What is Oblate spheroid?
[[File:OblateSpheroid.PNG|thumb|right|Diagram of an oblate spheroid]] An '''oblate spheroid''' is a [[sphere]]-like shape that gives the appearance of being flattened, to some degree, on the top and bottom of the object so that the [[diameter|circumference]] around the poles is less than the circumference around the [[equator]]. The [[Earth]] is an oblate spheroid, as are at least a few other planets. An oblate spheroid is a type of [[ellipsoid]]. Oblate [[spheroid]]s have rotational [[symmetry]] around an [[Axis (mathematics)|axis]] from pole to pole.<ref>{{cite web|url=http://mathworld.wolfram.com/OblateSpheroid.html |title=Oblate Spheroid - from Wolfram MathWorld |publisher=Mathworld.wolfram.com |date=2009-10-04 |accessdate=2009-10-23}}</ref> An example of an oblate spheroid is an [[M&M's|M&M]]. Many [[planet]]s, including the [[Earth]] and [[Saturn (planet)|Saturn]], are oblate spheroids. The difference between a sphere and the Earth's shape is small, only a difference of about 0.33%. == Flattening of [[Astronomical object|astronomical objects]] == [[Stars]] spin, and some spin very fast. The faster the spin, the flatter the oblate spheroid. The Sun rotates at 2 [[km|kilometers]] per second and [[neutron star]]s have speeds of thousands of kilometers per second.<ref>[http://www.astronomycafe.net/qadir/ask/a11838.html How fast do stars spin? Astronomy Café]</ref> In other words, the Earth is a spheroid because it rotates. [[Gas giant]]s like [[Jupiter]] and [[Saturn]] are flattened by rotation more than the Earth. === Origin === In 1687 [[Isaac Newton]] published the ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]''. He included a proof that a [[rotation|rotating]] self-gravitating fluid body in [[equilibrium]] takes the form of an oblate [[ellipsoid]] of revolution (a [[spheroid]]).<ref name=newton>Isaac Newton:''Principia'' Book III Proposition XIX Problem III, p. 407 in [https://archive.org/stream/ost-physics-newtonspmathema00newtrich/newtonspmathema00newtrich#page/n411/mode/2up Andrew Mott