Editing Earth radius (section)

==Extrema: equatorial and polar radii{{anchor|Fixed radius|Equatorial radius|Polar radius}}==
The following radii are derived from the [[World Geodetic System]] 1984 ([[WGS-84]]) [[reference ellipsoid]].<ref name=tr8350_2>{{cite web|url=https://nsgreg.nga.mil/doc/view?i=4085 |title=Department of Defense World Geodetic System 1984: Its Definition and Relationships with Local Geodetic Systems |access-date=2018-10-17}}</ref> It is an idealized surface, and the Earth measurements used to calculate it have an uncertainty of ±2&nbsp;m in both the equatorial and polar dimensions.<ref name="NGA">{{cite web |url=http://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350.2-a/Chapter%203.pdf |title=Info |website=earth-info.nga.mil |access-date=2008-12-31 |archive-date=2020-08-04 |archive-url=https://web.archive.org/web/20200804185855/https://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350.2-a/Chapter%203.pdf |url-status=dead }}</ref> Additional discrepancies caused by topographical variation at specific locations can be significant. When identifying the position of an observable location, the use of more precise values for WGS-84 radii may not yield a corresponding improvement in [[accuracy]].{{unclear-inline|date=April 2021}}

The value for the equatorial radius is defined to the nearest 0.1&nbsp;m in WGS-84. The value for the polar radius in this section has been rounded to the nearest 0.1&nbsp;m, which is expected to be adequate for most uses. Refer to the WGS-84 ellipsoid if a more precise value for its polar radius is needed.

* The Earth's ''equatorial radius'' {{mvar|a}}, or [[semi-major axis]],<ref name="Snyder manual"/>{{rp|11}} is the distance from its center to the [[equator]] and equals {{convert|6378.1370|km|mi|sigfig=8|abbr=on}}.<ref>{{cite web|url=http://maia.usno.navy.mil/NSFA/NSFA_cbe.html#EarthRadius2009|title=Equatorial Radius of the Earth|date=2012|website=Numerical Standards for Fundamental Astronomy: Astronomical Constants : Current Best Estimates (CBEs)|publisher=IAU Division I Working Group|access-date=2016-08-10|url-status=dead|archive-url=https://web.archive.org/web/20160826200953/http://maia.usno.navy.mil/NSFA/NSFA_cbe.html#EarthRadius2009|archive-date=2016-08-26}}</ref> The equatorial radius is often used to compare Earth with other [[Planet#Attributes|planets]].
* The Earth's ''polar radius'' {{mvar|b}}, or [[semi-minor axis]]<ref name="Snyder manual"/>{{rp|11}} is the distance from its center to the North and South Poles, and equals {{convert|6356.7523|km|mi|sigfig=8|abbr=on}}.