Editing Mercury (planet) (section)

== Observation ==
[[File:Mercury.jpg|thumb|Image mosaic by {{nowrap|''Mariner 10''}}, 1974]]
Mercury's [[apparent magnitude]] is calculated to vary between −2.48 (brighter than [[Sirius]]) around [[superior conjunction]] and +7.25 (below the limit of naked-eye visibility) around [[inferior conjunction]].<ref name="Mallama_and_Hilton" /> The mean apparent magnitude is 0.23 while the standard deviation of 1.78 is the largest of any planet. The mean apparent magnitude at superior conjunction is −1.89 while that at inferior conjunction is +5.93.<ref name="Mallama_and_Hilton" /> Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun's glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight.<ref name=Menzel1964P292-293>{{cite book |last=Menzel |first=Donald H. |author-link=Donald Howard Menzel |title=A Field Guide to the Stars and Planets |date=1964 |series=[[Peterson Field Guides|The Peterson Field Guide Series]] |location=Boston |publisher=[[Houghton Mifflin Co.]] |pages=292–293}}</ref>

Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. The [[Hubble Space Telescope]] cannot observe Mercury at all, due to safety procedures that prevent its pointing too close to the Sun.<ref>{{cite journal |last1=Baumgardner |first1=Jeffrey |last2=Mendillo |first2=Michael |last3=Wilson |first3=Jody K. |title=A Digital High-Definition Imaging System for Spectral Studies of Extended Planetary Atmospheres. I. Initial Results in White Light Showing Features on the Hemisphere of Mercury Unimaged by Mariner&nbsp;10 |journal=The Astronomical Journal |year=2000 |volume=119 |issue=5 |pages=2458–2464 |doi=10.1086/301323 |bibcode=2000AJ....119.2458B|s2cid=17361371 |doi-access=free }}</ref> Because the shift of 0.15 revolutions of Earth in a Mercurian year makes up a seven-Mercurian-year cycle (0.15 × 7 ≈ 1.0), in the seventh Mercurian year, Mercury follows almost exactly (earlier by 7 days) the sequence of phenomena it showed seven Mercurian years before.<ref name="elongation" />

Like the Moon and Venus, Mercury exhibits [[Lunar phase|phases]] as seen from Earth. It is "new" at [[inferior conjunction]] and "full" at superior conjunction. The planet is rendered invisible from Earth on both of these occasions because of its being obscured by the Sun,<ref name=Menzel1964P292-293/> except at its new phase during a transit. Mercury is technically brightest as seen from Earth when it is at a full phase. Although Mercury is farthest from Earth when it is full, the greater illuminated area that is visible and the [[opposition effect|opposition brightness surge]] more than compensates for the distance.<ref name="MallamaSky" /> The opposite is true for Venus, which appears brightest when it is a [[crescent]], because it is much closer to Earth than when [[gibbous]].<ref name="MallamaSky" /><ref>{{cite web | last=Espenak | first=Fred | year=1996 | url=http://eclipse.gsfc.nasa.gov/TYPE/venus2.html | title=NASA Reference Publication 1349; Venus: Twelve year planetary ephemeris, 1995–2006 | website=Twelve Year Planetary Ephemeris Directory | publisher=NASA | access-date=May 24, 2008 | archive-date=August 17, 2000 | archive-url=https://web.archive.org/web/20000817181616/http://sunearth.gsfc.nasa.gov/eclipse/TYPE/venus2.html | url-status=live }}</ref>
[[File:PIA19247-Mercury-NPolarRegion-Messenger20150316.jpg|thumb|left|False-color map showing the maximum temperatures of the north polar region]]

[[File:Mercury Venus Moon over San Jose 08 Jan 2024.png|thumb|right|Mercury (lower left) as seen from [[San Jose, California]] with Venus and the Moon.]]

Mercury is best observed at the first and last quarter, although they are phases of lesser brightness. The first and last quarter phases occur at greatest [[elongation (astronomy)|elongation]] east and west of the Sun, respectively. At both of these times, Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion.<ref name="elongation">{{cite web |title=Mercury Chaser's Calculator |publisher=Fourmilab Switzerland |first=John |last=Walker |url=http://www.fourmilab.ch/images/3planets/elongation.html |access-date=May 29, 2008 |archive-date=August 2, 2009 |archive-url=https://web.archive.org/web/20090802110601/http://www.fourmilab.ch/images/3planets/elongation.html |url-status=live }} (look at 1964 and 2013)</ref><ref name="MercHorizons">{{cite web |title=Mercury Elongation and Distance |url=http://home.surewest.net/kheider/astro/Mercury.txt |access-date=May 30, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20130511073623/http://home.surewest.net/kheider/astro/Mercury.txt |archive-date=May 11, 2013 }} – Numbers generated using the Solar System Dynamics Group, [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=1 Horizons On-Line Ephemeris System] {{Webarchive|url=https://web.archive.org/web/20150707172324/http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=1 |date=July 7, 2015 }}</ref> At greatest ''western'' elongation, Mercury rises at its earliest before sunrise, and at greatest ''eastern'' elongation, it sets at its latest after sunset.<ref name="RASC2007">{{cite book |title=Observer's Handbook 2007 |editor-first=Patrick |editor-last=Kelly |publisher=[[Royal Astronomical Society of Canada]] |date=2007 |isbn=978-0-9738109-3-6 |url=https://archive.org/details/observershandboo00raji_0 }}</ref>

[[File:PIA19422-Mercury-CarnegieRupes-MDIS-MLA-20150416.jpg|thumb|right|False-color image of [[Carnegie Rupes]], a tectonic landform—high terrain (red); low (blue).]]

Mercury is more often and easily visible from the [[Southern Hemisphere]] than from the [[Northern Hemisphere|Northern]]. This is because Mercury's maximum western elongation occurs only during early autumn in the Southern Hemisphere, whereas its greatest eastern elongation happens only during late winter in the Southern Hemisphere.<ref name="RASC2007" /> In both of these cases, the angle at which the planet's orbit intersects the horizon is maximized, allowing it to rise several hours before sunrise in the former instance and not set until several hours after sundown in the latter from southern mid-latitudes, such as Argentina and South Africa.<ref name="RASC2007" />

An alternate method for viewing Mercury involves observing the planet with a [[telescope]] during daylight hours when conditions are clear, ideally when it is at its greatest elongation. This allows the planet to be found easily, even when using telescopes with {{Convert|8|cm|abbr = on}} apertures. However, great care must be taken to obstruct the Sun from sight because of the extreme risk for eye damage.<ref>{{cite journal | title=Finding Venus or Mercury in daylight | last=Curtis | first=A. C. | journal=Journal of the British Astronomical Association | volume=82 | pages=438–439 | date=October 1972 | bibcode=1972JBAA...82..438C }}</ref> This method bypasses the limitation of twilight observing when the ecliptic is located at a low elevation (e.g. on autumn evenings). The planet is higher in the sky and less atmospheric effects affect the view of the planet. Mercury can be viewed as close as 4° to the Sun near superior conjunction when it is almost at its brightest.

Mercury can, like several other planets and the brightest stars, be seen during a total [[solar eclipse]].<ref name="eclipse">{{cite web |date=January 22, 2003 |title=Total Solar Eclipse of 2006 March 29 |publisher=Department of Physics at Fizik Bolumu in Turkey |first=Tunç |last=Tezel |url=http://www.physics.metu.edu.tr/~aat/TSE2006/TSE2006.html |access-date=May 24, 2008 |archive-date=September 12, 2016 |archive-url=https://web.archive.org/web/20160912153809/http://www.physics.metu.edu.tr/~aat/TSE2006/TSE2006.html |url-status=live }}</ref>