Editing Mercury (planet) (section)

=== Longitude convention ===
The longitude convention for Mercury puts the zero of longitude at one of the two hottest points on the surface, as described above. However, when this area was first visited, by {{nowrap|''Mariner 10''}}, this zero meridian was in darkness, so it was impossible to select a feature on the surface to define the exact position of the meridian. Therefore, a small crater further west was chosen, called [[Hun Kal]], which provides the exact reference point for measuring longitude.<ref>{{cite journal | last=Davies | first=M. E. | title=Surface Coordinates and Cartography of Mercury | journal=Journal of Geophysical Research | volume=80 | issue=B17 | pages=2417–2430 | date=June 10, 1975 | doi=10.1029/JB080i017p02417 | bibcode=1975JGR....80.2417D }}</ref><ref>{{cite book | last1=Davies | first1=M. E. | first2=S. E. | last2=Dwornik | first3=D. E. | last3=Gault | first4=R. G. | last4=Strom | title=NASA Atlas of Mercury | publisher=NASA Scientific and Technical Information Office | date=1978 }}</ref> The center of Hun Kal defines the 20°&nbsp;west meridian. A 1970 International Astronomical Union resolution suggests that longitudes be measured positively in the westerly direction on Mercury.<ref name="usgs">{{cite web |url=https://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |access-date=October 22, 2009 |title=USGS Astrogeology: Rotation and pole position for the Sun and planets (IAU WGCCRE) |archive-url=https://web.archive.org/web/20111024101856/http://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |archive-date=October 24, 2011 |url-status=dead}}</ref> The two hottest places on the equator are therefore at longitudes 0°&nbsp;W and 180°&nbsp;W, and the coolest points on the equator are at longitudes 90°&nbsp;W and 270°&nbsp;W. However, the ''MESSENGER'' project uses an east-positive convention.<ref name="ArchinalA'Hearn2010">{{cite journal |last1=Archinal |first1=Brent A. |display-authors=4 |last2=A'Hearn |first2=Michael F. |last3=Bowell |first3=Edward L. |last4=Conrad |first4=Albert R. |last5=Consolmagno |first5=Guy J. |last6=Courtin |first6=Régis |last7=Fukushima |first7=Toshio |last8=Hestroffer |first8=Daniel |last9=Hilton |first9=James L. |last10=Krasinsky |first10=George A. |last11=Neumann |first11=Gregory A. |last12=Oberst |first12=Jürgen |last13=Seidelmann |first13=P. Kenneth |last14=Stooke |first14=Philip J. |last15=Tholen |first15=David J. |last16=Thomas |first16=Peter C. |last17=Williams |first17=Iwan P. |title=Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009 |journal=Celestial Mechanics and Dynamical Astronomy |volume=109 |issue=2 |year=2010 |pages=101–135 |issn=0923-2958 |doi=10.1007/s10569-010-9320-4 |bibcode=2011CeMDA.109..101A|s2cid=189842666 }}</ref>