Litz's Description of Coriolis.

[bill123]

On page 97 of Applied Ballistics, Brian Litz writes the following:

"3. For latitudes north of the equator, the horizontal component of the Coriolis Effect is always to the right. For latitudes south of the equator, the deflection is to the left."

This does not make sense to me. I would think that the deflection would always be right if you were facing north and always left if you were facing south, regardless of your latitude. Latitude would only affect the magnitude of the deflection.

 

[Michael Courtney]

On page 97 of Applied Ballistics, Brian Litz writes the following:

"3. For latitudes north of the equator, the horizontal component of the Coriolis Effect is always to the right. For latitudes south of the equator, the deflection is to the left."

This does not make sense to me. I would think that the deflection would always be right if you were facing north and always left if you were facing south, regardless of your latitude. Latitude would only affect the magnitude of the deflection.

You must have the 2nd edition. It's on p. 101 in my copy. Litz is right. The Coriolis effect is always clockwise in the northern hemisphere. That makes it to the right regardless of the direction you are facing.

Likewise, counterclockwise in the southern hemisphere is always to the left.

It's not just that the earth is moving east under the projectile. It's that points closer to the equator are moving faster than points closer to the poles. When you shoot a bullet north the motion of the earth gives it a horizontal component of velocity to the east, which it retains in flight to the impact point. When it reaches the impact point, the target has not moved as far to the east (it has in longitude, but not in inches), so the bullet hits to the right. Hitting to the east facing north is clockwise, to the right.

Conversely, when you shoot to the south, the bullet gets a bit of eastward velocity from the motion of the earth. However, the target, being further to the south, has a greater eastward velocity and moves further east than the bullet while the bullet is in flight, thus the bullet hits slightly to the west. Hitting west facing south is clockwise, to the right.

This is an oversimplification that seems to fail the rightward motion when shooting due east or due west, but rest assured that the horizontal deflection is to the right in those cases also. The full explanation requires vectors and cross products and the whole bit.

 

[bill123]

It's not just that the earth is moving east under the projectile. It's that points closer to the equator are moving faster than points closer to the poles. When you shoot a bullet north the motion of the earth gives it a horizontal component of velocity to the east, which it retains in flight to the impact point. When it reaches the impact point, the target has not moved as far to the east (it has in longitude, but not in inches), so the bullet hits to the right. Hitting to the east facing north is clockwise, to the right.

So if I'm following you, the velocity that the earth is spinning relative to your location has a greater effect than the eastward motion? Sort of?

If I am standing 1000 yards north of the equator facing south and shooting at a target 1000 yards south if the equator, would the bullet impact at my point of aim with zero net Coriolis effect?

One more question about Litz's description. He writes:
4. The horizontal Coriolis deflection is minimal at the equator and grows greater as you move toward the poles.

Isn't that contradictory to what you said before? My understanding is that the earth is moving faster at the equator than at the poles.

 

[CB11WYO]

On page 97 of Applied Ballistics, Brian Litz writes the following:

"3. For latitudes north of the equator, the horizontal component of the Coriolis Effect is always to the right. For latitudes south of the equator, the deflection is to the left."...

Exactly right. I just watched a video on this Monday.

[ame=http://www.youtube.com/watch?v=hiRaP8qxqa4&list=PLJUaiRIEduNXoal2_PkBZi0vDCIcEPxUn]SNIPER 101 Part 73 - Coriolis Effects on Rifle Bullets - YouTube[/ame]

 

[Michael Courtney]

One more question about Litz's description. He writes:
4. The horizontal Coriolis deflection is minimal at the equator and grows greater as you move toward the poles.

Isn't that contradictory to what you said before? My understanding is that the earth is moving faster at the equator than at the poles.

No, there is no contradiction. The earth's speed is not the point. If all latitudes were moving at the same speed, there would be no effect. But the higher latitudes (further from the equator) are moving slower than the latitudes closer to the equator, and the difference in surface speeds is the issue.

If the earth were a big rotating cylinder with all points on the long sides moving at the same speed, there would be no horizontal Coriolis effect. But the flat circular top and bottom would have a big Coriolis effect, since the speed would change rapidly from the edge to the center.

 

[bill123]

No, there is no contradiction. The earth's speed is not the point. If all latitudes were moving at the same speed, there would be no effect. But the higher latitudes (further from the equator) are moving slower than the latitudes closer to the equator, and the difference in surface speeds is the issue.

If the earth were a big rotating cylinder with all points on the long sides moving at the same speed, there would be no horizontal Coriolis effect. But the flat circular top and bottom would have a big Coriolis effect, since the speed would change rapidly from the edge to the center.

I think I get it. Sort of. I'll have to watch the video. Thanks for your help with this.

 

[Michael Eichele]

Think of it this way:

You're 100 yards from a road. Your rifle is pointed at the road at car level. A car drives from your right to the left at 60 MPH. When the car is in your crosshairs you pull the trigger. Do you hit the car?

No.

Where did the bullet go? The builet went straight but the car moved out of its way. OR, impacted to the right of the car.

Can you picture that? Cool.

Now, rewind, imagine a passenger in that same car that you're about to shoot at. He has a rifle sticking out the window. When his crosshairs align with you, he pulls the trigger. Does he hit you?

No. Why?

Because you (the target) have been stationary. The bullet fired at you is moving laterally at 60 MPH and therefore impacts to the??? You guessed it. The right of you from the passengers perspective.

You fired and hit right from your perspective and he fired and hit right from his perspective.

When you sit on the North Pole and fire at a target on the equator, it's like you're shooting from a stationary position at a moving target.

When you sit on the equator and fire at a target at the North Pole, you're the passenger in the moving car shooting at a stationary target.

Now I understand that you're not taking shots to or from the equator to or from the poles but it does illustrate the principal. The point is that the rotational velocity is greater south of your position no matter where you are in the northern hemisphere at least to the point when you meet up with the equator.

If you sit 500 yards north of the equator and shoot at a target 500 yards south of the equator the correction is minimal. Why? Because your target and yourself both have the same rotational velocity, or surface velocity as Michael Courtney put it.

 

[bill123]

Think of it this way:

You're 100 yards from a road. Your rifle is pointed at the road at car level. A car drives from your right to the left at 60 MPH. When the car is in your crosshairs you pull the trigger. Do you hit the car?

No.

Where did the bullet go? The builet went straight but the car moved out of its way. OR, impacted to the right of the car.

Can you picture that? Cool.

Now, rewind, imagine a passenger in that same car that you're about to shoot at. He has a rifle sticking out the window. When his crosshairs align with you, he pulls the trigger. Does he hit you?

No. Why?

Because you (the target) have been stationary. The bullet fired at you is moving laterally at 60 MPH and therefore impacts to the??? You guessed it. The right of you from the passengers perspective.

You fired and hit right from your perspective and he fired and hit right from his perspective.

When you sit on the North Pole and fire at a target on the equator, it's like you're shooting from a stationary position at a moving target.

When you sit on the equator and fire at a target at the North Pole, you're the passenger in the moving car shooting at a stationary target.

Now I understand that you're not taking shots to or from the equator to or from the poles but it does illustrate the principal. The point is that the rotational velocity is greater south of your position no matter where you are in the northern hemisphere at least to the point when you meet up with the equator.

If you sit 500 yards north of the equator and shoot at a target 500 yards south of the equator the correction is minimal. Why? Because your target and yourself both have the same rotational velocity, or surface velocity as Michael Courtney put it.

Awesome description! Thanks everyone.