# the shape of the earth? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [February 25, 2010, 6:25am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371 "2010-02-25T06:25:40Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Todderbob](https://avatars.discourse-cdn.com/v4/letter/t/e47774/32.png) [@Todderbob](https://boards.straightdope.com/u/Todderbob)\ **Post date:** [February 25, 2010, 11:31pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/21 "2010-02-25T23:31:44Z")
> [@ivan\_astikov](#): > > Why should we assume it was the same size? The law of conservation of matter. Matter is not spontaneously created or destroyed, so where, exactly, did all of this extra matter come from to make the Earth larger than it was? > [@ivan\_astikov](#): > > What evidence? You’ve just said yourself that it couldn’t be accurately measured until the advent of satellites. I said the Earth couldn’t be accurately measured until the use of Satellites, I did not say that it could not be measured and I did not say that evidence could not be found that indicated that its size has not changed. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [February 25, 2010, 11:33pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/22 "2010-02-25T23:33:57Z")
> [@ivan\_astikov](#): > > Verifiably, then. Well, current measurements fall within the Greek’s error bars so there you go, verifiable. ---
**Author:** ![ivan\_astikov](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ivan\_astikov](https://boards.straightdope.com/u/ivan_astikov)\ **Post date:** [February 25, 2010, 11:38pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/23 "2010-02-25T23:38:58Z")
> [@Todderbob](#): > > The law of conservation of matter. Matter is not spontaneously created or destroyed, so where, exactly, did all of this extra matter come from to make the Earth larger than it was? > I said the Earth couldn’t be accurately measured until the use of Satellites, I did not say that it could not be measured and I did not say that evidence could not be found that indicated that its size has not changed. Exactly how many Earth-type planets have we observed for any significant amount of time, again? ---
**Author:** ![Todderbob](https://avatars.discourse-cdn.com/v4/letter/t/e47774/32.png) [@Todderbob](https://boards.straightdope.com/u/Todderbob)\ **Post date:** [February 25, 2010, 11:45pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/24 "2010-02-25T23:45:56Z")
> [@ivan\_astikov](#): > > Exactly how many Earth-type planets have we observed for any significant amount of time, again? Exactly what is your point? That we should disregard physics because you said _“and theeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeen?!”_? ---
**Author:** ![ivan\_astikov](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ivan\_astikov](https://boards.straightdope.com/u/ivan_astikov)\ **Post date:** [February 25, 2010, 11:55pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/25 "2010-02-25T23:55:14Z")
My point was, how would we know if the Earth was expanding? - say as a result of accretion beneath the sea at the tectonic plates - but I’m too lazy to start a new thread about it. It might take 100 years to ‘grow’ a metre - if we’ve not been monitoring it, how would we know it hadn’t happened? ---
**Author:** ![Todderbob](https://avatars.discourse-cdn.com/v4/letter/t/e47774/32.png) [@Todderbob](https://boards.straightdope.com/u/Todderbob)\ **Post date:** [February 26, 2010, 12:01am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/26 "2010-02-26T00:01:29Z")
> [@ivan\_astikov](#): > > My point was, how would we know if the Earth was expanding? - say as a result of accretion beneath the sea at the tectonic plates - but I’m too lazy to start a new thread about it. That wouldn’t cause the Earth to expand. The Earth is the size it is because of the mass it has, unless the mass changes significantly, the Earth’s size wouldn’t change significantly. (Note to earlier, my statement about the circumference changing was more about erosion and the movement of soil & water from point to point, _not_ about the actual overall mass of the Earth changing). What you’re talking about are [, which are counterbalanced by [url=[Subduction - Wikipedia](http://en.wikipedia.org/wiki/Subduction)]Subduction zones](]Oceanic trenches[/url). [Accretion](http://en.wikipedia.org/wiki/Accretion_(geology)), adds mass to one place _by taking it from another_, not through magic. Therefore, the overall mass of the Earth doesn’t change. ETA: > [@ivan\_astikov](#): > > It might take 100 years to ‘grow’ a metre - if we’ve not been monitoring it, how would we know it hadn’t happened? Because the Earth is a planet, not a magical creature that magically makes new material. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [February 26, 2010, 12:06am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/27 "2010-02-26T00:06:59Z")
In other words - propose a mechanism for progressive expansion of the Earth’s radius, propose various effects and then some means to test that your hypothesis is false. ---
**Author:** ![ivan\_astikov](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ivan\_astikov](https://boards.straightdope.com/u/ivan_astikov)\ **Post date:** [February 26, 2010, 12:08am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/28 "2010-02-26T00:08:55Z")
So, after the Earth’s crust pushed up to form all those mountain ranges, it wouldn’t have had made much difference to the planet’s overall circumference? ---
**Author:** ![Todderbob](https://avatars.discourse-cdn.com/v4/letter/t/e47774/32.png) [@Todderbob](https://boards.straightdope.com/u/Todderbob)\ **Post date:** [February 26, 2010, 12:22am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/29 "2010-02-26T00:22:35Z")
> [@ivan\_astikov](#): > > So, after the Earth’s crust pushed up to form all those mountain ranges, it wouldn’t have had made much difference to the planet’s overall circumference? not the average circumference, no. Those mountain ranges are made of material that made up the continental plates, for the most part. Once that material goes “up,” it’s no longer displacing the water in the oceans, which then lowers (although a very, very minor amount). ---
**Author:** ![ivan\_astikov](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ivan\_astikov](https://boards.straightdope.com/u/ivan_astikov)\ **Post date:** [February 26, 2010, 12:44am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/30 "2010-02-26T00:44:16Z")
> [@Todderbob](#): > > Because the Earth is a planet, not a magical creature that magically makes new material. Are you sure you don’t want to reconsider that, in the light of abiogenesis? ---
**Author:** ![Todderbob](https://avatars.discourse-cdn.com/v4/letter/t/e47774/32.png) [@Todderbob](https://boards.straightdope.com/u/Todderbob)\ **Post date:** [February 26, 2010, 12:45am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/31 "2010-02-26T00:45:39Z")
> [@ivan\_astikov](#): > > Are you sure you don’t want to reconsider that, in the light of abiogenesis? Quite. ---
**Author:** ![ivan\_astikov](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ivan\_astikov](https://boards.straightdope.com/u/ivan_astikov)\ **Post date:** [February 26, 2010, 12:47am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/32 "2010-02-26T00:47:02Z")
Just checking. ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [February 26, 2010, 12:51am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/33 "2010-02-26T00:51:43Z")
There is _some_ accretion from meteorites (which is, of course, how the Earth got to be as large as it is in the first place). But that mostly stopped nearly four billion years ago. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [February 26, 2010, 2:17am UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/34 "2010-02-26T02:17:15Z")
> [@ivan\_astikov](#): > > So, after the Earth’s crust pushed up to form all those mountain ranges, it wouldn’t have had made much difference to the planet’s overall circumference? If the world’s radius had increased by 5km (~0.08%) the overall circumference would have increased by around 30 km. 5km selected as the Tibetan plateau is around 4.5km ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [February 26, 2010, 3:42pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/35 "2010-02-26T15:42:02Z")
The answer to all of Ivan’s questions are contained in a single word: [Thomas Dolby][Margus Pyke]Science![/Magnus Pyke][/Thomas Dolby] ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [February 26, 2010, 7:18pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/36 "2010-02-26T19:18:38Z")
The Earth is slowly accreting mass from small meteorites. The Earth is slowly losing mass from atmospheric outgassing (i.e. molecules of the atmosphere are floating to the top of the atmosphere and being pushed away by the solar wind). Those two rates are not equal. I don’t know the numbers offhand and am not inclinded to go look them up. My intuitive feel is that we are slightly gaining matter on average. The rate of matter increase is a _tiny_ fraction of the mass of the Earth. Like 0.00001% or something. Again, I don’t know actual numbers here - I’m just wagging. If you wish to look up the numbers, feel free. 10 million years ago was nothing in geologic timeframes. 10 million years ago is still after the dinosaurs died out. 10 million years ago is after chimps and humans split. **ivan astikov** said: > [@](#): > > Why should we assume it was the same size? Define “same size”. As I stated, the Earth has accummulated some mass in that time frame, but the amount of mass change is insignificant compared to the total amount of mass. For all intents and purposes, the Earth is the same size because there is no mechanism for the Earth to have grown significantly. Shuffling of continents, pushing some points up and others down, does not affect the total size. It just redistributes what is there. The number you get is somewhat dependent on how you measure it (along the surface, does water count, around the equator or the poles, etc). Now if you can propose a mechanism for how the Earth might change size without increasing matter - say, some sort of “swelling principle” due to solar cycle heating and location in the plane of the galaxy - then you could propose a means to test that idea. And then we might have a _reason_ to think Earth’s size has changed substantially in the last 10 million years. Right now, though, there is no reason to think so. The burden is on you to show evidence that it was different, not on us to show evidence it is the same. ---
**Author:** ![troub](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troub/32/14802_2.png) [@troub](https://boards.straightdope.com/u/troub)\ **Post date:** [February 26, 2010, 7:57pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/37 "2010-02-26T19:57:35Z")
> [@ivan\_astikov](#): > > So, after the Earth’s crust pushed up to form all those mountain ranges, it wouldn’t have had made much difference to the planet’s overall circumference? No. [![](https://i.imgur.com/anzqB.jpg) ](https://i.imgur.com/anzqB.jpg) ---
**Author:** ![ivan\_astikov](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ivan\_astikov](https://boards.straightdope.com/u/ivan_astikov)\ **Post date:** [February 26, 2010, 8:16pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/38 "2010-02-26T20:16:10Z")
> [@troub](#): > > No. > [Imgur: The magic of the Internet](http://i.imgur.com/anzqB.jpg) That’s lovely. Did you paint it yourself? Only messing! I realise it is meant to be a cross section of the Earth, and I see your point. Still, say we put a marker on the ocean bed either side of a subduction zone, measured the distance and then 100 years later, went back and measured them again; would we expect them to be the same distance apart exactly? How long a time span would we need to know for definite whether the distance is changing, or not? ---
**Author:** ![Todderbob](https://avatars.discourse-cdn.com/v4/letter/t/e47774/32.png) [@Todderbob](https://boards.straightdope.com/u/Todderbob)\ **Post date:** [February 26, 2010, 8:26pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/39 "2010-02-26T20:26:48Z")
> [@ivan\_astikov](#): > > That’s lovely. Did you paint it yourself? > > Only messing! I realise it is meant to be a cross section of the Earth, and I see your point. > > Still, say we put a marker on the ocean bed either side of a subduction zone, measured the distance and then 100 years later, went back and measured them again; would we expect them to be the same distance apart exactly? How long a time span would we need to know for definite whether the distance is changing, or not? We know the distance is changing, the plates are moving. That’s a fundamental part of Plate Tectonics. ETA: But what gets added on side A, gets subtracted on Side B. ---
**Author:** ![troub](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troub/32/14802_2.png) [@troub](https://boards.straightdope.com/u/troub)\ **Post date:** [February 26, 2010, 8:31pm UTC](https://boards.straightdope.com/t/the-shape-of-the-earth/530371/40 "2010-02-26T20:31:29Z")
And the point of my posting that image (which I didn’t paint myself, and I don’t even know who did, by the way), is that even something “added” that’s the size of Mt. Everest is barely _significant_ with respect to how big around the planet is. The crust is such a tiny film around the sphere it’s ridiculous. [Previous page](https://boards.straightdope.com/t/the-shape-of-the-earth/530371.md?page=1) [Next page](https://boards.straightdope.com/t/the-shape-of-the-earth/530371.md?page=3)