# Is "oblate spheroid" the best description of the earth's shape?
**URL:** https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555
**Category:** Factual Questions
**Created:** [December 31, 2013, 11:00am UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555 "2013-12-31T11:00:32Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@davidmich](https://boards.straightdope.com/u/davidmich)
#### Post date: [December 31, 2013, 11:00am UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/1 "2013-12-31T11:00:32Z")
Hi
Is “oblate spheroid” the best description of the earth’s shape?
I look forward to your feedback.
davidmich
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### Author:  [@Nava](https://boards.straightdope.com/u/Nava)
#### Post date: [December 31, 2013, 11:23am UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/2 "2013-12-31T11:23:49Z")
Nope, geoid.
Literally, “Earth-shaped”. Nothing is more of a geoid than Earth itself.
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### Author:  [@eburacum45](https://boards.straightdope.com/u/eburacum45)
#### Post date: [December 31, 2013, 11:28am UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/3 "2013-12-31T11:28:41Z")
The geoid deviates from a perfect oblate spheroid by −105 m to about +85 m; see
> **[World Geodetic System](https://en.wikipedia.org/wiki/WGS_84#Main_parameters)**
>
> The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also describes the associated Earth Gravitational Model (EGM) and World Magnetic Model (WMM). The standard is published and maintained by the United States National Geospatial-Intelligence Agency.
> Efforts to supplement the various national surveying systems began in the 1...
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### Author:  [@the\_diego](https://boards.straightdope.com/u/the_diego)
#### Post date: [December 31, 2013, 12:21pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/4 "2013-12-31T12:21:09Z")
Geoid is not a shape. It’s an equipotential gravity outline. A weirdly shaped planet would have its own Geoid not the same as earth’s. Stick to oblate spheroid. That’s a shape.
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### Author:  [@Nava](https://boards.straightdope.com/u/Nava)
#### Post date: [December 31, 2013, 12:27pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/5 "2013-12-31T12:27:59Z")
It’s [both](http://www.ngs.noaa.gov/GEOID/geoid_def.html). And the property of words to have more than one meaning, polysemia.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [December 31, 2013, 2:43pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/6 "2013-12-31T14:43:42Z")
The Earth isn’t perfectly geoid-shaped, either, since its surface is not an equipotential (and if it were, it _would_ just be an oblate spheroid).
What’s the “best” description depends on what you want the description for. Sometimes the best description is the simple one, in which case “sphere” is probably what you’re looking for: It’s very, very close to being exactly true, while simultaneously being about as simple as possible.
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [December 31, 2013, 2:44pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/7 "2013-12-31T14:44:27Z")
> [@Nava](#):
>
> And the property of words to have more than one meaning, polysemia.
I don’t think that word [means](http://en.wikipedia.org/wiki/Polysemia_(moth)) what you think it does. 🙂
I am going to remember polysemia. It kind of sets me off when someone “corrects” someone else when they’re just using a secondary definition.
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### Author:  [@njtt](https://boards.straightdope.com/u/njtt)
#### Post date: [December 31, 2013, 5:00pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/8 "2013-12-31T17:00:59Z")
> [@Nava](#):
>
> It’s [both](http://www.ngs.noaa.gov/GEOID/geoid_def.html). And the property of words to have more than one meaning, polysemia.
> [@ZenBeam](#):
>
> I don’t think that word [means](http://en.wikipedia.org/wiki/Polysemia_(moth)) what you think it does. 🙂
>
> I am going to remember polysemia. It kind of sets me off when someone “corrects” someone else when they’re just using a secondary definition.
As **ZenBeam** points out, _polysemia_ is a moth. The actual word for the property, that some words have, of having more than one (unrelated, or only distantly related) meaning is _polysemy_.
I don’t think that is what is going on here though, even if **Nava** is right about _geoid_’s range of meaning. Even if it can mean either a geometrical shape or an “an equipotential gravity outline” (and if the latter is not, strictly, a shape, which point is not entirely clear to me), those meanings are much too closely related for it to reasonably be called a polysemous word.
And, as **Chronos** says, what is the _best_ description will depend on your purpose, and, in many contexts, describing the Earth as “Earth shaped” will not be particularly helpful.
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### Author:  [@cochrane](https://boards.straightdope.com/u/cochrane)
#### Post date: [December 31, 2013, 6:01pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/9 "2013-12-31T18:01:06Z")
> [@the\_diego](#):
>
> Geoid is not a shape. It’s an equipotential gravity outline. A weirdly shaped planet would have its own Geoid not the same as earth’s. Stick to oblate spheroid. That’s a shape.
An American football is often described as an “oblate spheroid.”
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### Author:  [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)
#### Post date: [December 31, 2013, 6:04pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/10 "2013-12-31T18:04:44Z")
> [@cochrane](#):
>
> An American football is often described as an “oblate spheroid.”
An American football is more a prolate spheroid than an oblate spheroid.
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### Author:  [@yabob](https://boards.straightdope.com/u/yabob)
#### Post date: [December 31, 2013, 6:28pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/11 "2013-12-31T18:28:55Z")
> [@Chronos](#):
>
> The Earth isn’t perfectly geoid-shaped, either, since its surface is not an equipotential (and if it were, it _would_ just be an oblate spheroid).
>
> What’s the “best” description depends on what you want the description for. Sometimes the best description is the simple one, in which case “sphere” is probably what you’re looking for: It’s very, very close to being exactly true, while simultaneously being about as simple as possible.
Particularly if you fudge the radius a bit to make great circle distances come out closer to the actual ellipsoid distances, while still being easier to compute.
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### Author:  [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)
#### Post date: [December 31, 2013, 6:33pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/12 "2013-12-31T18:33:53Z")
The “best” word to describe the earth is probably a sphere. It’s close enough to being perfectly round and smooth that if you scaled it down small enough it would pass all of the standards for a modern billiard ball.
The earth does bulge a bit due to its rotation, making it sort of an oblate spheroid, but the bulge is tiny. The diameter across the equator is 12,756 km and the diameter from pole to pole is 12,714 km (if the numbers I googled are correct). That’s a difference of only 42 km out of about 12,700 km, which is only about 0.33 percent away from being a true sphere.
The highest point on earth (Mt. Everest) is a bit less than 9 km above the surface and the lowest point (Marianas Trench, get out your once in 1960 references) is 11 km below it, so the entire variation in height is about 20 km out of 12,700 or so, or about 0.16 percent.
Those percentages are so small that they are within the modern manufacturing tolerances of a billiard ball, which everyone would just call a sphere. So basically, while the earth does bulge a bit, it doesn’t bulge enough (IMHO) to be called anything other than a slightly imperfect sphere. It’s closer to a sphere than anything else.
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### Author:  [@cochrane](https://boards.straightdope.com/u/cochrane)
#### Post date: [December 31, 2013, 6:36pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/13 "2013-12-31T18:36:53Z")
> [@Asympotically\_fat](#):
>
> An American football is more a prolate spheroid than an oblate spheroid.
Well, sportswriters aren’t really known for being rocket scientists. 🙂
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [December 31, 2013, 6:47pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/14 "2013-12-31T18:47:35Z")
> [@njtt](#):
>
> As **ZenBeam** points out, […]
I was actually joking. The joke was that “polysemia” was actually a case of polysemia/polysemy.
Polysemia redirects to polysemy on several webpages, so I didn’t think to check on it. I couldn’t find an online source that claims polysemia as an actual English word, but I will note that polysemy comes from _Latin_ polysemia.
At any rate, I’ll probably remember this now.
[/hijack]
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### Author:  [@Duckster](https://boards.straightdope.com/u/Duckster)
#### Post date: [December 31, 2013, 7:53pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/15 "2013-12-31T19:53:15Z")
> [@Asympotically\_fat](#):
>
> An American football is more a prolate spheroid than an oblate spheroid.
Ob-La-Di, Ob-La-Da.
😃
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### Author:  [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)
#### Post date: [December 31, 2013, 10:30pm UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/16 "2013-12-31T22:30:42Z")
I once read something about this. To a first approximation, the earth is a sphere. To a second approximation, it is an oblate spheroid. That is a degree 2 moment. There is a tiny degree 3 deviation from that. And higher degrees. Then you get into mountain ranges that are irregular.
I once read a book that described the expeditions the French sent out to try to measure this. They discovered that a degree of arc was about a km longer in northern Norway than in Peru. Go with oblate spheroid if you think sphere is too crude. For extra credit, to what place on the earth’s axis does a plumb line dropped at 45 deg north point? (I don’t know the quantitative answer; just say whether it points to the center or a point north of or south of that.)
From the oblateness and the fact that the plumb line always points in the direction perpendicular to the level surface (a standard principle of advanced calculus) you can readily see that it points to a place south of the center of the earth.
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 1, 2014, 1:22am UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/17 "2014-01-01T01:22:05Z")
**STEPHEN** (_turns_)
Eh? (_he disengages himself_) Why should I not speak to him or to any human being who walks upright upon this oblate orange?
—Joyce, _Ulysses_, Circe
* * *
On the eve of the day on which Victor had planned to arrive, Pnin entered a sport shop in Waindell’s Main Street and asked for a football. The request was unseasonable but he was offered one.
‘No, no,’ said Pnin, ‘I do not wish an egg or, for example, a torpedo. I want a simple football ball. Round!’
And with wrists and palms he outlined a portable world. It was the same gesture he used in class when speaking of the ‘harmonical wholeness’ of Pushkin.
The salesman lifted a finger and silently fetched a soccer ball.
‘Yes, this I will buy,’ said Pnin with dignified satisfaction.
—Nabokov, _Pnin_
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 1, 2014, 1:24am UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/18 "2014-01-01T01:24:12Z")
BTW, I believe comments on this thread are oblatory.
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### Author:  [@hibernicus](https://boards.straightdope.com/u/hibernicus)
#### Post date: [January 1, 2014, 1:39am UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/19 "2014-01-01T01:39:33Z")
> [@njtt](#):
>
> As **ZenBeam** points out, _polysemia_ is a moth.
But it is a geometer moth, which seems entirely appropriate.
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 1, 2014, 2:02am UTC](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555/20 "2014-01-01T02:02:22Z")
Well, the [guys who make maps](http://en.wikipedia.org/wiki/WGS-84) call it:
The coordinate origin of WGS 84 is meant to be located at the Earth’s center of mass; the error is believed to be less than 2 cm.[2]
The WGS 84 meridian of zero longitude is the IERS Reference Meridian,[3] 5.31 arc seconds or 102.5 metres (336.3 ft) east of the Greenwich meridian at the latitude of the Royal Observatory.[4][5]
The WGS 84 datum surface is an oblate spheroid (ellipsoid) with major (equatorial) radius a = 6378137 m at the equator and flattening f = 1/298.257223563.[6] The polar semi-minor axis b then equals a times (1−f), or 6356752.3142 m.[6]
Presently WGS 84 uses the EGM96 (Earth Gravitational Model 1996) geoid, revised in 2004. This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 360 (which provides about 100 km horizontal resolution).[7] The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about −105 m to about +85 m.[8] EGM96 differs from the original WGS 84 geoid, referred to as EGM84.
[Next page](https://boards.straightdope.com/t/is-oblate-spheroid-the-best-description-of-the-earths-shape/677555.md?page=2)