Editing Latitude (section)

===Ellipsoids===

{{main|Ellipsoid of revolution}}
In 1687 [[Isaac Newton]] published the ''[[Philosophiæ Naturalis Principia Mathematica]]'', in which he proved that a rotating self-gravitating fluid body in equilibrium takes the form of an [[Oblate spheroid|oblate]] ellipsoid.<ref name=newton>{{cite book|first=Isaac |last=Newton|title=Philosophiæ Naturalis Principia Mathematica|chapter=Book III Proposition XIX Problem III|page= [https://archive.org/details/100878576/page/407 407] |translator-first=Andrew |translator-last=Motte |url=https://archive.org/details/100878576}}</ref> (This article uses the term ''ellipsoid'' in preference to the older term ''spheroid''.) Newton's result was confirmed by geodetic measurements in the 18th century. (See [[Meridian arc]].) An oblate ellipsoid is the three-dimensional surface generated by the rotation of an ellipse about its shorter axis (minor axis). "Oblate ellipsoid of revolution" is abbreviated to 'ellipsoid' in the remainder of this article. (Ellipsoids which do not have an axis of symmetry are termed [[triaxial ellipsoid|triaxial]].)

Many different [[Figure of the Earth|reference ellipsoids]] have been used in the history of [[geodesy]]. In pre-satellite days they were devised to give a good fit to the [[geoid]] over the limited area of a survey but, with the advent of [[GPS]], it has become natural to use reference ellipsoids (such as [[WGS84]]) with centre at the centre of mass of the Earth and minor axis aligned to the rotation axis of the Earth. These geocentric ellipsoids are usually within {{convert|100|m|ft|abbr=on}} of the geoid. Since latitude is defined with respect to an ellipsoid, the position of a given point is different on each ellipsoid: one cannot exactly specify the latitude and longitude of a geographical feature without specifying the ellipsoid used. Many maps maintained by national agencies are based on older ellipsoids, so one must know how the latitude and longitude values are transformed from one ellipsoid to another. GPS handsets include software to carry out [[Datum (geodesy)|datum transformations]] which link WGS84 to the local reference ellipsoid with its associated grid.