[chris (OP)]

Saturn too is fatter around its middle. Why, in common with most people living in the Western world, should the Earth have an enlarging waistline?

Chris

[Heliotrope]

Basically the equatorial bulge of the planets, most notably the gas giants (Jupiter has a clearly visible equatorial bulge even in small telescopes), is caused by overconsumption of seasonal fare.
The leaner planets know what's good for them and they consume a minimum during their festive periods which considering they have a much shorter year is better for their health.
The larger planets overcompensate for their longer year by binge consumption during their festive period.
This leads to an imbalance of diet and exercise which causes the noticable equatorial bulges on the larger planets.
It has been suggested in assorted journals that perhaps counselling may be the answer to the issues raised by binge consumption and it's conspicuous lifestyle implications in the context of the outer planets especially.

[lightarrow]

The answer is: centrifugal force (from planet's rotation), but I prefer Heliotrope's answer.

[JimBob]

The shape of the earth is not bigger around the middle it is bigger around the 'hips.' The shape is not constant and the reasonson given in the article referenced ( http://www.nasa.gov/vision/earth/lookingatearth/earthshape.html ) the earth has a slight bulge around the bottom. Apple shaped it has been called, I believe; nothing to do with pectin though.

[chris (OP)]

That's really interesting. So it's El Nino affecting the Pacific Decadal Oscillation and hence the mass and water distribution around the equator.

Chris

[Heliotrope]

That's a part of it but not the whole story.
A rotating sphere composed mostly of fluid like the Earth : water, magma, liquid iron etc... will naturally bulge out at the equator.
Blow up a balloon and glue it to the centre of a turntable. Put the deck on 78rpm and watch the balloon.
Obviously here it's not going to be exactly an equatorial bulge in our balloon because it's being influenced by the Earth's gravity but you'll see the general principle.
It's better if you fill the balloon with water but I don't want to ruin a perfectly good record deck.

If you removed all the water from the oceans you'd still have a bulging equator due to the fluid nature of what most of the planet is made of.

[JimBob]

First, not to be boastful but to give you an idea of where I am coming from, I have been a geologist for 35+ years. I am not perfect but this is a subject I am rather clear about.

The "liquid" below a mile or three of ocean water is about the consistency of dried window putty. Bass, another guy here, says he thinks of this liquid as nearly fully crystalized oatmeal mush that has been on the table for 2 weeks. When you see liquid flowing down a volcano, that is the exception to the rule. A volcano sits atop a "hot spot" in the earth's crust; a place where there is a highly exagerated amount of heat.

In the article I quoted in the last answer it says: "Similar to El Nino, but lasting 20 to 30 years instead of months, the Pacific Decadal Oscillation is a long-term temperature fluctuation in the Pacific Ocean. The oscillation also brings changes in the location of the cold and warm water masses that can alter the path of the jet stream, which moves storms around the world.

"These changes redistribute water mass among the oceans, and water vapor in the atmosphere, and in soil on the continents, resulting in slight but detectable changes in the Earth's gravity field.

"Cheng and Tapley found that the variations in mass, which caused the shift in the gravity field, were predominantly over the continents, with a smaller contribution due to changes over the ocean."

Gravity measures the mass of the earth over a spot on the earth. The increase in gravity fields is due to water in soil on the continents. There is more water in the rainforest than in the ocean. That is the important information in the last two sentences quoted.

The caption for the bottom black and white illustration says it all: Image left: Factors that Affect Earth's Shape: This illustration depicts many of the factors that affect how the Earth's shape changes. They include such as: winds, earthquakes, post-glacial rebound, plate motion, melting of ice, atmospheric pressure and more. Click on image to enlarge. Credit: NASA (NOTE THAT THE ROTATION OF THE EARTH ISN'T MENTIONED. THE MOST IMPORTET ONES ARE)

The wind is the most important factor in oceanic water movement; the the fluidity of the rocks in the crust, the mantle, etc, has nothing to do with the earth's shape. "Geopotential" does include a vector for water but as stated in the article quoted, this is usually greater over land.

[Heliotrope]

First, not to be boastful but to give you an idea of where I am coming from, I have been a geologist for 35+ years. I am not perfect but this is a subject I am rather clear about.

I'm not interested in arguments from authority, real or supposed.
I am interested in the facts.

...the the fluidity of the rocks in the crust, the mantle, etc, has nothing to do with the earth's shape.

Incorrect.
Why do you think Jupiter has a massive bulge around it's middle ?
The planet is mostly gas ie. a fluid and rotates at high speed. It's day is only 10 hours long.
It bulges around the middle because of the redistribution of fluid due to it's angular momentum.

Anything fluid that rotates will naturally try to flatten out to the limits of it's material elasticity.

Clay on a potter's wheel.
A water filled balloon on a turntable.
A giant gas planet.
A massive molecular cloud of gas and dust.
Galaxies.

The Earth is mostly fluid.
When it spins some of the fluid is redistributed due to the angular momentum of the Earth.
It can't go that far because it's not that pliant. Hence the equatorial bulge of the Earth is a much lower percentage by radius than is Jupiter's.
Gas is easier to move than magma.

Read this : http://en.wikipedia.org/wiki/Equatorial_bulge

In comparison to the radius of the Earth the oceans are about the same proportion as the thickness of the paint on a football.
The oceans have almost nothing to do with it.

[Heliotrope]

The caption for the bottom black and white illustration says it all: Image left: Factors that Affect Earth's Shape: This illustration depicts many of the factors that affect how the Earth's shape changes. They include such as: winds, earthquakes, post-glacial rebound, plate motion, melting of ice, atmospheric pressure and more. Click on image to enlarge. Credit: NASA (NOTE THAT THE ROTATION OF THE EARTH ISN'T MENTIONED. THE MOST IMPORTET ONES ARE)

You have been misled because you have not seen the significance of a couple of words in the quotation.
The words : and more
You have also assumed that the quotation mentions everything that affects the shape of the Earth and therefore anything that is not mentioned is excluded.
That is an incorrect assumption.

"They include such as: winds, earthquakes, post-glacial rebound, plate motion, melting of ice, atmospheric pressure and more."

That diagram ignores the primary factors influencing the shape of the planet.
It is not comprehensive, nor is it forthcoming with it's omissions.
A poor diagram all around.

In fact, looking at it again it's an exceptionally poor diagram and is not at all representitive of the composition of the planet.
I can't really work out what it's supposed to be showing people but whatever it is they're going to come away with a very hazy and mostly incorrect impression of how the planet is put together.
It's precisely the sort of diagram I'd expect to see in a textbook for 8 year olds.
Actually I think I'll clarify that and dig out my old textbooks to see.

[Heliotrope]

Researchers at The University of Texas at Austin using NASA satellite data found that significant changes in the Earth's shape in the past 28 years may be linked to climate events such as the El Nino weather pattern.

The entire premise of the article is based on the last 28 years data !
Good lord.
Of course there are going to be changes due to environmental factors in that time.
The oceans move around etc... Tides go up and down.

None of that changes the shape of the planet.
Only the distribution of water around the planet.
It's like saying that the mud on a football changes it's shape.
Of course it doesn't.
But if you measure it with a laser scanner it's not going to be exactly football shaped.
Now go play another game with the same ball.
Measure it again and you'll find that miraculously the shape according to your laser scanner has changed !
Wow.

 []

[JimBob]

Please refer to http://www.gsfc.nasa.gov/gsfc/40th/firsts.htm - on this page at #24 it says "First use of satellite tracking data to improve our knowledge of Earth's gravity field, learning we have a pear shaped world. (Minitrack on Vanguard l)" This was in 1959. The actual place where the earth's bulge is the greatest is south of the equator.

The data you quoted from Wikipedia is from British Navy Observations before World War I. It has persisted because research has a time lag getting into text books. The British Naval data was proven wrong by the Minitrack experiment on the Vanguard satellite in 1959 and that knowledge has yet to get into the non-specialised fields. New data comes along and is incproprated into the public knowledgebase slowly.

The article I first cited - http://www.nasa.gov/vision/earth/lookingatearth/earthshape.html - is the newest knowledge and accounts for the gravity data discovered in 1959. The findings presented in this cited article are the state-of-the-art understanding.

And please, try to be civil when discussing an issue. Looks like someone came home tipsy from the pub. You stated in one of your diatribes "In fact, looking at it again it's an exceptionally poor diagram and is not at all representitive of the composition of the planet. I can't really work out what it's supposed to be showing people but whatever it is they're going to come away with a very hazy and mostly incorrect impression of how the planet is put together. It's precisely the sort of diagram I'd expect to see in a textbook for 8 year olds. Actually I think I'll clarify that and dig out my old textbooks to see."

I suppose you are so much smarter than NASA you can criticize the scientists who have spent their lives studying the earth and drew the diagram.   

[neilep]

NOTE:

Hi Dave (Heliotrope)
 
 I think it wouldn't be unfair to ask you what your credentials are..if only so that others may understand where you are coming from and that we can understand the substantiation of your reasoning . Do you have any other accredited data rather than a wikipedia entry  ? We just need you help in understanding your position that's all. Thank you so much .

We all need to emphasize the ability to interact with each other accordingly in a manner that is courteous and with all due respect by all parties to all parties.

[jysk]

This thread has become embarrassing and nearly depressing to follow, but I can't ignore some comic irony in the ranting.

I'm not interested in arguments from authority, real or supposed.

Heliotrope's same post suggests politely that JimBob should;

Read this : http://en.wikipedia.org/wiki/Equatorial_bulge

Another unreferenced and un-sourced article carrying with it all the credentials of the most recent author's (self-appointed) expertise.

(To be fair, the fellow that wrote this most recent version for Wikipedia does seem passionate about rotational physics but he doesn't reference any geologic background. This version has stood up for years but is overdue for a rethink.)

So after many milliseconds of critical thinking about Hoola-Hoops and Equatorial Bulge, and many minutes of unsportsmanlike ranting, Heliotrope finally states;

The oceans have almost nothing to do with it.

We're only looking for 26 odd miles of extra girth about the equator. My money is on Nasa's understanding of the model. The oceans have everything to do with it.

Mike

[woody000]

I just found this thread on Google and wanted to comment. I'm not an expert by any means; I'm just a third year physics student, studying these kinds of things myself right now.

Perhaps I'm just totally off-base, but aren't you talking about very different things here? If you take a Geoid of the Earth, then yes it's fatter around the middle, or pear shaped to be more precise. But a Geoid is an equipotential surface- in simple terms it's kind of like taking an average over an area. If you take the difference between the highest point and lowest point on a Geoid, you'll find it's MUCH smaller than the difference between the highest and lowest points on the actual surface of the Earth. The mentioned factors of "winds, earthquakes, post-glacial rebound, plate motion, melting of ice, atmospheric pressure" all have huge impacts on the LOCAL variation in the shape of the Earth. Mountains like Everest are obviously not due to gravitational effects. So if you stand in front of Everest- that's what one of those factors caused, and it's much larger than anything that the rotation of the Earth could produce locally.

But then if you look at the overall shape of the earth, the distance to the centre as it varies around the planet- if you produce a Geoid, you find that the OVERALL, GENERAL shape of the Earth is hugely affected by rotation.

One thing is talking about the elevation of land and the "sticky-out" bits, the other is talking about the distances of all those positions (at the top of mountains and the bottom) to the centre and finding the overall shape.

They're surely two very different things and can't be compared in such a simple way. Surely you can't say one's more significant than the other. That's nonsense, isn't it?

So in conclusion, feel free to correct me if I'm just being a silly inexperienced youth, but aren't you both right?

[daveshorts]

The earth isn't like an egg it is a very slightly flattened spheroid. eg see the much exagerated version below:
[diagram=165_0]

[BillJx]

My money is on Nasa's understanding of the model. The oceans have everything to do with it.

Mike

I'm no geologist, or geology student either, but I can read.  The NASA site doesn't refer to the causes of the equatorial bulge per se, but to causes of some changes in local gravities, and therefore to the Earth's shape, that have been detected by laser measurements.  It's talking about movement of air and water masses.  Miniscule compared to the mass of the Earth.  According to the article, the measurements they are using are accurate to within a millimeter.  Of course they will detect changes.  This article just discusses some of the reasons.  It's a fairly interesting article, but I don't see where it has any bearing on this thread.

[ukmicky]

As the earth is mainly comprised  of fluids ,molten rock, water etc its shape is mainly determined by the speed of the earths rotation and the differences in the strength of the earths gravity field at the poles and equator ,its a sort of a fight between rotation and gravity

However i also heard as the ice caps melt  the gravity field of the earth is changing causing  the strength of gravity to increase at the poles and decrease at the equator allowing  the central bulge to increase even more.

[daveshorts]

so if its squashed in that way its, Magnetics that puhes out from the north and south which creates a downward force and pushes out the sides. p.s. Thats an egg on its side. I thought it was squashed around the equator. If it is as you say then that in my opinion would explain it. If its not then its what I was thinking before (thats at the moment ofcourse, I'll have a new reason tomorrow no doubt). 

Sorry if I didn't make this clear, it isn't like an egg on it's side because the earth is symmetrical about it's axis - the line from the north to south pole. An egg on it's side most certainly wouldn't be.

On a different point:

As the ice caps melt, it will cause a net movement of not very dense stuff - water - from the poles to the equator. Which will cause the equator to bulge out a little more.

[daveshorts]

No again it isn't egg shape, here are some diagrams which may (or may not) help to enlighten you:
The earth from the top, a circle, with the north pole in the centre.
[diagram=171_0]

A cross section of the earth along one red line

[diagram=172_0]

A cross section of the earth along the other

[diagram=172_0]

Hence it is a sphere that has been squashed top to bottom, unlike an egg which is a sphere that has been stretched top to bottom.

[ukmicky]

 When you say the earth is egg shaped what type of bird are you talking about. You see birds eggs can be many different shapes and generally depends on the type of nest or lack of nest that the bird builds. Some like owls tend to be round, sea birds tend to be thin at one end so they cant roll away on the bare rocks and chickens tend to be oval. Personally i would prefer it to be like a peregrine falcons as i like peregrine falcons.  []

Also what makes you think there's a blackhole at the centre of the earth,how did it form and why hasn't the earth been pulled in.