Why is Earth considered round despite its bumpy surface?

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In summary: Africa?) - but it's not a true representation of the Earth's physical shape. It's an image meant to show something else.I don't think this looks very spherical? Looks more like my cats chew toy.
  • #1
Jacinta
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I mean it would be impossible for Earth to be fully spherical as it would be too bumpy to be a sphere shape.

If this is the case, why is Earth considered round?
 
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  • #2
Jacinta said:
... why is Earth considered round?
'Cause it's close enough for government work. You do the math. Figure out the tallest mountain as a percentage of the radius of the Earth.

Also, you forgot to mention tidal bulge and deformations due to (1) mass distribution differences and (2) the fact that the Earth is spinning.

Oh, and by the way, it is NOT considered round by scientists when they are being precise. That's just a pop-science simplification.
 
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  • #3
Jacinta said:
I mean it would be impossible for Earth to be fully spherical as it would be too bumpy to be a sphere shape.

If this is the case, why is Earth considered round?

Because it looks round?

hmmm... I think this would make an interesting experiment:

Find the highest resolution image of the Earth from outer space, and see if you can detect a difference in diameter when measuring along the equator vs from pole to pole.​

The highest resolution image I've found so far is 11,500 x 11,500 pixels, at NASA. [ref] Though, they claim it's a composite, and they claim there are even larger composites!
 
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  • #4
Hi Jacinta! :oldsmile:

On a scale where we can see the entire earth, it's pretty hard to see that the Earth is not exactly a sphere.
We should not be able to distinguish any mountain, since even the highest mountain is insignificant in comparison with Earth's size (0.15%).
If we see anything, it's that the Earth is about 0.5% bigger on the equator than on the poles as @OmCheeto mentioned.
The Earth is probably closer to a sphere than anything in every day life that we consider spherical. o_O
 
  • #5
I don't think this looks very spherical? Looks more like my cats chew toy.

geoide.png


This is a sphere.

giphy.gif
 

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  • #6
Because the Earth spins the radius is about 13 miles greater at the equator than the poles.
 
  • #7
phinds said:
Oh, and by the way, it is NOT considered round by scientists when they are being precise. That's just a pop-science simplification.

But it's dumb, shouldn't scientists do something about this? A sphere is something you can roll around, I don't think you can roll around the Earth.
 
  • #8
CWatters said:
Because the Earth spins the radius is about 13 miles greater at the equator than the poles.

So, in simple terms, you think that the Earth decreases in miles closer to the equator?
 
  • #9
Increases.
 
  • #10
CWatters said:
Increases.

How? Isn't the equator fatter than the north pole?

Edit: Nvm, read wrong
 
  • #11
Jacinta said:
I don't think this looks very spherical? Looks more like my cats chew toy.

Indeed. That doesn't look spherical at all.
It looks more like a model of the Earth that a cat chewed on! :nb)

For reference, Earth actually looks like this:
http://stream1.gifsoup.com/view1/1210397/earth-o.gif
 
  • #12
Jacinta said:
I mean it would be impossible for Earth to be fully spherical as it would be too bumpy to be a sphere shape.

If this is the case, why is Earth considered round?
Would you consider a billiard ball as being spherical? If so, if you were to scale the Earth down to the size of billiard ball, the roughness of it surface due to mountains a valleys would be less than the manufacturing tolerances for irregularities for a billiard ball. Even including the equatorial bulge leaves the Earth pretty close to that tolerance.
 
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  • #13
I like Serena said:
Indeed. That doesn't look spherical at all.
It looks more like a model of the Earth that a cat chewed on! :nb)

For reference, Earth actually looks like this:
http://stream1.gifsoup.com/view1/1210397/earth-o.gif

From a far it gives off a spherical illusion, but it actually looks like a cats chew toy.
 
  • #14
Janus said:
Would you consider a billiard ball as being spherical? If so, if you were to scale the Earth down to the size of billiard ball, the roughness of it surface due to mountains a valleys would be less than the manufacturing tolerances for irregularities for a billiard ball. Even including the equatorial bulge leaves the Earth pretty close to that tolerance.

Yes, because a billiard ball can roll around.

The Earth can't roll around but spin around, there is a difference.
 
  • #15
Rolling:

giphy.gif


Spinning:

spinning-gif-9.gif
 

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  • #16
But... a Simpson that can spin can also roll can't he?
Round or not.

giphy.gif
 

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  • #17
I like Serena said:
But... a Simpson that can spin can also roll can't he?
Round or not.

View attachment 215344

That's because his body sideways is rollable because of his cylinder shape.

We are discussing spheres.
 
  • #18
Jacinta @5 - this -

geoide-png.png


- is not an image of the Earth as it is, but is an artificially created image intended to show something other than the Earth's physical shape. Not sure exactly what it shows - gravitational potential of Earth with oceans removed maybe?

This is an image of the Earth - ( in this case, showing the shadow of the Moon during an eclipse in 2017)

StudyingEarth_3.jpg
 

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  • #19
Jacinta said:
But it's dumb, shouldn't scientists do something about this? A sphere is something you can roll around, I don't think you can roll around the Earth.
No, it's just standard pop-science. YOU do something about it if you actually imagine that you are going to change the culture of popular science reporting. Saying that popular science reporting is dumb is a tautology and a waste of breath.
 
  • #20
phinds said:
No, it's just standard pop-science. YOU do something about it if you actually imagine that you are going to change the culture of popular science reporting. Saying that popular science reporting is dumb is a tautology and a waste of breath.

Interesting.
 
  • #21
Jacinta said:
Yes, because a billiard ball can roll around.

The Earth can't roll around but spin around, there is a difference.
The Earth could roll if it had something to roll on. It's wobble would not be noticeable; it is way too small. It would roll better than a baseball or soccer ball.
 
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  • #22
Jacinta said:
Interesting.

Please read post #18. That's the answer to your question.

is not an image of the Earth as it is, but is an artificially created image intended to show something other than the Earth's physical shape
 
  • #23
Jacinta said:
If this is the case, why is Earth considered round?

Expand the shape of the Earth in terms of spherical harmonics. The leading term is a perfect sphere, all other terms are so small that to a first approximation, and a very good one at that, the Earth is a perfect sphere.
 
  • #24
I thought it might be an idea to find out where that first image in #5 came from and what it represents. That has proved difficult.

It seems to appear in press coverage of the launch of the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite in 2009, so clearly it's older than that.

Then I found it in a German article written and edited between March and November 2002 about the GRACE satellite which launched in March 2002. However all the images on the GRACE website appear to be higher resolution (I mean the show changes in gravity in more detail not the image resolution).

My guess is it's either an early image from GRACE or it might be from CHAMP which was launched in 2000 I think.

Anyway its most likely an image of the Earth modified to show how gravity varies due to changes in density and the like.
 
  • #25
Jacinta said:
I mean it would be impossible for Earth to be fully spherical as it would be too bumpy to be a sphere shape.

If this is the case, why is Earth considered round?
Read http://chem.tufts.edu/AnswersInScience/RelativityofWrong.htm
I've linked to this so many times that I should be getting royalties...
 
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  • #26
CWatters said:
Anyway its most likely an image of the Earth modified to show how gravity varies due to changes in density and the like.
Yes, and not just modified but also deliberately exaggerated.
 
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  • #27
russ_watters said:
The Earth could roll if it had something to roll on. It's wobble would not be noticeable; it is way too small. It would roll better than a baseball or soccer ball.
A ball has a relatively thick tough outer surface under tension, with air inside under pressure that makes it approximately spherical.

The Earth has a very thin and fragile crust on a fluid mantle and a solid core. The crust floats on the surface and cannot handle tension. If you tried to roll the Earth on a flat surface it would immediately collapse, (like a fried egg), into a glob of red hot stainless steel, in the middle of a puddle of lava.

By all means keep the Earth spinning in space on an axis, but try to avoid playing snooker with planets, or balloons full of liquid.
 
  • #28
Baluncore said:
A ball has a relatively thick tough outer surface under tension, with air inside under pressure that makes it approximately spherical.
The seams on a baseball and soccer ball are proportionally taller than the Himalayas and deeper than the Mariana Trench - and also larger than the equatorial bulge of the Earth.
The Earth has a very thin and fragile crust on a fluid mantle and a solid core. The crust floats on the surface and cannot handle tension. If you tried to roll the Earth on a flat surface it would immediately collapse, (like a fried egg), into a glob of red hot stainless steel, in the middle of a puddle of lava.
Agreed. This wasn't a statement about strength of materials, it was a statement about shape.
 
  • #29
Jacinta said:
A sphere is something you can roll around, I don't think you can roll around the Earth.
That's not how the term "sphere" is defined.
 
  • #30
There are 2D figures and coins that are not round, but that have a constant diameter and can roll between two lines with fixed separation. In polar coordinates, the radius would be a constant term + odd harmonics.

Is there a 3D version of that? A non-spherical solid with constant diameter that fits neatly between two planes with fixed separation.
 
  • #31
Baluncore said:
There are 2D figures and coins that are not round, but that have a constant diameter and can roll between two lines with fixed separation. In polar coordinates, the radius would be a constant term + odd harmonics.

Is there a 3D version of that? A non-spherical solid with constant diameter that fits neatly between two planes with fixed separation.
This?
https://en.wikipedia.org/wiki/Dodecahedron
 
  • #32
Baluncore said:
The Earth has a very thin and fragile crust on a fluid mantle and a solid core. The crust floats on the surface and cannot handle tension. If you tried to roll the Earth on a flat surface it would immediately collapse, (like a fried egg), into a glob of red hot stainless steel, in the middle of a puddle of lava.
The outer core is liquid. The mantle is solid that “flows” on geologic time scales. The mantle also makes up the vast majority of the earth’s volume. This is verified readily by seismometry: the S waves radiated from earthquakes do not propagate through liquid, which would have easily observable effects of the mantle were liquid.
 

1. Why is Earth considered round?

The Earth is considered round because it is an oblate spheroid, meaning it is slightly flattened at the poles and bulging at the equator. This shape is a result of the Earth's rotation, which causes the centrifugal force to push outwards at the equator.

2. How do we know that Earth is round?

There are several pieces of evidence that support the roundness of Earth. One of the earliest pieces of evidence was the observation of ships disappearing over the horizon, which can only happen if the Earth is curved. Additionally, modern technology such as satellite imagery and space travel have provided concrete evidence of Earth's round shape.

3. But why does Earth have a bumpy surface?

The Earth's surface may appear bumpy due to mountains, valleys, and other geological features. However, these bumps are relatively small compared to the overall size of the Earth and do not affect its overall round shape.

4. Is Earth a perfect sphere?

No, Earth is not a perfect sphere. As mentioned earlier, it is an oblate spheroid, meaning it is slightly flattened at the poles and bulging at the equator. This is due to the Earth's rotation and the centrifugal force it creates.

5. Does the round shape of Earth have any impact on our daily lives?

Yes, the round shape of Earth has a significant impact on our daily lives. It affects the Earth's rotation, which creates day and night, and also influences the Earth's climate and weather patterns. Additionally, the round shape of Earth allows for the existence of life as we know it, as it provides a stable gravitational pull and atmosphere.

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