B Why doesn't the Earth at the equator experience a sonic boom?

[bo reddude]

TL;DR Summary

Earth rotates faster than the speed of sound at the equator, but it doesn't produce a sonic boom, why?

The Earth's circumference at the equator is 24,901 miles, or 40.075 million meters. It rotates completely once a day or in 86400 seconds.

The speed of sound is 343 m/s and the Earth moves at the equator at 40.075 million m/86400 s = 463.831019 m / s

which is clearly faster than the speed of sound. and when an object moves through the atmosphere at speeds higher than the speed of sound, it will produce a sonic boom.

Why doesn't the Earth crust moving under all that air produce a sonic boom, or constant sonic boom, or something?

 

[DaveE]

Summary:: Earth rotates faster than the speed of sound at the equator, but it doesn't produce a sonic boom, why?

The Earth's circumference at the equator is 24,901 miles, or 40.075 million meters. It rotates completely once a day or in 86400 seconds.

The speed of sound is 343 m/s and the Earth moves at the equator at 40.075 million m/86400 s = 463.831019 m / s

which is clearly faster than the speed of sound. and when an object moves through the atmosphere at speeds higher than the speed of sound, it will produce a sonic boom.

Why doesn't the Earth crust moving under all that air produce a sonic boom, or constant sonic boom, or something?

Because the atmosphere is also moving at that speed. A sonic boom is caused by objects traveling through the air, faster than the air, above the speed of sound. You would need wind speed at the ground (relative to the ground) in excess of the speed of sound, which would make it really hard for people to live there. Picture yourself sitting on the wing of a fighter jet at Mach 1, then picture yourself sitting on a street in Quito; that is the difference.

 

[weirdoguy]

and when an object moves through the atmosphere at speeds higher than the speed of sound

Is Earth moving through the atmosphere? I would say it moves (simplifying things a little) with the atmosphere.

 

[bo reddude]

Because the atmosphere is also moving at that speed. A sonic boom is caused by objects traveling through the air, faster than the air, above the speed of sound. You would need wind speed at the ground (relative to the ground) in excess of the speed of sound, which would make it really hard for people to live there. Picture yourself sitting on the wing of a fighter jet at Mach 1, then picture yourself sitting on a street in Quito; that is the difference.

what causes the atmosphere to move with rotation of the earth? i understand the jetstream's movements are cause by the temperature differential at the poles and the equator, making undulate in wave like fashion, but it still moves west to east.

but why does the air gets dragged by the Earth's rotation? is it really as simple as the Earth crust colliding all those gas molecules and causing it to move in the same direction?

 

[A.T.]

Why doesn't the Earth crust moving under all that air ...

You know that there are humans living at the equator, right? How exactly did you envision them surviving in super sonic winds?

but why does the air gets dragged by the Earth's rotation? is it really as simple as the Earth crust colliding all those gas molecules and causing it to move in the same direction?

Yes, the motions are coupled by friction. Much of the atmosphere formed already rotating just like the rest of the Earth.

 

[PeroK]

but why does the air gets dragged by the Earth's rotation? is it really as simple as the Earth crust colliding all those gas molecules and causing it to move in the same direction?

There was a boy in my class at school who insited that the clouds came up and went down with the Sun. It took several days of cloud observations to persuade him to change his mind!

 

[bo reddude]

There was a boy in my class at school who insited that the clouds came up and went down with the Sun. It took several days of cloud observations to persuade him to change his mind!

i don't follow.

 

[PeroK]

i don't follow.

He assumed that the atmosphere didn't rotate with the Earth, that the clouds just sat up there while the Earth revolved under them.

 

[A.T.]

He assumed that the atmosphere didn't rotate with the Earth, that the clouds just sat up there while the Earth revolved under them.

I'm fascinated that the OP primarily worries about the noise pollution, not about say, how children would be able to walk to school in these conditions.

... a sonic boom, or constant sonic boom, or something?

If you had a constant supersonic wind, you would have shockwaves at fixed locations, like in a supersonic wind tunnel. So you wouldn't hear a distinct boom, unless you pass quickly though the shockwaves (e.g. being blown by the wind):

From: https://www.researchgate.net/figure/Schlieren-visualization-of-the-wind-tunnel-configuration-M-17-Re-35-10-6-m-1_fig1_324223394

 

[Nugatory]

but why does the air gets dragged by the Earth's rotation? is it really as simple as the Earth crust colliding all those gas molecules and causing it to move in the same direction?

That's pretty much it... and most of the gases in the Earth's atmosphere were emitted a molecule at a time from the solid material that coalesced to form the earth, so were already mostly moving in the right direction.

 

[Arjan82]

How the gas was moving during the formation is irrelevant. Let's say you have the power to 'stop' all the air such that it doesn't rotate with the Earth anymore (but still moves with the Earth in its path along the sun, so 'stop' is a relative remark) and then let it all go and see what happens.

[I wasn't finished yet..]

I think it will take in the order of a week or so (maybe a month?) before everything returns to normal again. Leave aside that the Earth is pretty much flattened near the equator... So that is very roughly the time it takes to dissipate all the energy again. It is many orders of magnitude smaller than the lifetime of the earth.

 

[sophiecentaur]

There was a boy in my class at school who insited that the clouds came up and went down with the Sun. It took several days of cloud observations to persuade him to change his mind!

There are many occasions when the Sun 'burns off' fog and low cloud in the morning and when the heating of the land near the coast induces an onshore wind, which can cause clouds to form on the coastline. If he happened to live near the sea then his observation could have been quite correct.

I live in Essex (UK) and it's very common to have bright sunshine at sunrise and the Sun climbs behind clouds for the rest of the day. That could support your class-mate's ideas.

Weather is just complicated.

 

[PeroK]

There are many occasions when the Sun 'burns off' fog and low cloud in the morning and when the heating of the land near the coast induces an onshore wind, which can cause clouds to form on the coastline. If he happened to live near the sea then his observation could have been quite correct.

Even at the seaside, I imagine, the Sun doesn't get blown along with the clouds on a windy day!

 

[sophiecentaur]

That's pretty much it... and most of the gases in the Earth's atmosphere were emitted a molecule at a time from the solid material that coalesced to form the earth, so were already mostly moving in the right direction.

And Newton's first law of motion tells us that there's no reason for them to slow down because there's no friction between the atmosphere and outer space.
In the distant past there was a lot of argument against a heliocentric universe, based on the comments in the OP. If you believe that 'things naturally slow up' then you can make all sorts of daft conclusions.

 

[sophiecentaur]

Even at the seaside, I imagine, the Sun doesn't get blown along with the clouds on a windy day!

How is that a relevant comment? This was not implied, either by my comment or what the schoolboy claims. There is a casual relationship between the Sun and what happens to the clouds. There is also some correlation, the other way round.
Edit:
I think I was just reading your original statement in a way that you didn't intend. If you had posted his actual words then I think we would be in agreement.

 

[PeroK]

How is that a relevant comment? This was not implied, either by my comment or what the schoolboy claims. There is a casual relationship between the Sun and what happens to the clouds. There is also some correlation, the other way round.

There was a windy day a day or two after the claim was made and the clouds were clearly moving across the sky faster than the Sun. He didn't argue with that.

It wasn't about cloud formation, it was about the clouds moving with the Sun.

In any case, we lived nowhere near the sea. This is silly!

 

[sophiecentaur]

This is silly!

Sure is.

 

[some bloke]

Perhaps an important thing is to change your perspective of the Earth & our atmosphere.

A lot of people think of the atmosphere as a separate entity to the planet, but it's really just a part of the planet. Look at Jupiter, a gas giant - If there is a solid mass in the middle of it, then the rest of it is the atmosphere. It's many orders of magnitude thicker than ours (both proportionally and directly) but you wouldn't consider the larger part of Jupiter to be not-really-Jupiter.

The atmosphere is a part of the earth, held in by gravity and awfully convenient for the planet to be as awesome as it is. Us thinking of the atmosphere as being something else would be like dolphins thinking the ocean wasn't a part of the planet!

 

[Arjan82]

In the distant past there was a lot of argument against a heliocentric universe, based on the comments in the OP. If you believe that 'things naturally slow up' then you can make all sorts of daft conclusions.

Before Newton they didn't really understand movement. They would argue that you would 'feel' it when you are moving. Back then the only reference was when you were moving using a horse / wagon / ship or your feet. In all cases you were very well aware that you were moving.

 

[vanhees71]

Well, even much later than Newton people were not aware of motion and particularly Galilei invariance. When trains came up in the 19th century some physicians warned against their use with the argument that very fast speeds could be unhealthy ;-)).

 

[Vanadium 50]

Do you see the same issue with the ocean? Why or why not?

 

[A.T.]

Well, even much later than Newton people were not aware of motion and particularly Galilei invariance. When trains came up in the 19th century some physicians warned against their use with the argument that very fast speeds could be unhealthy ;-)).

Weren't they merely worried of how the visual impression of a fast moving landscape would affect the brain?

 

[ardnog]

They once thought you would not be able to breathe if you moved faster than a horse could gallop.

 

[bo reddude]

Perhaps an important thing is to change your perspective of the Earth & our atmosphere.

A lot of people think of the atmosphere as a separate entity to the planet, but it's really just a part of the planet. Look at Jupiter, a gas giant - If there is a solid mass in the middle of it, then the rest of it is the atmosphere. It's many orders of magnitude thicker than ours (both proportionally and directly) but you wouldn't consider the larger part of Jupiter to be not-really-Jupiter.

The atmosphere is a part of the earth, held in by gravity and awfully convenient for the planet to be as awesome as it is. Us thinking of the atmosphere as being something else would be like dolphins thinking the ocean wasn't a part of the planet!

that's an interesting take.

 

[A.T.]

They once thought you would not be able to breathe if you moved faster than a horse could gallop.

Well, it depends how much faster, and if you are enclosed in a cabin.

 

[bo reddude]

Do you see the same issue with the ocean? Why or why not?

I do not follow. in air you can move faster than the speed of sound because it's not as dense. in liquid, the speed of sound is higher and it is much denser. so moving faster than the speed of sound under water seems impossible. unless you're talking about moving in a cavitation bubble, then you're not really moving in liquid, but in air. so... i have no idea

 

[A.T.]

... so moving faster than the speed of sound under water seems impossible.

While Ecuadorians living in supersonic wind seemed like a possibility to you?

 

[bo reddude]

While Ecuadorians living in supersonic wind seemed like a possibility to you?

i didn't say they were living in supersonic wind. i asked WHY they don't.

 

[PeroK]

I don't follow. what does that have to do with me asking why people do not experience supersonic conditions at the equator and

A.T asking me

"While Ecuadorians living in supersonic wind seemed like a possibility to you?"

and me asking

"i didn't say they were living in supersonic wind. i asked WHY they don't."

Sorry, I thought the question had already been answered several times in this thread. E.g. posts #2, #10 and #18.

 

[bo reddude]

Sorry, I thought the question had already been answered several times in this thread. E.g. posts #2, #10 and #18.

yes it has. but what you're referring to is equating supersonic movement in air to the supersonic movement in liquid which what A.T referred to

 

[PeroK]

yes it has. but what you're referring to is equating supersonic movement in air to the supersonic movement in liquid which what A.T referred to

The Earth and its atmosphere are in a dynamic, chaotic equilibrium. If the Earth was a giant, totally smooth billiard ball, with no geological or organic processes, then maybe it could rotate independent of its atmosphere.

The Earth doesn't sustain a "static" atmosphere, partly because of the way the atmosphere formed and partly because of the interactions between the Earth and the atmopshere.

What the Earth's rotation does sustain is the chaotic atmospheric conditions. For example, a region of high air pressure cannot easily reach equilibrium with an area of low pressure, as the Coriolis effect creates air currents (cyclones and anti-cyclones) instead. So we have weather systems instead of static equilibrium.

 

[bo reddude]

The Earth and its atmosphere are in a dynamic, chaotic equilibrium. If the Earth was a giant, totally smooth billiard ball, with no geological or organic processes, then maybe it could rotate independent of its atmosphere.

The Earth doesn't sustain a "static" atmosphere, partly because of the way the atmosphere formed and partly because of the interactions between the Earth and the atmopshere.

What the Earth's rotation does sustain is the chaotic atmospheric conditions. For example, a region of high air pressure cannot easily reach equilibrium with an area of low pressure, as the Coriolis effect creates air currents (cyclones and anti-cyclones) instead. So we have weather systems instead of static equilibrium.

that doesn't answer why it's impossible to have supersonic speed in liquid which what you and AT were referring to. many things are chaotic. life is chaotic. life is complex. but what you and AT refer to is equating the supersonic speed in air vs supersonic speed in liquid. now that you have, is that even possible?

 

[PeroK]

that doesn't answer why it's impossible to have supersonic speed in liquid which what you and AT were referring to. many things are chaotic. life is chaotic. life is complex. but what you and AT refer to is equating the supersonic speed in air vs supersonic speed in liquid. now that you have, is that even possible?

eh?

 

[bo reddude]

eh?

go read what you said.

 

[bo reddude]

Do you see the same issue with the ocean? Why or why not?

could you expound this please?

 

[Ibix]

could you expound this please?

The point, I think, is that the underlying problem you have with the atmosphere is that it's not nailed down, but rotates with the Earth anyway. The same is true of the oceans. So if you personally don't have the same problem with the oceans as with the air, there's a crack in your thinking that you might be able to lever open and gain insight.

The basic answer is friction. If the atmosphere were, in bulk, rotating at a different rate from the ground, each collision between an air atom and a ground atom would tend to result in the average rotation rates getting closer together. There would be a transfer of energy from the faster rotating part to the slower one. The same applies to the ocean.

As others have noted, a stationary atmosphere and a spinning Earth is not a realistic model of atmosphere formation. But this does explain why co-rotating Earth and atmosphere is stable. If some combination of events (billions upon billions of carefully coordinated and aimed micrometeorites burning up in the upper atmosphere, perhaps) made the atmosphere rotate a little faster, friction would slow it down again, passing some of its energy on to the Earth.

 

[russ_watters]

They once thought you would not be able to breathe if you moved faster than a horse could gallop.

Do you have a source for that? Surely nobody noticed breathing problems while riding a horse and people experienced faster winds (and horse + wind). Sounds like an old wive's tale to me.

 

[A.T.]

i didn't say they were living in supersonic wind. i asked WHY they don't.

Then I must have misunderstood. You seemed to assume the supersonic wind, and asking why there are no sonic booms:

Why doesn't the Earth crust moving under all that air produce a sonic boom, or constant sonic boom, or something?

 

[Office_Shredder]

He says why doesn't the Earth's crust move under the air, that's asking why the wind doesn't exist.Edit to add: with that said, I thought the atmosphere did move net west to east on average, because the rotation is not literally in sync with the earth. Is that true? Many of the responses here make it sound like that is false.

 

[Arjan82]

I believe that depends on the hemisphere you are on:

These are the trade winds. I'm not sure if they average out to zero however.

 

[Vanadium 50]

hese are the trade winds. I'm not sure if they average out to zero however.

If they don't, where does the air accumulate? And what part of Earth will soon be under vacuum?

 

[russ_watters]

I do not follow. in air you can move faster than the speed of sound because it's not as dense. in liquid, the speed of sound is higher and it is much denser. so moving faster than the speed of sound under water seems impossible. unless you're talking about moving in a cavitation bubble, then you're not really moving in liquid, but in air. so... i have no idea

This doesn't follow the logic of your opening post: your logic was that since the atmosphere is not firmly attached to the Earth, it should not rotate with the Earth. The same logic will take you to the same place with the oceans. But you seem to recognize the flaw regarding the oceans (drag), so...?

Also note it's not about the speed of sound itself, which is higher in water.

 

[russ_watters]

If they don't, where does the air accumulate? And what part of Earth will soon be under vacuum?

To the east.

(There is no need for them to accumulate.)

 

[russ_watters]

Also, it's believed that some of the water on Earth did in fact start off moving tens of thousands of mph, so it did need to stop somehow.

 

[jbriggs444]

If they don't, where does the air accumulate? And what part of Earth will soon be under vacuum?

The mean torque of Earth on atmosphere must be zero, else angular momentum would build without bound. That entails neither that the mean east-west surface wind velocity be zero nor that the net spinward flow rate be zero.

 

[Arjan82]

If they don't, where does the air accumulate? And what part of Earth will soon be under vacuum?

The Earth is round, so you can have an average westerly flow while avoiding a vacuum

 

[Arjan82]

Also, it's believed that some of the water on Earth did in fact start off moving tens of thousands of mph, so it did need to stop somehow.

You mean when it surged through the atmosphere on the back of a comet?

 

[Vanadium 50]

To the east.

Well, my comment was somewhat tongue-in-cheek, but the point is that the continuity condition constrains what the air can do. An average rotation is one solution. (I think the requirement is no net flow across lines of latitude). But this is kind of a digression - I think the difference between water and air is at the base of the OP's misunderstanding: if an ocean of water gets carried with the earth, why not an ocean of air?

 

[Office_Shredder]

http://adsabs.harvard.edu/full/1966SSRv...6..248KThis has some interesting things to say right on the first page. If you go up high enough you do get maybe west to east motion of about 100m/s measured by looking at satellite drift over time?, and also the upper atmosphere moving approximately with Earth is hypothesized to be at least in part driven because charged particles in the upper atmosphere get dragged along with Earth's magnetic field.

This was written in 1966, it feels like there must be something more up to date (I'm surprised they could measure satellite orbital drift back then...)

 

[hmmm27]

Earth rotates faster than the speed of sound at the equator, but it doesn't produce a sonic boom, why?

It does. Well, sortof.

Rather than the measly hundreds of meters per second that the Earth's atmosphere has as a speed-of-sound, the solar wind is trucking along at hundreds of kilometers per second. But, not many particles make it past the magnetic field to impact the atmosphere. So, you'd get a pretty lightweight "sonic hiss", maybe.