# The Naked Scientists Forum

### Author Topic: Why is the earth flat at the poles?  (Read 1512 times)

#### annie123

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##### Why is the earth flat at the poles?
« on: 28/02/2016 20:40:13 »
I'm not sure if this is a geology question - but perhaps someone can answer it anyway. Is there any way of explaining why the earth is flattened at each pole? And what effects does this have on climate, or any other aspect of earthly conditions?
« Last Edit: 28/02/2016 21:27:15 by chris »

#### Thebox

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##### Re: WHy is the earth flat at the poles?
« Reply #1 on: 28/02/2016 20:48:50 »
I'm not sure if this is a geology question - but perhaps someone can answer it anyway. Is there any way of explaining why the earth is flattened at each pole? And what effects does this have on climate, or any other aspect of earthly conditions?

Hi Annie, to describe the poles as ''flat'' is not really a term I would use.  The Earth's shape is an oblate shape.

http://math.stackexchange.com/questions/148839/what-is-the-geodesic-between-a-point-and-a-line-geodesic-between-two-points-on

The poles are sort  of rounded more to a ''point'' rather than ''flat''.

The reason for this shape is rotation, the ''sides'' as such are flung outwards, so the top and bottom pull inwards to compensate.

I have no idea how this affects climate change, maybe somebody else can answer that.

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#### chiralSPO

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##### Re: Why is the earth flat at the poles?
« Reply #2 on: 29/02/2016 00:50:51 »
TheBox is absolutely correct.

I will add: If we ignore the little bit of roughness that is geography (mountains, valleys etc.) the earth is essentially "flat" everywhere. This is a reasonable assumption--if the earth were the size of a marble, it would be smoother than the smoothest marble in existence. A marble will sit still on any flat surface that you place it on, no matter which point is on the bottom, because it is "flat" at every point. The same applies to the Earth.

Now, as TheBox pointed out, the Earth is spinning and therefore is every-so-slightly distorted such that it is wider across the equator than it is from pole to pole.

This distortion of shape of the Earth doesn't really have much effect on the climate that I know of. But the spinning that causes the distortion is also responsible for the Coriolis effect, which has some effects on the weather (not on the climate though, as far as I know) (https://en.wikipedia.org/wiki/Coriolis_force#Meteorology)

#### Thebox

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##### Re: Why is the earth flat at the poles?
« Reply #3 on: 29/02/2016 09:57:26 »
TheBox is absolutely correct.

I will add: If we ignore the little bit of roughness that is geography (mountains, valleys etc.) the earth is essentially "flat" everywhere. This is a reasonable assumption--if the earth were the size of a marble, it would be smoother than the smoothest marble in existence. A marble will sit still on any flat surface that you place it on, no matter which point is on the bottom, because it is "flat" at every point. The same applies to the Earth.

Now, as TheBox pointed out, the Earth is spinning and therefore is every-so-slightly distorted such that it is wider across the equator than it is from pole to pole.

This distortion of shape of the Earth doesn't really have much effect on the climate that I know of. But the spinning that causes the distortion is also responsible for the Coriolis effect, which has some effects on the weather (not on the climate though, as far as I know) (https://en.wikipedia.org/wiki/Coriolis_force#Meteorology)

It's interesting though Chiral , that the oblate of the Earth  not seemingly  having two points of external force/pressure applied ,  the work done apparently internally.    If we squeeze something for example, we have to apply an equal and opposing force to the direction of force applied from one of the external  force , source point(s).   In motion i.e pushing a car, the single force point overpowering the inertia of the vehicle at relative rest mass, where as when a moving vehicle encounters an opposing an equal or greater force the vehicle comes to an abrupt halt before it contracts (squeezes) relative to speed.   The rear of the vehicle continuing forward in momentum relative to the ''stopped'' front of the vehicle.   Then it is interesting to consider the space-time curvature and to consider the rubber sheet analogy.  However the rubber sheet analogy provides only one point of pressure/force, it may be a presumption , but can we presume space-time surrounds the Earth and atmosphere giving us all the points of pressure/force?

« Last Edit: 29/02/2016 10:07:11 by Thebox »

#### evan_au

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##### Re: Why is the earth flat at the poles?
« Reply #4 on: 29/02/2016 10:27:26 »
Quote from: Annie123
Is there any way of explaining why the earth is flattened at each pole?
If you ignore mountain chains, etc, the shape of the Earth would be the Reference Ellipsoid, which is 21km shorter from center to the Poles than it is from center to equator. This is only a 0.3% difference, which you would not notice if you were looking at it.

This ellipsoid shape affects all spinning bodies, including Jupiter and the Sun - the faster it spins, the more pronounced the equatorial bulge. This would probably be classified as Physics; Newton described this in 1687.

If you want to get more precise about the shape of the Earth, subduction zones produce areas of higher density rocks, and higher local gravity. This produces variations in sea level compared to the Reference Ellipsoid - this is captured by the Geoid (such as those used by GPS to measure height above sea level).

Quote from: TheBox
the Earth not seemingly having two points of external force/pressure applied, the work done apparently internally.
Yes; the shape is not determined by external forces, but it is due to internal hydrostatic equilibrium.

If you ignore tides from the Moon and Sun (which do represent external forces)...

The definition of a planet (or dwarf planet) is that gravity has pulled it into the shape of a sphere (which is easier if the insides get hot enough to melt at some point in its history).

If a planet were not spinning, it would form an accurate sphere. Any point that was above the sphere would be subject to weathering, and would fall into the lower points (at less than the radius of the sphere) and would reform the spherical shape. If the interior is liquid, the molten rock will flow to form the sphere. If there is a liquid ocean or a gaseous/plasma atmosphere, it will form a sphere.

However, for a spinning object, centrifugal force slightly counteracts gravity as you get closer to the equator, so material will tend to flow towards the equator, forming the classic oblate spheroid shape.

#### annie123

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##### Re: Why is the earth flat at the poles?
« Reply #5 on: 01/03/2016 22:12:35 »

#### The Naked Scientists Forum

##### Re: Why is the earth flat at the poles?
« Reply #5 on: 01/03/2016 22:12:35 »