Editing Planet (section)

==== Size and shape ====
{{see also|Earth#Size and shape|Astronomical object#Shape|Planetary coordinate system}}
Gravity causes planets to be pulled into a roughly spherical shape, so a planet's size can be expressed roughly by an average radius (for example, [[Earth radius (unit)|Earth radius]] or [[Jupiter radius]]). However, planets are not perfectly spherical; for example, the [[Earth's rotation]] causes it to be slightly flattened at the poles with a [[equatorial bulge|bulge around the equator]].<ref name=milbert_smith96>{{cite web |last1=Milbert |first1=D. G. |last2=Smith |first2=D. A |url=http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html |title=Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model |publisher=National Geodetic Survey, NOAA |access-date=7 March 2007 |archive-date=20 August 2011 |archive-url=https://web.archive.org/web/20110820090214/http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html |url-status=live }}</ref> Therefore, a better approximation of Earth's shape is an [[oblate spheroid]], whose equatorial diameter is {{convert|43|km|mi|sp=us}} larger than the [[Geographical pole|pole]]-to-pole diameter.<ref name="ngdc2006">{{cite web |last1=Sandwell |first1=D. T. |last2=Smith |first2=Walter H. F. |author-link2=Walter H. F. Smith|date=7 July 2006 |url=http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML |title=Exploring the Ocean Basins with Satellite Altimeter Data |publisher=NOAA/NGDC |access-date=21 April 2007|archive-url=https://archive.today/20140715142212/http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML|archive-date=15 July 2014}}</ref> Generally, a planet's shape may be described by giving polar and equatorial radii of a [[spheroid]] or specifying a [[reference ellipsoid]]. From such a specification, the planet's flattening, surface area, and volume can be calculated; its [[normal gravity]] can be computed knowing its size, shape, rotation rate, and mass.<ref>{{Citation |last=Wieczorek |first=M. A. |title=10.05 – Gravity and Topography of the Terrestrial Planets |date=2015 |url=https://www.sciencedirect.com/science/article/pii/B978044453802400169X |work=Treatise on Geophysics |edition=2nd |pages=153–193 |editor-last=Schubert |editor-first=Gerald |place=Oxford |publisher=Elsevier |language=en |isbn=978-0-444-53803-1 |access-date=13 May 2022 |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513203935/https://www.sciencedirect.com/science/article/pii/B978044453802400169X |url-status=live }}</ref>