# Just how smooth is the Earth? **URL:** \ **Category:** Factual Questions\ **Created:** [May 22, 2009, 4:52am UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272 "2009-05-22T04:52:32Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![The\_Controvert](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@The\_Controvert](https://boards.straightdope.com/u/The_Controvert)\ **Post date:** [May 22, 2009, 4:52am UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/1 "2009-05-22T04:52:32Z")
According to this [post](http://boards.straightdope.com/sdmb/showpost.php?p=10444701&postcount=18), the smoothness of the Earth is compared to ball bearings and billiard balls. Yet, I have a hard time believing this statement. The human finger is pretty sensitive to small imperfections, if I ran my finger over a perfect scale replica of the Earth, would I or would I not be able to feel the mountains and other features? How large would the replica need to be before this happens? Or how small does it have to be in order to feel perfectly smooth to the human finger? ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [May 22, 2009, 5:01am UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/2 "2009-05-22T05:01:22Z")
From the surface of the ocean, the Marianas Trench is 6.8 miles deep and Mt. Kilimanjaro is 3.7 miles high. So in total, that’s a maximum difference of 10.5 miles for the surface of the planet. The Earth is about 4000 miles in radius. A pool ball is maybe an inch to an inch and a half in radius. Do you think you can perceive 2.6 thousandths of an inch (6.6 hundredths of a millimeter)? And really we should take out the Marianas Trench since the spherocity of the Earth is aided by the ocean. So our 10.5 should be shrunk back to the 3.7 above the ocean. (Though actually the bulge of the equator gives a total of 13 miles difference, which is the greatest overall difference, but is a smooth gradient rather than a bump like a mountain or trench.) ---
**Author:** ![exastris](https://avatars.discourse-cdn.com/v4/letter/e/b38774/32.png) [@exastris](https://boards.straightdope.com/u/exastris)\ **Post date:** [May 22, 2009, 5:05am UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/3 "2009-05-22T05:05:35Z")
> [@The\_Controvert](#): > > The human finger is pretty sensitive to small imperfections, if I ran my finger over a perfect scale replica of the Earth, would I or would I not be able to feel the mountains and other features? > > How large would the replica need to be before this happens? Or how small does it have to be in order to feel perfectly smooth to the human finger? I’ve seen (and more importantly, felt) an exhibit like this. The Earth in question was about the size of a standard desk globe (what’s that, maybe a foot in diameter or so?), and I was barely able to feel the smaller mountains. Definitely could feel the larger ones, but even they were surprisingly small. It was astonishing. I’m sorry I don’t have exact numbers for you! ---
**Author:** ![jovan](https://avatars.discourse-cdn.com/v4/letter/j/0ea827/32.png) [@jovan](https://boards.straightdope.com/u/jovan)\ **Post date:** [May 22, 2009, 7:52am UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/4 "2009-05-22T07:52:38Z")
This question has been answered by none other than the **Bad Astronomer** himself: [http://blogs.discovermagazine.com/badastronomy/2008/09/08/ten-things-you-dont-know-about-the-earth/](http://blogs.discovermagazine.com/badastronomy/2008/09/08/ten-things-you-dont-know-about-the-earth/) > [@](#): > > According to the World Pool-Billiard Association, a pool ball is 2.25 inches in diameter, and has a tolerance of +/- 0.005 inches. > (…) > The Earth has a diameter of about 12,735 kilometers (on average, see below for more on this). Using the smoothness ratio from above, the Earth would be an acceptable pool ball if it had no bumps (mountains) or pits (trenches) more than 12,735 km x 0.00222 = about 28 km in size. > > The highest point on Earth is the top of Mt. Everest, at 8.85 km. The deepest point on Earth is the Marianas Trench, at about 11 km deep. > > Hey, those are within the tolerances! ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [May 22, 2009, 11:11am UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/5 "2009-05-22T11:11:25Z")
> [@Sage\_Rat](#): > > From the surface of the ocean, the Marianas Trench is 6.8 miles deep and Mt. Kilimanjaro is 3.7 miles high. So in total, that’s a maximum difference of 10.5 miles for the surface of the planet. The Earth is about 4000 miles in radius. A pool ball is maybe an inch to an inch and a half in radius. Do you think you can perceive 2.6 thousandths of an inch (6.6 hundredths of a millimeter)? > > And really we should take out the Marianas Trench since the spherocity of the Earth is aided by the ocean. So our 10.5 should be shrunk back to the 3.7 above the ocean. > > (Though actually the bulge of the equator gives a total of 13 miles difference, which is the greatest overall difference, but is a smooth gradient rather than a bump like a mountain or trench.) Why Kilimanjaro, out of interest? You know, what with Everest being way higher and all that. ---
**Author:** ![R.P.McMurphy](https://avatars.discourse-cdn.com/v4/letter/r/7993a0/32.png) [@R.P.McMurphy](https://boards.straightdope.com/u/R.P.McMurphy)\ **Post date:** [May 22, 2009, 11:14am UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/6 "2009-05-22T11:14:37Z")
> [@jovan](#): > > This question has been answered by none other than the **Bad Astronomer** himself: > [http://blogs.discovermagazine.com/badastronomy/2008/09/08/ten-things-you-dont-know-about-the-earth/](http://blogs.discovermagazine.com/badastronomy/2008/09/08/ten-things-you-dont-know-about-the-earth/) And who said you can’t get an education by hanging out at the pool hall? ---
**Author:** ![RickJay](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@RickJay](https://boards.straightdope.com/u/RickJay)\ **Post date:** [May 22, 2009, 12:48pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/7 "2009-05-22T12:48:51Z")
> [@Malacandra](#): > > Why Kilimanjaro, out of interest? You know, what with Everest being way higher and all that. Everest is still far, far under the percentage tolerance allowable in an official pool ball. It does seem hard to believe, I admit, but that’s mostly because the enormousness of the Earth itself is kind of hard to comprehend. Everest looks really big when you’re standing below it, but on the scale of the Earth it doesn’t even qualify as a pimple. The difference of .066 millimetres between lowest and highest points you’d be on a billiard-ball sized Earth is beyond visual perception and, since any specific relief point would actually be smaller than that, likely not perceptible to the touch. You can’t even measure that with a ruler; you’d need a micrometer or a vernier or some such instrument that measures differences the eye cannot perceive. ---
**Author:** ![notfrommensa](https://avatars.discourse-cdn.com/v4/letter/n/f14d63/32.png) [@notfrommensa](https://boards.straightdope.com/u/notfrommensa)\ **Post date:** [May 22, 2009, 1:01pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/8 "2009-05-22T13:01:48Z")
> [@Malacandra](#): > > Why Kilimanjaro, out of interest? You know, what with Everest being way higher and all that. Everest is the highest point in terms of sealevel, but because of the bulge of the earth (not a perfect sphere) that another mountain is the furthest from the center of the earth? Chimborazo, in Ecuador, is the furthest point from the Earth Center, according to this admittedly [vague article](http://en.wikipedia.org/wiki/Chimborazo_(volcano)) in wiki. Since we are talking about “perceived roughness” of the Earth Kilimanjaro might be the highest peak, relative to its nearby surroundings. But I would think mountainous islands like Hawaii would have that beat. ---
**Author:** ![Hampshire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hampshire/32/4887_2.png) [@Hampshire](https://boards.straightdope.com/u/Hampshire)\ **Post date:** [May 22, 2009, 1:33pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/9 "2009-05-22T13:33:15Z")
I’m still impressing people with the following fact we discussed here about a month ago in regards to the surface of the earth: Run a cable around the circumference of the earth. Run another cable loop on top of that one but a foot higher. How much longer is the outer loop? ~6 feet. ---
**Author:** ![pipper](https://avatars.discourse-cdn.com/v4/letter/p/c2a13f/32.png) [@pipper](https://boards.straightdope.com/u/pipper)\ **Post date:** [May 22, 2009, 1:41pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/10 "2009-05-22T13:41:52Z")
> [@Malacandra](#): > > Why Kilimanjaro, out of interest? You know, what with Everest being way higher and all that. notfrommensa is on the right track. It’s because Kili is the world’s largest/ tallest freestanding mountain (that is on land anyway. I think there are some underwater volcanoes that are larger). In other words, the difference between the summit and the surrounding land is the greatest. Everest is higher, but since the surrounding land is also very high, the absolute difference is less. ---
**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\ **Post date:** [May 22, 2009, 2:19pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/11 "2009-05-22T14:19:55Z")
> [@Hampshire](#): > > I’m still impressing people with the following fact we discussed here about a month ago in regards to the surface of the earth: > > Run a cable around the circumference of the earth. Run another cable loop on top of that one but a foot higher. How much longer is the outer loop? ~6 feet. Well, that’s true of Jupiter or the Moon, too. Or a beach ball. It doesn’t have anything to do with the circumference of the Earth, but with the fact that C = 2_pi_r. ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [May 22, 2009, 2:27pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/12 "2009-05-22T14:27:12Z")
> [@notfrommensa](#): > > Everest is the highest point in terms of sealevel, but because of the bulge of the earth (not a perfect sphere) that another mountain is the furthest from the center of the earth? > > Chimborazo, in Ecuador, is the furthest point from the Earth Center, according to this admittedly [vague article](http://en.wikipedia.org/wiki/Chimborazo_(volcano)) in wiki. > > Since we are talking about “perceived roughness” of the Earth Kilimanjaro might be the highest peak, relative to its nearby surroundings. But I would think mountainous islands like Hawaii would have that beat. My understanding (no cite, sorry, maybe another poster will confirm) is that Everest is the highest point above sea level, Kilimanjaro is the highest mountain on land measured from the surrounding land to the peak, and that one of the Hawaiian islands (maybe Hawaii itself) is the tallest measured from base to peak if you accept that most of it is underwater. ---
**Author:** ![tdn](https://avatars.discourse-cdn.com/v4/letter/t/94ad74/32.png) [@tdn](https://boards.straightdope.com/u/tdn)\ **Post date:** [May 22, 2009, 2:37pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/13 "2009-05-22T14:37:58Z")
> [@DrCube](#): > > one of the Hawaiian islands (maybe Hawaii itself) is the tallest measured from base to peak if you accept that most of it is underwater. Yep, Mauna Kea on the Big Island. ---
**Author:** ![Shamozzle](https://avatars.discourse-cdn.com/v4/letter/s/9f8e36/32.png) [@Shamozzle](https://boards.straightdope.com/u/Shamozzle)\ **Post date:** [May 22, 2009, 4:00pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/14 "2009-05-22T16:00:05Z")
> [@pipper](#): > > notfrommensa is on the right track. It’s because Kili is the world’s largest/ tallest freestanding mountain (that is on land anyway. I think there are some underwater volcanoes that are larger). > > In other words, the difference between the summit and the surrounding land is the greatest. Everest is higher, but since the surrounding land is also very high, the absolute difference is less. I believe Mount McKinley/Denali takes the cake in that department, with a relative vertical relief of around 5.5 km. ---
**Author:** ![Shoeless](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/shoeless/32/7099_2.png) [@Shoeless](https://boards.straightdope.com/u/Shoeless)\ **Post date:** [May 22, 2009, 6:19pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/15 "2009-05-22T18:19:01Z")
Also, it has been determined by geologists that [Kansas really is flatter than a pancake](http://www2.ljworld.com/news/2003/jul/27/holy_hotcakes_study/). ---
**Author:** ![RickJay](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@RickJay](https://boards.straightdope.com/u/RickJay)\ **Post date:** [May 22, 2009, 6:43pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/16 "2009-05-22T18:43:28Z")
> [@DrCube](#): > > My understanding (no cite, sorry, maybe another poster will confirm) is that Everest is the highest point above sea level, Kilimanjaro is the highest mountain on land measured from the surrounding land to the peak, and that one of the Hawaiian islands (maybe Hawaii itself) is the tallest measured from base to peak if you accept that most of it is underwater. I guess it depends where you measure from, but according to the resources I can find online, the prominence of Mount Logan (17224 feet) is quite a bit higher than Kilmanjaro (15100 feet, or possibly 16700 feet.) Spome resources also cite Mount Aconcagua as higher, and some McKinley as higher. I can’t find a really clear explanation of who’s measuring from what. ---
**Author:** ![Drum\_God](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drum_god/32/44_2.png) [@Drum\_God](https://boards.straightdope.com/u/Drum_God)\ **Post date:** [May 22, 2009, 6:48pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/17 "2009-05-22T18:48:25Z")
Since we’ve established that Earth is smoother than a billiard ball, is it also dryer? If I were to shoot my Earth across the pool table, would it leave a wet trail behind? If I pick it up in my hand, will it feel bone dry, damp, moist, or like my dog just brought it to me? ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [May 22, 2009, 6:50pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/18 "2009-05-22T18:50:04Z")
> [@Malacandra](#): > > Why Kilimanjaro, out of interest? You know, what with Everest being way higher and all that. Poor memory ---
**Author:** ![Captain\_Carrot](https://avatars.discourse-cdn.com/v4/letter/c/f0a364/32.png) [@Captain\_Carrot](https://boards.straightdope.com/u/Captain_Carrot)\ **Post date:** [May 22, 2009, 6:59pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/19 "2009-05-22T18:59:23Z")
> [@Shoeless](#): > > Also, it has been determined by geologists that [Kansas really is flatter than a pancake](http://www2.ljworld.com/news/2003/jul/27/holy_hotcakes_study/). That study found that a whole lot of things were flatter than a pancake, though, partly because a pancake is only flat from our perspective (lots of little bubbles from cooking), and partially because they didn’t exclude enough of the edge of the pancake, so they put in a good bit of slope. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [May 22, 2009, 6:59pm UTC](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272/20 "2009-05-22T18:59:30Z")
Well, I saw it chatting up Venus once… [Next page](https://boards.straightdope.com/t/just-how-smooth-is-the-earth/497272.md?page=2)