Why is Earth considered round despite its bumpy surface?

[Jacinta]

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?

 

[phinds]

... 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.

 

[OmCheeto]

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!

 

[I like Serena]

Hi Jacinta!

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.

 

[Jacinta]

I don't think this looks very spherical? Looks more like my cats chew toy.

This is a sphere.

 

[CWatters]

Because the Earth spins the radius is about 13 miles greater at the equator than the poles.

 

[Jacinta]

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.

 

[Jacinta]

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?

 

[CWatters]

Increases.

 

[Jacinta]

Increases.

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

Edit: Nvm, read wrong

 

[I like Serena]

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!

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

 

[Janus]

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.

 

[Jacinta]

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

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.

 

[Jacinta]

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.

 

[Jacinta]

Rolling:

Spinning:

 

[I like Serena]

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

 

[Jacinta]

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.

 

[Ken Fabos]

Jacinta @5 - this -

- 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)

 

[phinds]

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.

 

[Jacinta]

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.

 

[russ_watters]

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.

 

[gmax137]

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

 

[Spinnor]

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.

 

[CWatters]

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.

 

[Nugatory]

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...

 

[phinds]

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.

 

[Baluncore]

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.

 

[russ_watters]

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.

 

[Mark44]

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.

 

[Baluncore]

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.