Talk:Figure of the Earth

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Geodesy[edit]

This text is taken from chapter 1 of the public domain resource Geodesy for the Layman at http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003A.HTM#ZZ0 -- please Wikify as necessary. —Preceding unsigned comment added by The Anome (talkcontribs) 10:19, 25 May 2003

I've been reading Gordon Lauf's Geodesy and Map Projections and some of the information is different. e.g. he has Krassowsky with a major semi axis of 6,378,295 rather than the 6,378,245m quoted here, and a reciprocal flattening of 298.4 rather than the 298.3 quoted here.

Also an issue which particularly concerns me is Lauf's suggestion that in general "the geodesic has an infinite number of windings on the spherical earth", oscillating between a given latitude north and south. WHilst this may be true "in general", I would infer that there are particular examples where this is not the case and relatively short periodicitites of return can be found. Harry Potter.

article name[edit]

Since figure of the earth is an expression, shouldn't the name if this article be Figure of the earth? Kingturtle 03:12, 13 Dec 2003 (UTC)

I believe the 245.0, 298.3 values to be the correct ones. 85.76.129.149 20:27, 28 Feb 2005 (UTC)

I don't understand the title. Why isn't it called "Shape and size of the Earth"? This would be a more normal English expression. Borock (talk) 13:21, 1 December 2009 (UTC)
We must use the term most often used in the references, which is "figure of the Earth". This figure is indeed its shape and size. — Joe Kress (talk) 19:32, 1 December 2009 (UTC)
Cool. You learn something every day. Which is one reason to visit WP. :-) Borock (talk) 05:07, 15 December 2009 (UTC)

from PNA/Geology[edit]

  • Figure of the Earth - This has been lifted from a public domain web page [1]. No problem there, but it refers to a number of non-existent diagrams. These could probably be borrowed from the same source, but they are of poor quality. --Heron 17:01, 27 Nov 2004 (UTC)

The actual figure of the earth[edit]

The article describes many ways of approximating the shape of the earth, but other than the triaxial bit or the pear shape thing, the article doesn't mention what the figure of the earth is to any appreciable degree. Where are the relative high and low points of the geoid. —Preceding unsigned comment added by 71.102.144.27 (talkcontribs) 03:00, 14 March 2007

According to NASA (http://www.nasa.gov/worldbook/earth_worldbook.html) the earth is pear-shaped. ig 12.110.164.222 (talk) 21:49, 10 December 2010 (UTC)

Deflection of the vertical[edit]

"The angle between the plumb line which is perpendicular to the geoid (sometimes called "the vertical") and the perpendicular to the ellipsoid (sometimes called "the ellipsoidal normal") is defined as the deflection of the vertical."

But what is the sign convention? I believe the deflection is the angle from the vertical to the ellipsoid normal. Right? Jrvz (talk) 20:41, 19 October 2010 (UTC)

The reference direction is usually the ellipsoidal normal, hence from the ellipsoidal normal to the plumb line. In addition, the deflection of the vertical relative to the ellipsoidal normal has a north-south component ξ and an east-west component η, where north and east are positive. See Geodesy pages 218–219. — Joe Kress (talk) 08:35, 20 October 2010 (UTC)

CopyVio - Geoid[edit]

The Geoid section appears to have been copied verbatim from "Geodesy for the Layman". Item 3 of that document's Foreword says "[this document] contains no copyrighted material...". Is that enough to make this copy/paste OK? Even if the copy/paste is OK, we should acknowledge the source.

A Google search for the phrase "The geoid is a surface along which the gravity potential is everywhere equal" will find other instances, and it's not obvious which is the original. 203.176.108.99 (talk) 06:02, 11 October 2012 (UTC)

"Geodesy for the Layman" should certainly be cited. Documents prepared by employees of the US federal government are in the public domain. Sometimes the federal government published documents that contain copyrighted work prepared by others, used by permission. But the statement "[this document] contains no copyrighted material..." seems to mean that it was prepared only by federal employees in this case.
I don't think a single sentence, "The geoid is a surface..." is a copyright problem, so I think we can leave it alone unless we can establish that it was first published outside of Wikipedia. Jc3s5h (talk) 12:48, 11 October 2012 (UTC)

Flat Earth[edit]

Who knew; the Earth really is flat!JoelDick (talk) 13:22, 26 May 2014 (UTC)

Spherical[edit]

No, the Earth is spherical. The radius is approximately 6.4 thousand km, with minimal variation. Yes, there is some very slight flattening at the poles, but it is pretentious, pedantic, and misleading to say it is an "oblate spheroid".--Jack Upland (talk) 20:48, 30 December 2015 (UTC)

The article starts with a sphere and then discusses refinements. What is pedantic about that? Geodesists are very interested in the refinements. RockMagnetist(talk) 21:47, 30 December 2015 (UTC)
You're right, actually. I made my comment here because it seemed the most appropriate place, as the root of the topic. But I was wrong: it is not the root of problem. The presentation here is actually fine. The problem is with associated articles.
  • Spherical Earth lead says: "The realization that the figure of the Earth is more accurately described as an ellipsoid dates to the 18th century".
  • Earth radius lead says: "The Earth is only approximately spherical, so no single value serves as its natural radius." It adds that "..."radius" normally is a characteristic of perfect spheres..." which is pointlessly pedantic and highly misleading.
  • Earth#Shape says: "The shape of Earth approximates an oblate spheroid". If you go to oblate spheroid you see a diagram with extreme flattening that has no relationship to the Earth's shape at all.
  • WP:Truth which started me on this quest says: "The article which describes the shape of the Earth simply asserts that it is an oblate spheroid."
In fact, they should all reflect what it says here in the lead: "the sphere is a close approximation of the true figure of the Earth and satisfactory for many purposes".--Jack Upland (talk) 23:54, 30 December 2015 (UTC)
Agreed; please go ahead and harmonize the leads. There's even the story of the Earth being rounder/smoother than a billiard ball to support your point: [2] fgnievinski (talk) 16:05, 3 March 2016 (UTC)
Yes, I've heard rounder than a ping-pong ball and less wet...--Jack Upland (talk) 18:26, 3 March 2016 (UTC)
Jack Upland, your comments are hyperbolic (viz., "no relationship at all", "pointlessly pedantic", "highly misleading"). The comments about smoothness are irrelevant, and the comment about "flattening at the poles" is incorrect. The earth isn't only "flattened at the poles"; it is an ellipsoid, to within a hundred meters of mean sea level everywhere. Its deviation from a sphere is three orders of magnitude greater. The fact of the earth's ellipsoidness is crucial to geodesy, and it is also fundamental to the value chosen for "a radius", which you would understand if you understood the Earth radius article. I do not support these proposed edits yet, not without a lot more nuance on verbiage and purpose, and will revert any that reflect this hyperbolic crusade. Strebe (talk) 19:50, 3 March 2016 (UTC)
The difference between polar and equatorial radii is less than half a percent of the global mean radius, see Earth radius. fgnievinski (talk) 19:56, 3 March 2016 (UTC)
Why is that relevant Strebe (talk) 20:06, 3 March 2016 (UTC)
As I admitted above, this article – "Figure of the Earth" – is OK. It's the other article that are problematic. For most intents and purposes, the Earth is spherical. I am not going to argue the point whether the "true" shape is oblate spheroid or an ellipsoid. I stand by my other comments. The descriptions of the Earth as not a perfect sphere are pedantic and highly misleading. These articles are supposed to be read by laypeople.--Jack Upland (talk) 20:27, 3 March 2016 (UTC)
Then fix the Earth#Shape description, if that's what bugs you most. I disagree with the rest of your comments. All the other cited articles deal responsibly with the sphere/ellipsoid distinction. Improvements are always welcome. Just be aware that your own need to think in two decimal places doesn't reflect mapping needs over the past two hundred years. Strebe (talk) 20:57, 3 March 2016 (UTC)
In re "why is that relevant": it depends on the map scale. If you consider visual acuity is no better than 0.5 mm on a sheet of paper, then you can safely make a world map assuming a spherical earth. It's only for maps covering small areas (larger scale for the same sheet of paper) that ellipsoidal earth matters. If you find the explanation in terms of paper maps outfashioned, most numerical weather models still assume a spherical earth. It's only for topographical and cadastral mapping (air- and space-based photogrammetry and ground-based surveying) that the ellipsoidal shape applies. fgnievinski (talk) 23:58, 3 March 2016 (UTC)
That's all true, and still not relevant. Just because some maps don't care does not mean the topic is not important; does not mean distinguishing between ellipsoidal and spherical is pointlessly pedantic or highly misleading; does not mean an image of an oblate spheroid "has nothing to do with" the real shape of the earth. I agree Earth#Shape ought to start with the fact that a sphere approximates the shape of the earth closely... but on the other hand, everyone already knows that, so who's pointlessly pedantic here? Strebe (talk) 00:23, 4 March 2016 (UTC)
I think most maps -- not just some -- need no ellipsoidal complication. Take school maps, for example. In truth, the earth is not even ellipsoidal, but more deformed -- but not so exaggerated as often conveyed to the layperson, e.g., [3]. I think we can reasonably say it's only for geodesists and surveyors that ellipsoidal earth matter. For most everyone else -- geographers, meteorologists, aviation folks -- spherical earth suffices. fgnievinski (talk) 00:53, 4 March 2016 (UTC)
No. It matters to everyone, in that the products and services depending on precise mensuration of the earth matter to everyone. As for “not ellipsoidal”, that’s the truly pointless observation. •Obviously• there are mountains and seas and trenches. None of that detracts from the fact that mean sea level deviates from an ellipsoid by less than a hundred meters anywhere on earth. It’s the simplest regular shape that describes the earth’s surface with high accuracy. I’m rather done discussing whether or not that’s “important for most people”. It’s important. Nobody advocates emphasizing how spherical the earth is not. The question is whether the articles are balanced. Strebe (talk) 03:00, 4 March 2016 (UTC)
No, it doesn't matter to everyone, in that the products and services depending on precise mensuration of the earth do not matter to everyone. Most people only take a glance at maps, don't ever measure distances or areas or azimuths. As for “not ellipsoidal”, that’s the truly essential observation, in that you should not insist that the earth is actually an ellipsoid. In order of increasing accuracy, it's a sphere, a spheroid, the geoid, then the topographical/bathymetric relief. Which one is best? That depends on the application, if you're interested in horizontal positions only or vertical positions too, etc. You can't say "[The ellipsoid] It’s the simplest regular shape that describes the earth’s surface with high accuracy", but you could say "The spheroid is the simplest shape that describes the earth’s surface with 100-m accuracy" as well as "The sphere is the simplest shape that describes the earth’s surface with 0.3% accuracy." fgnievinski (talk) 16:55, 4 March 2016 (UTC)
Accurate property lines, airplanes that fly where they are supposed to (and using minimal fuel), GPS navigation, all matter to "most" people who are likely to read these pages. I think, at this point, you are arguing for the sake of arguing, and therefore I am done. The purpose of this page is how to improve the article, not to provide a forum for endless blathering. I stand by my assertion that the objections to characterizing the earth as ellipsoidal are hyperbolic. If you have edits to make, make them, and if those edits dilute the purpose of the pages, I will dispute them. Strebe (talk) 19:44, 4 March 2016 (UTC)
My perspective is that as a kid I "discovered" that the Earth was an oblate spheroid, then I found that actually it was more spherical than a ping pong ball. In most cases, saying that the Earth is not a sphere is excessively pedantic to be point of being misleading. These articles should be written for laypeople.--Jack Upland (talk) 01:18, 4 March 2016 (UTC)
It’s not “more spherical than a ping-pong ball”. Where did you get that notion? Meanwhile these articles are written for lay people. How many articles do you want whose purpose is to say, “The earth is a sphere?” The reason those articles exist is because the earth is not a sphere. Strebe (talk) 03:00, 4 March 2016 (UTC)
According to the article the deviation is only approximately 0.3%, and see the link about billiard balls above. I guess most ping pong balls have seams so they are noticeably less smooth than the Earth. Is there a factual here that is in dispute?--Jack Upland (talk) 04:35, 4 March 2016 (UTC)
Indeed, if you were to fit an ellipsoid (or rather, a spheroid) to a bunch of billiard balls -- estimate polar and equatorial semi-major axes from metrology measurements -- the flattening or eccentricity would be greater than the Earth's. Therefore, the Earth is more spherical than a brand-new billiard ball. Smoothness/roughness is a different property, as the finish might have small-scale random surface topography, although it doesn't affect the best-fitting ellipsoid. fgnievinski (talk) 16:55, 4 March 2016 (UTC)