Editing Earth radius (section)

{{for multi|its historical development|Spherical Earth|its determination|Arc measurement}}
{{short description|Distance from the Earth surface to a point near its center}} 
{{Infobox quantity
| name = Earth radius
| othernames = terrestrial radius
| width =
| background =
| image = WGS84_mean_Earth_radius.svg
| caption = Equatorial (''a''), polar (''b'') and arithmetic mean Earth radii as defined in the 1984 [[World Geodetic System]] revision (not to scale)
| unit = meters
| otherunits =
| symbols   = ''R''<sub>🜨</sub>, ''R''<sub>E</sub>, ''a'', ''b'', ''a''<sub>E</sub>, ''b''<sub>E</sub>, ''R''<sub>''e''E</sub>, ''R''<sub>''p''E</sub>
| baseunits = [[Metre|m]]
| dimension = <math>\mathsf{L}</math>
| extensive = 
| intensive =
| conserved =
| transformsas = scalar
| derivations =
| value = ''Equatorial radius'': {{mvar|a}} = ({{val|6378137.0|u=m}}) <br/> ''Polar radius'': {{mvar|b}} = ({{val|6356752.3|u=m}})
}}
{{Infobox unit
| name = Nominal Earth radius
| image = EarthPieSlice.png
| caption = Cross section of Earth's Interior
| standard     = [[astronomy]], [[geophysics]]
| quantity     = [[distance]]
| symbol       = <math>\mathcal{R}^\mathrm N_\mathrm{E}</math>
| symbol2      = <math>\mathcal{R}^\mathrm N_{e\mathrm{E}}</math>, <math>\mathcal{R}^\mathrm N_{p\mathrm{E}}</math>
| units1       = [[SI base unit]]
| inunits1     = {{val|6.3781|e=6|u=m}}<ref name = "IAU XXIX">{{cite arXiv |eprint=1510.07674|last1=Mamajek|first1=E. E|title=IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties| last2=Prsa|first2=A|last3=Torres|first3=G|last4=Harmanec|first4=P|last5=Asplund|first5=M|last6=Bennett|first6=P. D|last7=Capitaine|first7=N|last8=Christensen-Dalsgaard|first8=J|last9=Depagne|first9=E|last10=Folkner|first10=W. M|last11=Haberreiter|first11=M|last12=Hekker|first12=S|last13=Hilton|first13=J. L|last14=Kostov|first14=V|last15=Kurtz|first15=D. W|last16=Laskar|first16=J|last17=Mason|first17=B. D|last18=Milone|first18=E. F|last19=Montgomery|first19=M. M|last20=Richards|first20=M. T|last21=Schou|first21=J|last22=Stewart|first22=S. G|class=astro-ph.SR|year=2015|display-authors= 3}}</ref>
| units2       = [[Metric system]]
| inunits2     = {{val|6357|to|6378|u=km|fmt=commas}}
| units3       = [[English units]]
| inunits3     = {{val|3950|to|3963|u=mi|fmt=commas}}
}}
{{Geodesy}}

'''Earth radius''' (denoted as ''R''<sub>🜨</sub> or ''R''<sub>E</sub>) is the distance from the center of [[Earth]] to a point on or near its surface. Approximating the [[figure of Earth]] by an [[Earth spheroid]] (an [[oblate ellipsoid]]), the radius ranges from a maximum ('''equatorial radius''', denoted ''a'') of about {{cvt|6378|km|mi}} to a minimum ('''polar radius''', denoted ''b'') of nearly {{cvt|6357|km|mi}}. 

A globally-average value is usually considered to be {{convert|6371|km|mi}} with a 0.3% variability (±10 km) for the following reasons.
The [[International Union of Geodesy and Geophysics]] (IUGG) provides three reference values: the ''mean radius'' (''R''{{sub|1}}) of three radii measured at two equator points and a pole; the ''authalic radius'', which is the radius of a sphere with the same surface area (''R''{{sub|2}}); and the ''volumetric radius'', which is the radius of a sphere having the same volume as the ellipsoid (''R''{{sub|3}}).<ref name="Moritz" /> All three values are about {{convert|6371|km|mi}}.

Other ways to define and measure the Earth's radius involve either the spheroid's [[radius of curvature]] or the actual [[topography]]. A few definitions yield values outside the range between the [[geographical pole|polar]] radius and [[equator]]ial radius because they account for localized effects.

A ''nominal Earth radius'' (denoted <math>\mathcal{R}^\mathrm N_\mathrm{E}</math>) is sometimes used as a [[unit of measurement]] in [[astronomy]] and [[geophysics]], a [[conversion factor]] used when expressing planetary properties as multiples or fractions of a constant terrestrial radius; if the choice between equatorial or polar radii is not explicit, the equatorial radius is to be assumed, as recommended by the [[International Astronomical Union]] (IAU).<ref name="IAU XXIX" />