Editing Extraterrestrial sky (section)

===Pluto and Charon===
[[Pluto]], accompanied by its largest moon [[Charon (moon)|Charon]], orbits the Sun at a distance usually outside the orbit of [[Neptune]] except for a twenty-year period in each orbit.

From Pluto, the Sun is point-like to human eyes, but still very bright, giving roughly 150 to 450 times the light of the full Moon from Earth (the variability being due to the fact that Pluto's orbit is highly elliptical, stretching from just 4.4 billion km to over 7.3 billion km from the Sun).<ref>{{cite web |url=http://blogs.discovermagazine.com/badastronomy/2012/03/15/bafact-math-how-bright-is-the-sun-from-pluto/ |title=BAFact math: How bright is the Sun from Pluto? |last=Plait |first=Phil |date=15 March 2012 |publisher=Discover Magazine |access-date=6 July 2015 |archive-date=July 7, 2015 |archive-url=https://web.archive.org/web/20150707045843/http://blogs.discovermagazine.com/badastronomy/2012/03/15/bafact-math-how-bright-is-the-sun-from-pluto/ |url-status=dead }}</ref> Nonetheless, human observers would notice a large decrease in available light: the solar illuminance at Pluto's average distance is about 85 [[lux#Illuminance|lx]], which is equivalent to the lighting of an office building's hallway or restroom.

Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide gases, all of which are derived from the ices of these substances on its surface. When Pluto is close to the Sun, the temperature of Pluto's solid surface increases, causing these ices to sublimate into gases. This atmosphere also produces a noticeable blue haze that is visible at sunset and possibly other times of the Plutonian day.<ref>{{Cite web|url=https://www.newscientist.com/article/dn28309-new-horizons-shows-pluto-sporting-blue-skies-and-red-water-ice/|title=New Horizons shows Pluto sporting blue skies and red water ice|website=New Scientist|accessdate=6 October 2023}}</ref>

Pluto and Charon are [[Tidal locking|tidally locked]] to each other. This means that Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon. Observers on the far side of Charon from Pluto would never see the dwarf planet; observers on the far side of Pluto from Charon would never see the moon. Every 124 years, for several years it is mutual-eclipse season, during which Pluto and Charon each alternately eclipse the Sun for the other at intervals of 3.2 days. Charon, as seen from Pluto's surface at the sub-Charon point, has an angular diameter of about 3.8°, nearly eight times the Moon's angular diameter as seen from Earth and about 56 times the area. It would be a very large object in the night sky, shining about 8%<ref group="nb">Charon's radius is 606 km to the Moon's 1737 (surface ratio 0.12), its albedo is 0.35 to the Moon's 0.14 (2.6 ratio), its semi-major axis is 19591 km to the Moon's 384,400 (385 ratio of illumination), and the Sun is 39.482 times farther (0.00064 ratio of solar illumination). Multiplying these together yields an illuminance 0.077 times the Moon's.</ref> as bright as the Moon (it would appear darker than the Moon because its lesser illumination comes from a larger disc). Charon's illuminance would be about 14 mlx (for comparison, a moonless clear night sky is 2 mlx while a full Moon is between 300 and 50 mlx).
<gallery class="center">
File:Plutonian system.jpg|View from [[Hydra (moon)|Hydra]]. [[Pluto]] and [[Charon (moon)|Charon]] (right); [[Nix (moon)|Nix]] (left) (artist concept).
File:ESO-L. Calçada - Pluto (by).jpg|View from Pluto. [[Sun]] (right-top); [[Charon (moon)|Charon]] (left) (artist concept).
File:PIA19682-Pluto-Charon-Sun-ArtistConcept-20150608.jpg|View from Pluto of Charon and the Sun (artist concept).
File:NH-PlutoByMoonlight-ArtistConcept-20150710.jpg|Pluto by moonlight<br />(artist concept).
</gallery>
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| footer            = [[Pluto]] – [[Tenzing Montes]] (left-foreground); [[Hillary Montes]] (left-skyline); [[Sputnik Planitia]] (right)<br />Near-sunset view includes several layers of [[Atmosphere of Pluto|atmospheric haze]].
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