chief of the Defence Staff. Canada, 1992.

    Mast, Gregory and Halberstadt, Hans. To Be a Military Sniper St. Paul, MN: Zenith Press, 2007.

    Michaelis, Dean Complete .50 Caliber Sniper Course: Hard Target Interdiction Boulder, Colorado: Paladin Press, 2000.

    Plaster, Maj. John L. The Ultimate Sniper: An Advanced Training Manual for Military and Police Snipers Boulder, Colorado: Paladin Press, 1993.

    Telander, Rick “The Way of the Sniper” Men’s Journal 30 November, 2009.

    The Straight Dope: Why does weather move west to east? http://www.straightdope.com/columns/read/1191/why-does-weather-move-west-to-east

    United States Army Field Manual: FM23-10 Department of the Army, Washington, D.C., 17 August, 1994.
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    Interesting thread.

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    I'd be more concerned with the curvature of the earth (we are talking long-range shooting, aren't we?) more than rotational effect. Unless, of course, we're taking pot-shots at the moon rover.
    Retired USAF E-8. Lighten up and enjoy life because:
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    I particularly like the line in the quoted article where the author states that heating metal CONTRACTS. Someone needs to go back to 8th grade Earth Science...
    A man fires a rifle for many years, and he goes to war. And afterward he turns the rifle in at the armory, and he believes he's finished with the rifle. But no matter what else he might do with his hands - love a woman, build a house, change his son's diaper - his hands remember the rifle.

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    I dont see this as being an issue.

    Even big game hunters that have a clue will fire a few shots upon arrival to confirm zero.

    Target shooters do the same.

    Snipers will dial it in also.

    The average shooter will care less, and they arent accurate enough with their stuff to matter anyway.

    Now, if you are shooting at an Insurgent at say...+2500 yards, it would be good to know that you will automatically be several inches off. So you do it like everyone else has been doing it since day one...take a shot and then correct.
    Or you could do it like Hathcock did and zero at the assumed point of impact, so that there is no correction because you have already done it.
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    Quote Originally Posted by OPFOR View Post
    I particularly like the line in the quoted article where the author states that heating metal CONTRACTS. Someone needs to go back to 8th grade Earth Science...
    Questionably written.

    But does the diameter of the bore contract/reduce because of the metal of the barrel expanding? Or does the whole barrel expand enough that the bore diameter itself also expands?

    One of the truths about rifles and guns is their erratic shooting or walking after heating up. However, I tend to attribute that to flexing/bending along the length or to bore expansion.
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    Quote Originally Posted by OPFOR View Post
    I particularly like the line in the quoted article where the author states that heating metal CONTRACTS. Someone needs to go back to 8th grade Earth Science...
    The article was correct. Heating metal expands, or swells, which causes the bore to contract. The bore is open space, into which the metal expands. Smaller bore equals more pressure behind the bullet. Like swelling h-roids cause something else to contract, not open up.

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    Does the time zone have any effect? I mean, if I were shooting from the ETZ at a deer standing in a field in the CTZ, would I have to shoot an hour earlier or later to have the bullet arrive at the same time as the deer?
    Retired USAF E-8. Lighten up and enjoy life because:
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    its barrel heats up, the metal contracts
    This is NOT a correct statement. The conclusions drawn were correct, but this is not "quetionably written" - it's just wrong.

    I could say "gravity makes everything fall upwards, causing us to not fly off into space." My conclusion is correct, but my premise is 180 degrees wrong...
    A man fires a rifle for many years, and he goes to war. And afterward he turns the rifle in at the armory, and he believes he's finished with the rifle. But no matter what else he might do with his hands - love a woman, build a house, change his son's diaper - his hands remember the rifle.

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    Quote Originally Posted by QKShooter View Post
    Interesting thread.
    Indeed. But I'm going to put my boots on just in case the river begins to rise. I always keep an open mind. Science.

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    Quote Originally Posted by OldVet View Post
    Does the time zone have any effect? I mean, if I were shooting from the ETZ at a deer standing in a field in the CTZ, would I have to shoot an hour earlier or later to have the bullet arrive at the same time as the deer?
    Nice!. I LOL-ed.
    I haven’t heard any of the journalists who volunteered to be waterboarded asking to have their fingernails wrenched out with pliers, or electrodes attached to their genitals.

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    Hummmm....I'm possibly looking at it as a great new excuse to blame those flyers on.
    Whoops! & Doggoneit! I Plumb Forgot To Factor In The Rotation of the Earth On That One!

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    Great.... now my wife says I don't need a new scope. Thanks for postin' pal.

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    Heat from shooting causes more warping of the barrel than changes in diameter of the bore though.....hence differences in 'cold bore' shots than hot ones.

    One thing I didn't see addressed was SPIN DRIFT. With long distance shooting spin drift is a bigger factor than rotation of the earth. Spin drift is caused by the rifling in the barrel, it makes most rounds curve slightly to the right & downward. (think of a football spiral).
    Last edited by goawayfarm; July 5th, 2010 at 09:07 PM.
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    No no no - the bore diameter contracting due to heating up is BS.... think about it - you're wanting to slide a tight bushing onto a shaft. You heat the bushing up to make it easier.

    ID gets bigger, just like the OD, when a hollow tube, like a barrel, is heated up.

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