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

• B
• bo reddude
In summary: 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):
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?

bo reddude said:
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.

rsk, Klystron and davenn
bo reddude said:
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.

DaveE said:
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?

bo reddude said:
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?
bo reddude said:
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.

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bo reddude said:
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!

rsk and davenn
PeroK said:
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.

bo reddude said:
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.

davenn
PeroK said:
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.

bo reddude said:
... 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

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Klystron
bo reddude said:
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.

vanhees71
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.

PeroK said:
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.

russ_watters and PeroK
sophiecentaur said:
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
Nugatory said:
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.

PeroK
PeroK said:
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.

sophiecentaur said:
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!

PeroK said:
This is silly!
Sure is.

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!

nasu, Vanadium 50, vanhees71 and 3 others
sophiecentaur said:
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 ;-)).

Arjan82
Do you see the same issue with the ocean? Why or why not?

vanhees71 said:
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?

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

anorlunda
some bloke said:
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.

ardnog said:
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.

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

bo reddude said:
... so moving faster than the speed of sound under water seems impossible.
While Ecuadorians living in supersonic wind seemed like a possibility to you?

A.T. said:
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.

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

"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 said:
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

bo reddude said:
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.

PeroK said:
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?

bo reddude said:
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?

PeroK said:
eh?

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

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