Not so much air travel as air waiting.
March 1, 2012 4:11 PM   Subscribe

How (im)practical of a travel method would it be to go up, hang out, and wait for the Earth's rotation to align with where you want to be?

Imagine going up in a balloon somewhere along 45 degrees latitude, somehow maintaining position (easier said than done, maybe) while Earth rotates, and then coming back down some hours later, letting the planet do most of the work in the "traveling".

This feels crank science-y so I'm sure I'm missing something really obvious.
posted by curious nu to Science & Nature (20 answers total) 1 user marked this as a favorite
 
Air rotates, too.
posted by thewumpusisdead at 4:14 PM on March 1, 2012 [2 favorites]


In the heights you could reach in a balloon, one would think wind would be a factor.
posted by gimonca at 4:14 PM on March 1, 2012


A balloon wouldn't do it, because the atmosphere rotates along with the surface of the Earth (otherwise think of the strong winds relative to the surface!) -- Of course there are some winds like the jet stream that would influence things, but basically, the air rotates too.
posted by brainmouse at 4:14 PM on March 1, 2012


Well, it would take you no more than 24 hours to get to any point at the same latitude. That's pretty fast at the equator (about 1000mph), but less and less efficient at higher latitudes (e.g. right at the poles you could walk there faster).

Air rotates, too.

Hence the hand-waving regarding maintaining position.
posted by jedicus at 4:16 PM on March 1, 2012


What do you mean by position? You will have a velocity upon going up (ignore wind, etc.) simply from the earth's motion. Stopping that motion could be seen from the surface of the earth as giving yourself velocity moving west. Given that the earth rotates at 1674.4 km/h, you've got a lot of work to do to slow yourself down to have no angular velocity from the earth. You'd still have it from the revolution of the earth around the sun, but that's for later.

So, yes, you could go up under a combination of rocket and balloon and then spend a great deal of energy to bring yourself to a "rest" state above the earth (you'd have to be in space really), and then you'd have to keep spending energy to stay there. You wouldn't be in orbit, so the second your rocket switched off, you'd be descending.

You could park yourself at the L1 and L2 lagrange points, but that's just getting silly.
posted by Hactar at 4:17 PM on March 1, 2012 [8 favorites]


The other thing not yet mentioned here is that you need to accelerate to "maintaining position". When you're tied to the surface of the earth (at least near the equator), you're moving pretty fast.
posted by straw at 4:18 PM on March 1, 2012


The surface of the earth at the equator rotates about one thousand miles per hour. If the air did not go along for the ride, we would have thousand mile per hour winds all day and all night.
posted by weapons-grade pandemonium at 4:19 PM on March 1, 2012 [5 favorites]


Well, it's conservation of momentum. Even if you go into space, you are still traveling with the Earth's rotational velocity. In order to come to a "stationary" orbit (i.e., flying over a point on Earth once a day), you need to counter that initial velocity.
posted by supercres at 4:19 PM on March 1, 2012


Response by poster: "Given that the earth rotates at 1674.4 km/h, you've got a lot of work to do to slow yourself down to have no angular velocity from the earth."

Okay, but surely it takes less energy to stand still against a force than it does to move counter to that force, right? Standing up in a strong wind might be tough, but walking against it has to be tougher. So it seems like even if you had to spend a bunch of energy, you'd still be spending less than if you were flying (in a commercial airliner, for instance).
posted by curious nu at 4:22 PM on March 1, 2012


This is essentially what LEO satellites do, isn't it?
posted by DarlingBri at 4:23 PM on March 1, 2012


Best answer: No, because you're already moving at 1674.4 km/h by virtue of standing still on the surface of the earth. To go to zero requires acceleration. To go 300 km/h faster (with the rotation) or slower (against the rotation) requires less acceleration.
posted by supercres at 4:24 PM on March 1, 2012


Best answer: Commercial airliners travel at around 920 km/h (top speed of a 747). To maintain a relatively "still" position with regard to the earth and the sun, you're accelerating yourself (or decelerating, whichever) to 1674.4 km/h. Also, they get lift out of their movement. You would have to generate lift some other way (unless you want to get in a plane and fly it at sufficient speed to maintain that position. You'd need some kind of fighter jet though.)
posted by Hactar at 4:26 PM on March 1, 2012


Best answer: Your idea of "standing still" is traveling at a constant 1674.4 km/h going due west (relative to the surface of the earth). Relative to you, the atmosphere is a ~1674.4 km/h headwind.

Frames of reference are what's missing from your mental model, i.e., velocity relative to what?
posted by supercres at 4:32 PM on March 1, 2012 [4 favorites]


Yeah, you're confusing 'standing still' with 'moving really, really fast'.
posted by Sebmojo at 4:58 PM on March 1, 2012 [6 favorites]


Hang on a second...

While on the ground you have that 1674 km/h linear velocity. Let's say you're on the equator, and there is no atmosphere. Now get in your rocket, and fly straight up, to the altitude of 6394km - doubling your distance from the earth's center. Now you're still moving at 1674km/h, but now your path's circumference is twice as long as it is for those on the ground, so if you can maintain that altitude for 24 hours, the earth would get half a rotation ahead of you. (If you don't start at the equator, you'll wind up on some orbit that cross the equator)

So this all is caused by movement towards or away from the axis of rotation. On the Earth's surface, traveling North or South moves you closer of further from the poles, though the extent decreases as you near the equator (and is 0 on the equator itself). So, in the northern hemisphere, if you have air moving in towards a low pressure region, the air coming from the south will wind up going faster than the earth (going east) and the air from the north will be left behind a bit. This results in a counter-clockwise spiral, like we see in northern hemisphere hurricanes!
posted by aubilenon at 5:23 PM on March 1, 2012 [1 favorite]


Aubilenon, that ignores the effect of gravity. Even at an altitude, the earth prevents your rocket from only traveling a linear 1674 km/h through gravitational acceleration. Say you jumped straight up and then the earth disappeared. All your rotational velocity would be transferred into linear velocity- the equivalent is spinning a yoyo in an around-the-world trick (in zero gravity) and cutting the string.
posted by supercres at 5:44 PM on March 1, 2012


It ignores the effect of gravity because you are pointing your rocket straight in the direction gravity is pulling you and you're pushing back the same amount. You would have to realign your thrusters over the course of 24 hours to keep them pointed at the center of earth's mass.
posted by aubilenon at 5:50 PM on March 1, 2012


I'm using rockets to lengthen the yoyo's string, not cut it. Now my yoyo goes around the world less often in the same amount of time as the big blue-green with the original length string.
posted by aubilenon at 5:54 PM on March 1, 2012


You're right; sorry. Forgot about the position vector term crossed with the linear momentum term in the equation for angular momentum.
posted by supercres at 6:38 PM on March 1, 2012


Best answer: Why I joined Metafilter
posted by pianomover at 3:59 AM on March 2, 2012


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