# Regarding the diameter of the earth **URL:** \ **Category:** Factual Questions\ **Created:** [May 28, 2019, 5:22pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660 "2019-05-28T17:22:56Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [May 28, 2019, 5:22pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/1 "2019-05-28T17:22:56Z")
1. is the equatorial diameter of the Earth measured at sea level? 2. If the Earth was all smoothed out like a billiard ball, mountains leveled and holes filled in, would the diameter be much different? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 28, 2019, 5:25pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/2 "2019-05-28T17:25:04Z")
Don’t know the answer to #1, but as for #2 - the Earth is a more perfect sphere than any billiard ball. Up until recently, it was more perfectly spherical than any man-made object, but the new kilogram standard is more precise. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [May 28, 2019, 5:27pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/3 "2019-05-28T17:27:02Z")
> [@beowulff](#): > > Don’t know the answer to #1, but as for #2 - the Earth is a more perfect sphere than any billiard ball. > Up until recently, it was more perfectly spherical than any man-made object, but the new kilogram standard is more precise. I think it was _smoother_, but not _more spherical_ - the oblateness is a fair bit greater than the roughness. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 28, 2019, 5:28pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/4 "2019-05-28T17:28:04Z")
It’s smoother than a billiard ball, but more oblate than most of them. And I’m pretty sure there have been smoother spheres, for precise experiments, before the new kilogram standard. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [May 28, 2019, 5:44pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/5 "2019-05-28T17:44:46Z")
> [@Czarcasm](#): > > 1. is the equatorial diameter of the Earth measured at sea level? > 2. If the Earth was all smoothed out like a billiard ball, mountains leveled and holes filled in, would the diameter be much different? 1. I assume you meant circumference rather than diameter. Either way it depends on who measures. The Wikipedia article on Earth for instance uses two different sources for equatorial radius and equatorial circumference and the two don’t match up perfectly. If you calculate the circumference from the radius you get a number a few hundred meters shorter. 2. When giving a single number for the diameter, or circumference, one simplifies and gives the number for an averaged circle. The radius is pretty much at least 3 orders of magnitudes larger than any bumps you may encounter on a trip around the equator, so smoothing things out wouldn’t change much. ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [May 28, 2019, 5:48pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/6 "2019-05-28T17:48:36Z")
I meant diameter. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [May 28, 2019, 5:49pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/7 "2019-05-28T17:49:08Z")
[Sea level](https://en.wikipedia.org/wiki/Sea_level#Measurement) is not a very precise measurement. Sea Level, the earth’s circumference and diameter are all approximations. ---
**Author:** ![Doyle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doyle/32/439_2.png) [@Doyle](https://boards.straightdope.com/u/Doyle)\ **Post date:** [May 28, 2019, 5:57pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/8 "2019-05-28T17:57:36Z")
Centrifugal force. ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [May 28, 2019, 6:04pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/9 "2019-05-28T18:04:55Z")
> [@Doyle](#): > > Centrifugal force. Gesundheit. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [May 28, 2019, 6:09pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/10 "2019-05-28T18:09:20Z")
> [@Chronos](#): > > It’s smoother than a billiard ball, but more oblate than most of them. And I’m pretty sure there have been smoother spheres, for precise experiments, before the new kilogram standard. Not only that, but the Avogadro project sphere doesn’t even hold the record. That belongs to the gyroscopes made for the satellite [Gravity Probe B](https://en.wikipedia.org/wiki/Gravity_Probe_B). They were designed to measure an effect called “frame-dragging”, which is essentially the effect that the Earth’s rotation has on the local properties of spacetime. To do this, they needed some very precisely machined spheres to set into rotation. > [@](#): > > At the time of their manufacture, the gyroscopes were the most nearly spherical objects ever made. (Two gyroscopes still hold that record, but third place has been taken by the silicon spheres made by the Avogadro project.) Approximately the size of ping pong balls, they were perfectly round to within forty atoms (less than 10 nm). If one of these spheres were scaled to the size of the Earth, the tallest mountains and deepest ocean trench would measure only 2.4 m (8 ft) high. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [May 28, 2019, 6:19pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/11 "2019-05-28T18:19:56Z")
Roughly speaking, Earth is out of round by about 1 part in 300 (comparing polar to equatorial radius), and its extreme surface roughness is about 1 part in 1600 (based on the height of Mt Everest). Perfectly ordinary ball bearings improve on these tolerances by a factor of 10 to 20. ---
**Author:** ![digs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/digs/32/14889_2.png) [@digs](https://boards.straightdope.com/u/digs)\ **Post date:** [May 28, 2019, 6:26pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/12 "2019-05-28T18:26:03Z")
> [@Chronos](#): > > It’s smoother than a billiard ball, but more oblate than most of them. And I’m pretty sure there have been smoother spheres, for precise experiments, before the new kilogram standard. I’ve always heard that we exaggerate the oblate-ness, and that the earth does fit within cue ball tolerance guidelines. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [May 28, 2019, 6:28pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/13 "2019-05-28T18:28:38Z")
Here is a [useful site](https://www.universetoday.com/15055/diameter-of-earth/) > [@](#): > > According to the 2004 Working Group of the International Earth Rotation and Reference Systems Service (IERS), Earth experiences a flattening of 0.0033528 at the poles. This flattening is due to Earth’s rotational velocity – a rapid 1,674.4 km/h (1,040.4 mph) – which causes the planet to bulge at the equator. Because of this, the diameter of the Earth at the equator is about 43 kilometers (27 mi) larger than the pole-to-pole diameter. As a result, the latest measurements indicate that the Earth has an equatorial diameter of 12,756 km (7926 mi), and a polar diameter of 12713.6 km (7899.86 mi). > > The most notable exceptions are the Mariana Trench – the deepest place on Earth, at 10,911 m (35,797 ft) below local sea level – and Mt. Everest, which is 8,848 meters (29,029 ft) above local sea level. However, these two geological features represent a very minor variation when compared to Earth’s overall shape – 0.17% and 0.14% respectively. > > Meanwhile, the highest point on Earth is Mt. Chiborazo. The peak of this mountain reaches an attitude of 6,263.47 meters (20,549.54 ft) above sea level. But because it is located just 1° and 28 minutes south of the equator (at the highest point of the planet’s bulge), it receives a natural boost of about 21 km. Some stats from [wikipedia](https://en.wikipedia.org/wiki/Earth#Surface) > [@](#): > > The elevation of the land surface varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top of Mount Everest. The mean height of land above sea level is about 797 m (2,615 ft). Given that 71% of the earths surface is water, leveling all continents to fill underwater trenches would raise the sea level by about 758 feet. So absolutely negligible relative to the overall diameter. ---
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [May 28, 2019, 6:32pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/14 "2019-05-28T18:32:32Z")
> [@beowulff](#): > > Up until recently, [Earth] was more perfectly spherical than any man-made object, but the new kilogram standard is more precise. Am I missing something? I thought the new kilogram standard was based on Plank’s constant and not on a physical artifact. Is this a metrological in-joke that went over my head? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 28, 2019, 6:41pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/15 "2019-05-28T18:41:38Z")
Coordinate systems such as WGS84 define a “reference ellipsoid”. It’s not supposed to be exactly at sea level AFAIK, which we can see since mean sea level varies from place to place and is defined in that model by a bumpy [“geoid”](https://gisgeography.com/wp-content/uploads/2016/02/geoid-678x322.png); the diameter to sea level would depend on exactly where you took the measurement. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 28, 2019, 6:48pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/16 "2019-05-28T18:48:49Z")
> [@74westy](#): > > Am I missing something? I thought the new kilogram standard was based on Plank’s constant and not on a physical artifact. Is this a metrological in-joke that went over my head? > **[Kilogram: Silicon Spheres and the International Avogadro Project](https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project)** > > Though measurement scientists chose the Planck constant as the basis for redefinition, other constants of nature can also be used, if for ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 28, 2019, 6:57pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/17 "2019-05-28T18:57:33Z")
> [@beowulff](#): > > [https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project](https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project) Those are nice, smooth spheres, but why would I want one in my lab? Seems like a pain to use in an analytical balance; it would keep trying to roll away… ---
**Author:** ![dorvann](https://avatars.discourse-cdn.com/v4/letter/d/df705f/32.png) [@dorvann](https://boards.straightdope.com/u/dorvann)\ **Post date:** [May 28, 2019, 6:58pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/18 "2019-05-28T18:58:41Z")
This only tangential related to the OP’s question but it is an interesting read none the less: What would a bowling ball look like if it were blown up to the size of the Earth? (The first half does discuss how smooth the Earth is compared to a bowling ball.) [https://what-if.xkcd.com/46/](https://what-if.xkcd.com/46/) ---
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [May 28, 2019, 7:20pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/19 "2019-05-28T19:20:21Z")
> [@beowulff](#): > > [https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project](https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project) Thanks. Looks to me like that project is making hella-smooth one kilogram spheres but they are not the new standard kilogram. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [May 28, 2019, 8:13pm UTC](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660/20 "2019-05-28T20:13:21Z")
> [@Czarcasm](#): > > I meant diameter. Then I don’t understand your question 2 at all. “The diameter” of an approximate circle is of course an average of sort. What kind of smoothing of the Earth were you imagining that would change things? [Next page](https://boards.straightdope.com/t/regarding-the-diameter-of-the-earth/834660.md?page=2)