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| {{original research|date=November 2013}}
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| [[File:NASA-Apollo8-Dec24-Earthrise.jpg|thumb|350px|A historic extraterrestrial sky—the [[Earth]] viewed from the [[Moon]], [[Apollo 8|Apollo 8 mission]], [[Lunar orbit]], December 24, 1968]]
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| In [[astronomy]], the term '''extraterrestrial sky''' refers to a view of [[outer space]] from the surface of a world other than [[Earth]].
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| The [[sky]] of the [[The Moon|Moon]] has been directly observed or photographed by [[astronaut]]s, while those of [[Titan (moon)|Titan]], [[Mars]], and [[Venus]] have been observed indirectly by [[space probe]]s designed to land on the surface and transmit images back to Earth.
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| Characteristics of extraterrestrial skies appear to vary substantially due to a number of factors. An [[extraterrestrial atmosphere]], if present, has a large bearing on visible characteristics. The [[atmosphere]]'s density and [[chemical composition]] can contribute to differences in [[colour]], [[Opacity (optics)|opacity]] (including [[haze]]) and the presence of [[cloud]]s. Astronomical objects may also be visible and can include [[natural satellite]]s, [[planetary ring|ring]]s, [[star system]]s and [[nebula]]s and other [[planetary system]] bodies.
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| For skies that have not been directly or indirectly observed, their appearance can be simulated based on known parameters such as the position of astronomical objects relative to the surface and atmospheric composition.
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| ==Mercury==
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| Because [[Mercury (planet)|Mercury]] has no [[Atmosphere of Mercury|atmosphere]], a view of the planet's skies would be no different from viewing space from orbit. Mercury has a southern [[pole star]], [[Alpha Pictoris|α Pictoris]], a magnitude 3.2 star. It is fainter than Earth's [[Polaris]] (α Ursae Minoris).<ref>[http://www.windows.ucar.edu/tour/link=/mercury/Atmosphere/atmosphere.html Windows planets—Mercury's atmosphere]</ref>
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| ===The Sun from Mercury===
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| On average, the visible diameter of the Sun on Mercury is 2.5 times as large as it appears from Earth, and its total brightness is more than 6 times as great. Because of the planet's eccentric orbit, the Sun's apparent size in the sky would vary from 2.2 times that from Earth at [[Apsis|aphelion]] (with total brightness 4.8 times as great), to 3.2 times at [[Apsis|perihelion]] (with total brightness 10.2 times as great).
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| Mercury has a 3:2 spin–orbit resonance. This means that although a [[sidereal day]] (the period of rotation) lasts ~58.7 Earth days, a [[solar day]] (the length between two [[meridian (astronomy)|meridian]] [[Astronomical transit|transit]]s of the Sun) lasts ~176 Earth days.
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| Mercury's spin–orbit resonance generates an unusual effect in which the Sun appears to briefly reverse its usual east to west motion once every Mercurian year. The effect is visible wherever one happens to be on Mercury, but there are certain points on Mercury's surface where an observer would be able to see the Sun rise about halfway, reverse and set, and then rise again, all within the same Mercurian day. This is because approximately four days prior to perihelion, the angular speed of Mercury's orbit exactly equals its [[rotational velocity]], so that the Sun's [[improper motion|apparent motion]] ceases; at perihelion, Mercury's orbital angular velocity then exceeds the rotational velocity; thus, the Sun appears to be [[retrograde motion|retrograde]]. Four days after perihelion, the Sun's normal apparent motion resumes. Because of its spin–orbit resonance, Mercury presents one of two spots of its surface to the Sun on alternate perihelia; one of these subsolar points is [[Caloris Planitia]] ("hot basin"), appropriately named because an observer near its centre would see the Sun loop around the zenith once per Mercurian day, and hence experience a very hot day indeed.
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| ===Other planets seen from Mercury===
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| After the Sun, the second brightest object in the Mercurian sky is [[Venus]], which is much brighter than for terrestrial observers. The reason for this is that when Venus is closest to Earth, it is between the Earth and the Sun, so we see only its night side. Indeed, even when Venus is brightest in the Earth's sky, we are actually seeing only a narrow crescent. For a Mercurian observer, on the other hand, Venus is closest when it is in [[Astronomical opposition|opposition]] to the Sun and is showing its full disk. The [[apparent magnitude]] of Venus is as bright as −7.7.<ref name=Perelman>{{cite book
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| |author = Yakov Perelman; Arthur Shkarovsky-Raffe
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| |year = 2000
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| |title = Astronomy for Entertainment
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| |url = http://books.google.com/books?id=2Sc_eaR9c9UC&pg=PA147&lpg=PA147&dq=%22Earth+from+Mercury%22+magnitude|publisher = University Press of the Pacific
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| |isbn = 0-89875-056-3 }}</ref>
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| The Earth and the Moon are also very prominent, their apparent magnitudes being about −5<ref name=Perelman/> and −1.2 respectively. The maximum apparent distance between the Earth and the Moon is about 15′. All other planets are visible just as they are on Earth, but somewhat less bright at opposition.
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| The [[zodiacal light]] is probably more prominent than it is from Earth.
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| ==Venus==
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| The atmosphere of [[Venus]] is so thick that the Sun is not distinguishable in the daytime sky, and the [[star]]s are not visible at night. Color images taken by the Soviet [[Venera program|Venera]] probes suggest that the sky on Venus is orange-red.<ref>[http://www.windows.ucar.edu/tour/link=/venus/images/Venus_Clouds_image.html Venus's atmosphere layers]</ref> If the Sun could be seen from Venus's surface, the time from one sunrise to the next (a [[solar day]]) would be 116.75 Earth days. Because of Venus's [[retrograde motion|retrograde rotation]], the Sun would appear to rise in the west and set in the east.<ref>{{cite web |url=http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html |archiveurl=http://www.webcitation.org/616VoDl1i |archivedate=2011-08-21 |title=The Terrestrial Planets |publisher=The Planetary Society |accessdate=2007-08-03}}</ref>
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| An observer aloft in Venus's cloud tops, on the other hand, would whip around the planet in about four days and be treated to a sky in which Earth and the Moon shine brightly (about magnitudes −6.6<ref name=Perelman/> and −2.7, respectively) because their maximum approach occurs at opposition. Mercury would also be easy to spot, because it is closer and brighter, at up to magnitude −2.7,<ref name=Perelman/> and because its maximum [[Elongation (astronomy)|elongation]] from the Sun is considerably larger (40.5°) than when observed from Earth (28.3°).
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| ==The Moon==
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| [[File:Eclipse_from_moon.jpg|thumb|An artist's conception of a solar eclipse from the moon, during a [[lunar eclipse]] as viewed from the earth.]]
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| The [[Moon]] has no atmosphere, so its sky is always black. However, the Sun is so bright that it is impossible to see stars during the daytime, unless the observer is well shielded from sunlight (direct or reflected from the ground). The Moon has a southern polar star, [[Delta Doradus|δ Doradus]], a magnitude 4.34 star. It is better aligned than Earth's [[Polaris]] (α Ursae Minoris), but much fainter.
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| ===The Sun from the Moon===
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| The Sun looks the same from the Moon as it does from Earth's orbit, somewhat brighter than it does from the Earth's surface, and colored pure white, due to the lack of atmospheric scattering and absorption.
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| Since the Moon's axial tilt relative to its orbit around the Sun is nearly zero, the Sun traces out almost exactly the same path through the Moon's sky over the course of a year. As a result there are craters and valleys near the Moon's poles that never receive direct sunlight, and there may exist mountains and hilltops that are never in shadow (see [[Peak of eternal light]]).
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| ===The Earth from the Moon===
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| [[File:Apollo17.jpg|thumb|left|[[Apollo 17]] commander [[Eugene Cernan]] on the Moon, with Earth visible in the sky]]
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| Among the most prominent features of the Moon's sky is Earth. Its visible diameter (1.9°) is four times the diameter of the Moon as seen from Earth, although because the Moon's orbit is eccentric, Earth's apparent size in the sky varies by about 5% either way (ranging between 1.8° and 2.0° in diameter). Earth shows [[planetary phase|phases]], just like the Moon does for the terrestrial observer, but they are opposite: when the terrestrial observer sees the full Moon, the lunar observer sees a "new Earth", and vice versa. Earth's [[albedo]] is three times as high as that of the Moon, and coupled with the increased area the full Earth glows over 50 times brighter than the full Moon at zenith does for the terrestrial observer.
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| As a result of the Moon's [[synchronous rotation]], one side of the Moon (the "[[near side of the Moon|near side]]") is permanently turned towards Earth, and the other side, the "[[Far side (Moon)|far side]]", mostly cannot be seen from Earth. This means, conversely, that Earth can only be seen from the near side of the Moon and would always be invisible from the far side.
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| If the Moon's rotation were purely synchronous, Earth would not have any noticeable movement in the Moon's sky. However, due to the Moon's [[libration]], Earth does perform a slow and complex wobbling movement. Once a month, as seen from the Moon, Earth traces out an approximate oval of diameter 18°. The exact shape and orientation of this oval depend on one's location on the Moon. As a result, near the boundary of the near and far sides of the Moon, Earth is sometimes below the [[horizon]] and sometimes above it.
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| ===Eclipses from the Moon===
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| The Earth and the Sun sometimes meet in the lunar sky, causing an [[eclipse]]. On the Earth, one then sees a [[lunar eclipse]], in which the Moon passes through the Earth's shadow, but on the Moon, one would see the Sun go behind the Earth—causing a [[solar eclipse]]. As the apparent diameter of the Earth would be four times larger than that of the Sun, the Sun would be hidden behind the Earth for hours. The [[Earth's atmosphere]] would be visible as a reddish ring. An attempt was made to use the [[Apollo 15]] [[Lunar rover (Apollo)|Lunar rover]] TV camera to view such an eclipse, but the camera or its power source failed after the astronauts left for Earth.<ref>[http://www.hq.nasa.gov/alsj/a15/a15.launch.html Return to Orbit<!-- Bot generated title -->]</ref>
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| Terrestrial [[solar eclipse]]s, on the other hand, would not be as spectacular for lunar observers because the Moon's umbra nearly tapers out at the Earth's surface. A soft, slightly darkened patch would barely be visible. The effect would be comparable to the shadow of a golf ball cast by sunlight on an object 15 feet away. Lunar observers with telescopes might be able to discern the region of totality as a very black spot at the center of a soft shadow travelling across the full Earth's disk.
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| In summary, whenever an eclipse of some sort is occurring on the Earth, an eclipse of ''another'' sort is occurring on the Moon. Eclipses occur for both Earth and Lunar observers whenever the two bodies and the Sun align in a straight line.
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| ==Mars==
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| {{see also|Astronomy on Mars}}
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| [[Mars]] has only a thin atmosphere; however, it is extremely dusty and there is much light that is scattered about. The sky is thus rather bright during the daytime and stars are not visible. The Martian northern pole star is [[Deneb]]<ref>Burgess, E. & Singh, G., ''To the Red Planet'' Columbia University Press 1978; see [http://adsabs.harvard.edu/full/1993Ap&SS.201..160B review] in Astrophysics and space SCI. V.201, NO. 1/FEB(I), P.160, 1993</ref> (although the actual pole is somewhat offset in the direction of [[Alpha Cephei]]).
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| ===The color of the Martian sky===
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| [[File:Mars violet sky.jpg|right|thumb|Mars's sky turned violet by water ice clouds (August, 1997).]]
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| [[File:Mars sky at noon PIA01546.jpg|left|thumb|Mars sky at noon, as imaged by [[Mars Pathfinder]] (June, 1999).]]
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| [[File:Mars sunset PIA01547.jpg|left|thumb|Mars sky at sunset, as imaged by [[Mars Pathfinder]] (June, 1999).]]
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| [[File:MarsSunset losslesscrop.jpg|left|thumb|Mars sky at sunset, as imaged by the [[Spirit rover]] (May, 2005).]]
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| [[File:Martian-Sunset-O-de-Goursac-Curiosity-2013.jpg|left|thumb|Mars sky at [[Sunset#Planets|sunset]], as imaged by the [[Curiosity rover]] (February 2013; sun simulated by artist).]]
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| [[File:Mars sunset PIA00920.jpg|right|thumb|Close-up of Mars sky at sunset, showing more color variation, as imaged by [[Mars Pathfinder]] (August, 1997).]]
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| Generating accurate true-color images from Mars's surface is surprisingly complicated.<ref>[http://www.badastronomy.com/bad/misc/hoagland/mars_colors.html Phil Plait's Bad Astronomy: Misconceptions: What Color is Mars?<!-- Bot generated title -->]</ref> To give but one aspect to consider, there is the [[Purkinje effect]]: the human eye's response to color depends on the level of ambient light; red objects appear to darken faster than blue objects as the level of illumination goes down. There is much variation in the color of the sky as reproduced in published images, since many of those images have used filters to maximize their scientific value and are not trying to show true color. For many years, the sky on Mars was thought to be more pinkish than it is now believed to be.
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| It is now known that during the Martian day, the sky is a [[scarlet (color)|scarlet]] or bright orangeish-red color.{{fact|date=May 2012}} Around sunset and sunrise, the sky is [[rose (color)|rose]] in color, but in the vicinity of the setting Sun it is blue. This is the opposite of the situation on Earth. At times, the sky takes on a purplish color, due to the scattering of light by very small water ice particles in clouds.<ref>[http://starryskies.com/The_sky/events/mars/opposition08.html The Martian Sky: Stargazing from the Red Planet<!-- Bot generated title -->]</ref> Twilight lasts a long time after the Sun has set and before it rises because of the dust high in Mars's atmosphere.
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| On Mars, [[Rayleigh scattering]] is usually a very weak effect; the red color of the sky is caused by the presence of [[iron(III) oxide]] in the airborne dust particles.
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| ===The Sun from Mars===
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| {{see also|Timekeeping on Mars}}
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| The [[Sun]] as seen from Mars appears to be 5/8 the size as seen from Earth (0.35°), and sends 40% of the light, approximately the brightness of a slightly cloudy afternoon on Earth.
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| ===Mars's moons as seen from Mars===
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| [[File:15-ml-06-phobos2-A067R1.jpg|thumb|right|Phobos [[astronomical transit|transits]] the [[Sun]], as seen by Mars Rover ''[[MER-B|Opportunity]]'' on March 10, 2004]]
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| Mars has two small moons: [[Phobos (moon)|Phobos]] and [[Deimos (moon)|Deimos]]. From the Martian surface, Phobos has one-third to one-half the [[angular diameter]] of the Sun while Deimos is barely more than a dot (only 2' angular diameter). The orbital period of Phobos is so much shorter than the Martian day that the apparent motion of Phobos is in reverse: it rises in the west and sets in the east. Phobos orbits so close (in a low-inclination equatorial orbit) that it cannot be seen north of 70.4°N or south of 70.4°S latitude; high-latitude observers would also notice a decrease in Phobos's apparent size, the additional distance being non-negligible. Phobos's apparent size varies by up to 45% as it passes overhead, due to its proximity to Mars's surface. For an equatorial observer, for example, Phobos is about 0.14° upon rising and swells to 0.20° by the time it reaches the zenith. It crosses the sky swiftly, in about 4.24 hours, every 11.11 hours.<!--- Phobos can be seen from the equator for 68.725° on either side of the zenith; this doesn't match the 70.4° latitude limit because of Mars's oblateness, I presume --->
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| Deimos rises in the east and sets in the west, like a "normal" moon, although its appearance is star-like (angular diameter between 1.8' and 2.1'). Its brightness would vary between that of Venus and of the star [[Vega]] (as seen from Earth). Being relatively close to Mars, Deimos cannot be seen from Martian latitudes greater than 82.7°. Finally, Deimos's orbital period of about 30.3 hours exceeds the Martian rotation period (of about 24.6 hours) by such a small amount that it rises every 5.5 days and takes 2.5 days between rising and setting for an equatorial observer.<!--- Deimos can be seen from the equator for 81.662° on either side of the zenith; this doesn't match the 82.7° latitude limit because of Mars's oblateness, I presume ---> Thus Phobos crosses the Martian skies nearly 12 times<!--- 11.836 ---> whilst Deimos crosses them just once.
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| Phobos and Deimos can both [[eclipse]] the Sun as seen from Mars, although neither can completely cover its disk and so the event is in fact a [[Astronomical transit|transit]], rather than an eclipse. For a detailed description of such events see the articles [[Transit of Phobos from Mars]] and [[Transit of Deimos from Mars]].
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| From Phobos, Mars would appear very large in the sky, comparable to the view of a basketball from a mere 21 centimeters from its surface. From Deimos, Mars would appear comparable to the view of a basketball from about 70 centimeters from its surface.
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| ===Earth from Mars===
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| The Earth is visible from Mars as a double star; the Moon would be visible alongside it as a fainter companion. The maximum visible distance between the Earth and the Moon would be about 25′, at [[Conjunction (astronomy and astrology)|inferior conjunction]] of the Earth and the Sun (for the terrestrial observer, this is the [[Opposition (planets)|opposition]] of Mars and the Sun). Near maximum [[Elongation (astronomy)|elongation]] (47.4°), the Earth and Moon would shine at apparent magnitudes −2.5 and +0.9, respectively.<ref name=Perelman/><ref name=EarthfromMars>{{cite web|date=2003-05-08|title=Earth and Moon as Viewed from Mars|publisher=Earth Observatory|url=http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=15293|accessdate=2008-06-03}} (JPL Horizons shows: 0.9304AU from Earth; Phase 43%; Sun Elongation 43°)</ref>
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| ===Venus from Mars===
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| [[Venus]] as seen from Mars (when near the maximum elongation from the Sun of 31.7°) would have an apparent magnitude of about −3.2.<ref name=Perelman/>
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| ===The skies of Mars's moons===
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| From Phobos, Mars appears 6,400 times larger and 2,500 times brighter than the full Moon as seen from Earth, and appears as a sphere of angular diameter roughly 42 degrees—taking up a quarter of the width of a celestial hemisphere
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| From Deimos, Mars appears 1,000 times larger and 400 times brighter than the full Moon as seen from Earth, and appears as a sphere of angular diameter roughly 16 and a half degrees—taking up an eleventh of the width of a celestial hemisphere.
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| ==Asteroids==
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| The [[asteroid belt]] is sparsely populated and most asteroids are very small, so that an observer situated on one asteroid would be unlikely to be able to see another without the aid of a telescope. Occasional "close approaches" do occur, but these are spread out over eons. One movie to accurately show this is ''[[2001: A Space Odyssey (film)|2001: A Space Odyssey]]''.
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| Some asteroids that cross the orbits of planets may occasionally get close enough to a planet or asteroid so that an observer from that asteroid can make out the disc of the nearby object without the aid of binoculars or a telescope. For example, in September 2004, [[4179 Toutatis]] came about four times the distance from the Earth that the Moon does. At the closest point in its encounter, the Earth would have appeared about the same size as the Moon appears from Earth. The Moon would also be easily visible as a small shape in Toutatis's sky at that time.
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| Asteroids with unusual orbits also offer a lot to the imagination. For instance, the asteroid (or more likely, [[extinct comet]]) [[3200 Phaethon]] has one of the most eccentric orbits: its distance from the Sun varies between 0.14 and 2.4 [[astronomical unit|AU]]. At perihelion, the Sun would loom over 7 times larger than it does in our sky, and blast the surface with over 50 times as much energy; at aphelion, the Sun would shrink to less than half its apparent diameter on Earth, and give little more than a sixth as much illumination.
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| ===87 Sylvia and its moons Romulus and Remus===
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| The asteroid [[87 Sylvia]] is one of the largest main-belt asteroids and the first asteroid observed to have two moons. These moons, [[Romulus (moon)|Romulus]] and [[Remus (moon)|Remus]], would appear roughly the same size. Romulus, the farther one, would be about 0.89° across, slightly bigger than the closer but smaller Remus, which would be about 0.78° across. Because Sylvia is far from spherical, these values can vary by about a little more than 10%, depending on where the observer is on Sylvia's surface. Since the two asteroidal moons appear to orbit (as best we can tell) in the same plane, they would occult each other once every 2.2 days. When the season is right, twice during Sylvia's 6.52 year orbital period, they would eclipse the Sun, which, at 0.15° across, is much smaller than when seen from Earth (0.53°). From Remus, the inner moon, Sylvia is huge, roughly 30°×18° across, while its view of Romulus varies between 1.59 and 0.50° across. From Romulus, Sylvia measures 16°×10° across, while Remus varies between 0.62° and 0.19°.
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| ===243 Ida and its moon Dactyl===
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| The asteroid [[243 Ida]] was visited by the [[Galileo (spacecraft)|Galileo spacecraft]] on its way to Jupiter. It discovered a small moon named Dactyl. Although the exact orbital characteristics of Dactyl are not fully known, it was 90 kilometers from Ida when Galileo passed by. This would mean that from Ida, Dactyl would appear as large as 1.76 degrees across, which is huge for such a small moon (1.4 km).
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| ==Jupiter==
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| Although no images from within [[Jupiter]]'s atmosphere have ever been taken, artistic representations typically assume that the planet's sky is blue, though dimmer than Earth's, since the sunlight there is on average 27 times fainter, at least in the upper reaches of the atmosphere. The planet's narrow [[planetary ring|rings]] might be faintly visible from latitudes above the equator. Further down into the atmosphere, the Sun would be obscured by clouds and haze of various colors, most commonly blue, brown, and red. While theories abound on the cause of the colors, there is currently no unambiguous answer.<ref name=bagenal>{{cite web
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| |year=2005
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| |title=Class 17 – Giant Planets
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| |publisher=Laboratory for Atmospheric and Space Physics
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| |author=Fran Bagenal
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| |url=http://lasp.colorado.edu/~bagenal/3720/CLASS17/17GiantPlanets1.html
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| |accessdate=2008-09-05}}</ref>
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| From Jupiter, the Sun appears to cover only 5 arc minutes, less than a quarter of its size as seen from Earth.
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| ===Jupiter's moons as seen from Jupiter===
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| [[File:Jupiter's moons.jpg|thumb|left|Simulated view of Io, Europa, and the rings of Jupiter seen from their parent planet<ref>This and other simulated images on this page were made with the [[Celestia]] space simulation software.</ref>]]
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| Aside from the Sun, the most prominent objects in Jupiter's sky are the four [[Galilean moon]]s. [[Io (moon)|Io]], the nearest to the planet, would be slightly larger than the full Moon in Earth's sky, though less bright, and would be the largest moon in the Solar System as seen from its parent planet. The higher [[albedo]] of [[Europa (moon)|Europa]] would not overcome its greater distance from Jupiter, so it would not outshine Io. In fact, the low [[solar constant]] at Jupiter's distance (3.7% Earth's) ensures that none of the Galilean satellites would be as bright as the full Moon is on Earth; from Io to Callisto their [[apparent magnitude]]s would be: −11.2, −9.7, −9.4, and −7.0.{{Citation needed|date=February 2007}}
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| [[File:PIA17659-Europa-WaterPlume-ArtistConcept-20131212.jpg|thumb|250px|right|[[Water vapor|Water Vapor Plume]] on [[Europa (moon)|Europa]] (artist concept) (December 12, 2013).<ref name="NASA-20131212-EU">{{cite web|last1=Cook |first1=Jia-Rui C. |last2=Gutro |first2=Rob|last3=Brown|first3=Dwayne|last4=Harrington |first4=J.D. |last5=Fohn |first5=Joe |title=Hubble Sees Evidence of Water Vapor at Jupiter Moon |url=http://www.jpl.nasa.gov/news/news.php?release=2013-363|date=December 12, 2013 |work=[[NASA]] |accessdate=December 12, 2013 }}</ref>]]
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| [[Ganymede (moon)|Ganymede]], the largest moon and third from Jupiter, is almost as bright as Io and Europa, but appears only half the size of Io. However, its great size means that it appears about 2" larger than Europa. Also, a triple conjunction of Io, Europa and Ganymede would be impossible, as the Laplace Resonance that they share prevents this. When two of the Galilean Moons are aligned, the third is never less than 90 degrees to the left or right of these moons in its orbit. [[Callisto (moon)|Callisto]], still further out, would appear only a quarter the size of Io.
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| None of the surface features on Jupiter's moons would appear as prominent as the lunar maria do when the moon is viewed from Earth. Dark and light patches would be visible on Io's surface due to the coloration of sulfur that covers Io, and the largest volcanoes would be designated by dark points, but the lack of large, contrasting features results in a poor view of the moon. Europa, however, would appear as a completely featureless white orb. Even when viewed from nearby, most spacecraft images use contrast enhancements to clearly show the cracks in the Europan ice, which are only about 10–15 percent darker than the rest of the surface. Vague dark and light patches would be visible on Ganymede, while Callisto is much too distant for any features to be made out.
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| All four Galilean moons stand out because of the swiftness of their motion, compared to the Earth's Moon. They are all also large enough to fully [[eclipse]] the Sun.<ref name=CallistoSun>{{cite web
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| |date = 2009-Jun-03 00:30 UT
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| |title = Pre-eclipse of the Sun by Callisto from the center of Jupiter
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| |publisher = [http://space.jpl.nasa.gov/ JPL Solar System Simulator]
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| |url = http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=504&vbody=599&month=6&day=3&year=2009&hour=00&minute=20&fovmul=1&rfov=0.5&bfov=30&brite=1
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| |accessdate = 2008-06-04}}</ref>
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| Since Jupiter's axial tilt is minimal, and the Galilean moons all orbit in the plane of Jupiter's equator, solar eclipses are quite common.
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| Jupiter's small inner moons appear only as starlike points except Amalthea, which can occasionally appear as large as Callisto. However, they would all be brighter than any star, ranging from mag. -1.5 to -4.9. The outer moons would be invisible except for [[Himalia (moon)|Himalia]], which would appear as a dim, starlike point to the naked eye (5.5) only under favorable circumstances (if its periapsis happens to coincide exactly with its full phase).
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| ===The skies of Jupiter's moons===
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| None of Jupiter's moons have more than traces of atmosphere, so their skies are black or very nearly so. For an observer on one of the moons, the most prominent feature of the sky by far would be Jupiter. For an observer on [[Io (moon)|Io]], the closest large moon to the planet, Jupiter's apparent diameter would be about 20° (38 times the visible diameter of our Moon, covering 1% of Io's sky). An observer on [[Metis (moon)|Metis]], the innermost moon, would see Jupiter's apparent diameter increased to 68° (130 times the visible diameter of our Moon, covering 18% of Metis's sky). A "full Jupiter" over Metis shines with about 4% of the Sun's brightness (light on Earth from our full Moon is 400 thousand times dimmer than sunlight).
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| Since the inner moons of Jupiter are in [[synchronous rotation]] around Jupiter, the planet always appears in nearly the same spot in their skies (Jupiter would wiggle a bit because of the non-zero eccentricities). Observers on the sides of the Galilean satellites facing away from the planet would never see Jupiter, for instance.
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| From the moons of Jupiter, [[solar eclipse]]s caused by the Galilean satellites would be spectacular, as an observer would see the circular shadow of the eclipsing moon travel across Jupiter's face.<ref name=shadow>{{cite web
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| |title=Jupiter Moon Shadow Transit
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| |publisher=Jim Thommes Astrophotography
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| |author=Jim Thommes
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| |url=http://www.jthommes.com/Astro/JupiterShadowSeq.htm
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| |accessdate=2008-12-03}}</ref>
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| ==Saturn==
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| [[File:Earth-Moon system as seen from Saturn (PIA17171).jpg|left|thumb|250px|NASA's [[Cassini spacecraft]] photographs the [[Earth]] and [[Moon]] (visible bottom-right) from [[Saturn]] (July 19, 2013).]]
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| The sky in the upper reaches of [[Saturn]]'s atmosphere is probably blue, but the predominant color of its cloud decks suggests that it may be yellowish further down. Observations from spacecraft show that seasonal smog develops in Saturn's southern hemisphere at its perihelion due to its axial tilt. This could cause the sky to become yellowish at times. As the northern hemisphere is pointed towards the sun only at aphelion, the sky there would likely remain blue. The [[rings of Saturn]] are almost certainly visible from the upper reaches of its atmosphere. The rings are so thin that from a position on Saturn's equator, they would be almost invisible. From anywhere else on the planet, they could be seen as a spectacular arc stretching across half the celestial hemisphere.<ref name=bagenal/>
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| [[File:Saturn sunset.jpg|thumb|right|Simulated view of Saturn's rings seen from its equator]]
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| [[File:Saturn rings.jpg|thumb|right|Simulated view of Saturn's rings seen from a latitude above its equator]]
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| Saturn's moons would not look particularly impressive in its sky, as most are fairly small, and the largest are a long way from the planet. Even [[Titan (moon)|Titan]], the largest moon of Saturn, appears only half the size of Earth's moon, and only grows as bright as -7. In fact, Titan is the dimmest of Saturn's moons due to its great distance and low albedo; Mimas, Enceladus, Tethys, Dione, and Rhea are all brighter, at magnitudes -7.8, -7.9, -8.8, -8, and -7.8, respectively. Here are the approximate [[angular diameter]]s of the main moons (for comparison, Earth's moon has an angular diameter of 31'): Mimas: 7–11', Enceladus: 7–9', Tethys: 12–15', Dione: 10–12', Rhea: 8–11', Titan: 14–15', Iapetus: 1'. Most of the inner moons would appear as bright, starlike points (with the exception of [[Janus (moon)|Janus]], which would appear 7 arcseconds across at its zenith), although most would shine brighter than any star; for example, Janus and Epimetheus would both shine as bright as magnitude -6. Meanwhile, none of the outer moons would be visible except Phoebe, which nears the naked eye limit (6.2-6.6) even at full phase.
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| Saturn has a southern polar star, [[δ Octantis]], a magnitude 4.3 star. It is much fainter than Earth's [[Polaris]] (α Ursae Minoris).
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| ===The skies of Saturn's moons===
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| Since the inner moons of Saturn are all in [[synchronous rotation]], the planet always appears in the same spot in their skies. Observers on the sides of those satellites facing away from the planet would never see Saturn.
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| In the skies of Saturn's inner moons, Saturn is an enormous object. For instance, Saturn seen from [[Pan (moon)|Pan]] has an apparent diameter of ~50°, 104 times larger than our Moon and occupying 11% of Pan's sky. Because Pan orbits along the [[Encke division]] within Saturn's rings, they are visible from anywhere on Pan, even on its side facing away from Saturn.
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| ====The rings from Saturn's moons====
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| Saturn's rings would not be prominent from most of the moons. This is because the rings, though wide, are not very thick, and most of the moons orbit almost exactly (within 1.5°) in the planet's ring plane. Thus, the rings are edge-on and practically invisible from the inner moons. From the outer moons, starting with [[Iapetus (moon)|Iapetus]], a more oblique view of the rings would be available, although the greater distance would make Saturn appear smaller in the sky; from [[Phoebe (moon)|Phoebe]], the largest of Saturn's outermost moons, Saturn would appear only as big as the full Moon does from Earth. The play of distance and angle is quite sensitive to the values used, but calculations show the best view of the rings would be achieved from the inner moon [[Mimas (moon)|Mimas]], which lies a full 1.5° off Saturn's equatorial plane ''and'' is fairly near the rings. At their widest opening, when Mimas is at its maximum distance from Saturn's equatorial plane, the edges of the rings (from B to A) would be separated by 2.7 degrees. The co-orbitals [[Epimetheus (moon)|Epimetheus]] and [[Janus (moon)|Janus]] would also get a good view, with maximum opening angles ranging between 1.5 and 2.9°. [[Tethys (moon)|Tethys]] gets the next best view, with nearly half a degree. Iapetus achieves 0.20°, which is more than any of the outer moons can claim.
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| ====The sky of Titan====
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| [[File:Huygens surface color.jpg|thumb|right|Image of the surface of Titan from the Huygens probe]]
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| [[Titan (moon)|Titan]] is the only moon in the solar system to have a thick atmosphere. Images from the [[Huygens probe]] show that the Titanian sky is a light tangerine color. However, an astronaut standing on the surface of Titan would see a hazy brownish/dark orange colour. Titan receives 1/3000 of the sunlight Earth does, so under the thick atmosphere, plus the much greater distance from the Sun, daytime on Titan is only as bright as twilight on the Earth. It seems likely that Saturn is permanently invisible behind orange smog, and even the Sun would only be a lighter patch in the haze, barely illuminating the surface of ice and methane lakes. However, in the upper atmosphere, the sky would have a blue color and Saturn would be visible.<ref>[http://www.beugungsbild.de/huygens/povray/titan_rendered.html POV-Ray renderings of Huygens descending to Titan<!-- Bot generated title -->]</ref> With its thick atmosphere and methane rain, Titan is the only celestial body other than Earth upon which [[rainbow]]s could form. However, given the extreme opacity of the atmosphere in visible light, the vast majority would be in the infrared.<ref>{{cite web|title=Rainbows on Titan|publisher=NASA|year=http://science.nasa.gov/science-news/science-at-nasa/2005/25feb_titan2/|accessdate=2011-10-08}}</ref>
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| ====The sky of Enceladus====
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| [[File:Saturn seen from Enceladus (artist concept).jpg|thumb|250px|left|An artist's view of Enceladus's sky]]
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| Seen from [[Enceladus (moon)|Enceladus]], Saturn would have a visible diameter of almost 30°, sixty times more than the Moon visible from Earth. Moreover, since Enceladus rotates synchronously with its orbital period and therefore keeps one face pointed toward Saturn, the planet never moves in Enceladus's sky (albeit with slight variations coming from the orbit's [[eccentricity (orbit)|eccentricity]]), and cannot be seen from the far side of the satellite.
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| Saturn's rings would be seen from an angle of only 0.019° and would be almost invisible, but their shadow on Saturn's disk would be clearly distinguishable. Like our own Moon from Earth, Saturn itself would show regular [[Lunar phase|phases]]. From Enceladus, the Sun would have a diameter of only 3.5 minutes of arc, one ninth that of the Moon as seen from Earth.
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| An observer located on Enceladus could also observe [[Mimas (moon)|Mimas]] (the biggest satellite located inside Enceladus's orbit) transit in front of Saturn every 72 hours on average. Its apparent size would be at most 26 minutes of arc, about the same size as the Moon seen from Earth. [[Pallene (moon)|Pallene]] and [[Methone (moon)|Methone]] would appear nearly star-like (maximum 30 seconds of arc). [[Tethys (moon)|Tethys]], visible from Enceladus's anti-Saturnian side, would reach a maximum apparent size of about 64 minutes of arc, about twice the Moon as seen from the Earth.
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| {{clear}}
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| ==Uranus==
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| Judging by the colour of its atmosphere, the sky of [[Uranus]] is probably a light blue, i.e. [[Cyan (color)|cyan]] color. It is unlikely that the planet's rings can be seen from its surface, as they are very thin and dark. Uranus has a northern polar star, [[Sabik]] (η Ophiuchi), a magnitude 2.4 star. Uranus also has a southern polar star, [[15 Orionis]], an unremarkable magnitude 4.8 star. Both are fainter than Earth's [[Polaris]] (α Ursae Minoris), although Sabik only slightly.<ref name=bagenal/>
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| <!-- Deleted image removed: [[File:UranusSky-1-.jpg|thumb|right|200px|Simulated view of Uranus's upper cloud decks]] -->
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| Uranus is unusual in that the obliquity of its ecliptic is 82° (angle between the orbital and rotational poles). The North Pole of Uranus<!--- R.A. 17 h 9 min 15 s, Dec. −15° 10' 30"; the orbital pole lies at R.A. 18 h 7 min 23 s, Dec. +66° 20' 26" ---> points to somewhere near [[Sabik|η Ophiuchi]], about 15° northeast of [[Antares]], and its South Pole halfway between [[Betelgeuse]] and [[Aldebaran]]. Uranus's "tropics" lie at 82° latitude and its "Arctic circles" at 8° latitude. On December 17, 2007, the Sun passed the Uranian celestial equator to the North and in 2029 the North Pole of Uranus will be nearly pointed at the Sun.
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| None of Uranus's moons would appear as large as a full moon on Earth from the surface of their parent planet, but the large number of them would present an interesting sight for observers hovering above the cloudtops. The [[angular diameter]]s of the five large moons are as follows (for comparison, Earth's moon measures 31' for terrestrial observers): Miranda, 11–15'; Ariel, 20–23'; Umbriel, 15–17'; Titania, 11–13'; Oberon, 8–9'. Unlike on the other gas giants, many of the inner moons can be seen as disks rather than starlike points; the moons [[Portia (moon)|Portia]] and [[Juliet (moon)|Juliet]] can appear around the size of Miranda at times, and a number of other inner moons appear larger than Oberon. Several others range from 6' to 8'. The outer irregular moons would not be visible to the naked eye.
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| However, the low light levels at such a great distance from the sun ensure that the moons appear very dim; the brightest, Ariel, would only shine at magnitude -7.4, more than 100 times dimmer than the moon as seen from Earth. Meanwhile, the outer large moon Oberon would only be as bright as magnitude -4.9, about the same as Venus despite its proximity.
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| ==Neptune==
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| Judging by the color of its atmosphere, the sky of [[Neptune]] is probably an [[Azure (color)|azure or sky blue]], similar to [[Uranus]]'s. It is unlikely that the planet's rings can be seen from its surface, as they are very thin and dark.
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| Aside from the Sun, the most impressive object in Neptune's sky is its large moon [[Triton (moon)|Triton]], which would appear slightly smaller than a full Moon on Earth. It moves more swiftly than our Moon, because of its shorter period (5.8 days) compounded by its [[retrograde orbit]]. The smaller moon [[Proteus (moon)|Proteus]] would show a disk about half the size of the full Moon. Surprisingly, Neptune's small inner moons all cover, at some point in their orbits, more than 10' in Neptune's sky. At some points, Despina's angular diameter rivals that of Ariel from Uranus and Ganymede from Jupiter. Here are the angular diameters for Neptune's moons (for comparison, Earth's moon measures 31' for terrestrial observers): Naiad, 7–13'; Thalassa, 8–14'; Despina, 14–22'; Galatea, 13–18'; Larissa, 10–14'; Proteus, 12–16'; Triton, 26–28'. An alignment of the inner moons would likely produce a spectacular sight. Neptune's large outer satellite, [[Nereid (moon)|Nereid]], is not large enough to appear as a disk from Neptune, and is not noticeable in the sky, as its brightness at full phase varies from magnitude 2.2-6.4, depending on which point in its eccentric orbit it happens to be. The other irregular outer moons would not be visible to the naked eye, although a dedicated telescopic observer could potentially spot some at full phase.
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| As with Uranus, the low light levels cause the major moons to appear very dim. The brightness of Triton at full phase is only -7.11, which is disappointing considering the fact that Triton is more than 4 times as intrinsically bright as our moon and orbits much closer to Neptune.
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| ===The sky of Triton===
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| [[File:Tritonian sky.jpg|thumb|left|Simulated view of Neptune in the sky of Triton]]
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| [[Triton (moon)|Triton]], Neptune's largest moon, has an atmosphere, but it is so thin that the moon's sky is still black, perhaps with some pale haze at the horizon. Because Triton orbits with [[synchronous rotation]], Neptune always appears in the same position in its sky. Triton's rotation axis is inclined 130° to Neptune's orbital plane and thus points within 40° of the [[Sun]] twice per Neptunian year, much like [[Uranus]]'s. As Neptune orbits the Sun, Triton's polar regions take turns facing the Sun for 82 years at a stretch, resulting in radical seasonal changes as one pole then the other moves into the sunlight.
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| Neptune itself would span 8 degrees in Triton's sky, though with a maximum brightness roughly comparable to that of the full Moon on Earth it would appear only about 1/256th as bright as the full Moon, per unit area. Due to its eccentric orbit, [[Nereid (moon)|Nereid]] would vary considerably in brightness, from fifth to first magnitude; its disk would be far too small to see with the naked eye. [[Proteus (moon)|Proteus]] would also be difficult to resolve at just 5–6 arcminutes across, but it would never be fainter than first magnitude, and at its closest would rival [[Canopus]].
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| {{clear}}
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| ==Pluto and Charon==
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| [[File:Plutonian system.jpg|thumb|230px|Artist's concept of the surface of [[Moons of Pluto|Pluto's small satellite]] [[Hydra (moon)|Hydra]]. [[Pluto]] and [[Charon (moon)|Charon]] are to the right and [[Nix (moon)|Nix]] is the bright dot on the left.]]
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| [[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.
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| From Pluto, the Sun is 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). Nonetheless, human observers would find a large decrease in available light.
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| [[File:ESO-L._Calçada_-_Pluto_(by).jpg|thumb|230px|left|Artist's concept of the view from the Plutonian surface. The bright spot on right-top is the [[Sun]] and the satellite [[Charon (moon)|Charon]] is shown on the left.]]
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| Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide gases, which are derived from the ices of these substances on its surface. When Pluto is closer to the Sun, the temperature of Pluto's solid surface increases, causing the transition of ices directly into gas, an anti-greenhouse effect. This transition cools the surface of Pluto similar to the cooling effect caused on human body due to sweat. The reflection of Sun light on the Plutonian surface(as shown in the image below) is caused because of this phase transition ([[Sublimation (phase transition)|Sublimation]]).<ref>http://en.wikipedia.org/wiki/Pluto</ref>
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| 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, when Pluto and Charon each eclipse the Sun for the other, at intervals of 3.2 days.
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| ==Comets==
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| The sky of a [[comet]] changes dramatically as it nears the Sun. During perihelion, a comet's ices begin to [[Sublimation (phase transition)|sublime]] from its surface, forming tails of gas and dust, and a [[Coma (cometary)|coma]]. An observer on a comet nearing the Sun might see the stars slightly obscured by a milky haze, which could create interesting [[Halo (optical phenomenon)|halo]] effects around the Sun and other bright objects.
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| ==Extrasolar planets==
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| {{OR section|date=April 2013}}
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| For observers on [[extrasolar planet]]s, the [[constellation]]s would be quite different. The Sun would be visible to the [[naked eye|naked human eye]] only at distances below 20–25 [[parsec]]s (65–80 light years). The star [[Beta Comae Berenices|β Comae Berenices]] is slightly more luminous than the Sun, but even over its relatively close distance of 27 light years, appears quite faint in our sky.
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| If the Sun were observed from the [[Alpha Centauri]] system, the nearest [[star system]] to ours, it would appear to be a bright star in the constellation [[Cassiopeia (constellation)|Cassiopeia]]. It would be almost as bright as [[Capella (star)|Capella]] is in our sky. Due to the proximity of the Alpha Centauri system, the constellations would, for the most part, appear similar. However, there are some notable differences; for example, [[Sirius]] would appear about one degree from the star [[Betelgeuse]] in the constellation [[Orion (constellation)|Orion]]. Also, [[Procyon]] would appear in the constellation [[Gemini (constellation)|Gemini]], about 13 degrees below [[Pollux (star)|Pollux]].
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| <!--- To compute the following figures, use masses 1.1 and 0.92 Suns, luminosities 1.57 and 0.51 Suns (Sun visual magnitude -26.73), and an orbit of 11.2 to 35.6 AUs; min luminosity adds the planet's orbital radius to the max A-B distance (conjunction), max luminosity subtracts the planet's orbital radius from the min A-B distance (opposition) --->A planet around either α Centauri A or B would see the other star as a very bright secondary. For example, an Earth-like planet at 1.25 [[astronomical unit]]s from α Cen A (with a revolution period of 1.34 years) would get Sun-like illumination from its primary, and α Cen B would appear 5.7 to 8.6 magnitudes dimmer (−21.0 to −18.2), 190 to 2700 times dimmer than α Cen A but still 2100 to 150 times brighter than the full Moon. Conversely, an Earth-like planet at 0.71 AUs from α Cen B (with a revolution period of 0.63 years) would get Sun-like illumination from its primary, and α Cen A would appear 4.6 to 7.3 magnitudes dimmer (−22.1 to −19.4), 70 to 840 times dimmer than α Cen B but still 5700 to 470 times brighter than the full Moon. In both cases the secondary sun would, in the course of the planet's year, appear to circle the sky. It would start off right beside the primary and end up, half a period later, opposite it in the sky (a "midnight sun"). After another half period, it would complete the cycle. Other planets orbiting one member of a binary system would enjoy similar skies. The angular separation between the two stars would not be exactly the same after one orbit of the planet however, because during that time the parent star will have completed part of its orbit around the other star in the system.
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| From [[40 Eridani]], 16 light years away, the Sun would be an average looking star of about [[apparent magnitude]] 3.3 in the constellation [[Serpens Caput]]. At this distance most of the [[List of nearest stars|stars nearest to us]] would be in different locations than in our sky, including [[Alpha Centauri]] and [[Sirius]].
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| From a planet orbiting [[Aldebaran]], 65 light years away, the Sun would appear slightly above [[Antares]] in our constellation [[Scorpius]], and at magnitude 6.4 would barely be visible to the naked eye. Constellations made of bright, far-away stars would look somewhat similar (such as [[Orion (constellation)|Orion]]), but much of the night sky would seem unfamiliar to someone from Earth. Even Orion would appear somewhat different; viewed from this position, [[Alnilam]] and [[Mintaka]] would appear to be on top of each other, only 15 arcminutes apart, thus reducing the belt to two stars.
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| ===Alpha Centauri Bb===
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| The planet orbiting Alpha Centauri B, [[Alpha Centauri Bb]], orbits just 0.04 AU from its star. Although surface and atmospheric conditions are not yet known, if the planet has negligible axial tilt and its orbit has elliptical characteristics, if an observer could stand on the planet's surface and had a clear atmosphere to observe, the vista would be thus (referencing Alpha Centauri B as the "home sun"):
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| Day side—the home sun (Alpha Centauri B) would dominate the sky. During the 77-hour 39.4 minute year, the home sun would shift from east to west and back, unless the planet's orbit is exactly circular, in which event the home sun would remain fixed.
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| Night side—the home sun (Alpha Centauri B) would never be seen. Alpha Centauri A would rise and be visible for an average, over 80 Earth years, of about 38 hours 50 minutes, then set for 38 hours 50 minutes. Alpha Centauri A's position would shift over the 80 Earth years, so it would be at different positions as the planet accelerates and decelerates along its orbit: if Alpha Centauri A is rising as the planet decelerates, A would spend more than half the planet's year in the sky before setting.
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| Terminator zone—the home sun (Alpha Centauri B) would rise then set, at the same horizon, once in every 77 hours 39.4 minutes. If the planet's orbit is exactly circular, the home sun would remain virtually motionless at the horizon.
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| ==Calculating apparent magnitudes==
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| The brightness of an object varies as the inverse square of the distance. The apparent magnitude scale varies as −2.5 times the (base-10) logarithm of the brightness. Thus if an object has apparent magnitude <math> m_1 </math> at distance <math> d_1 </math> from the observer, then all other things being equal, it will have magnitude <math> m_2 = m_1 - 2.5\log(d_1^2/d_2^2) = m_1 + 5\log(d_2/d_1) </math> at distance <math> d_2 </math>.{{Citation needed|date=September 2007}}
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| ==See also==
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| *[[Large Magellanic Cloud#View from the LMC]]
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| *[[Sky]]
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| ==References==
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| {{reflist}}
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| ==Further reading==
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| {{Refbegin}}
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| *{{cite book|last=Carroll|first=Michael|title=Space art : how to draw and paint planets, moons, and landscapes of alien worlds|year=2007|publisher=Watson-Guptil Publications|location=New York|isbn=9780823048762}}
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| *{{cite book|last=Carroll|first=Michael|title=Drifting on Alien Winds Exploring the Skies and Weather of Other Worlds|year=2010|publisher=Springer Science+Business Media, LLC|location=New York, NY|isbn=9781441969170}}
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| *{{cite journal |first=Lincoln |last=Barnett |title=The starry universe |journal=Life |date=20 December 1954 |pages=44–64 |url=http://books.google.com/books?id=WlMEAAAAMBAJ&pg=PA44#v=onepage&q&f=false |accessdate=12 April 2013}}
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| *{{cite book|last=Perelman|first=Y.|chapter=Lunar heavens |title=Astronomy for entertainment|year=2000|publisher=University Press of the Pacific|location=Honolulu|isbn=9780898750560|pages=78–84}}
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| {{Refend}}
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| ==External links==
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| *[http://www.orionsarm.com/xcms.php?r=oa-page&page=gen_skyonalienworlds Essay on the possible sky colours of alien worlds.]
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| *[http://space.jpl.nasa.gov/ JPL Solar System Simulator]
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| *[http://www.boulder.swri.edu/~buie/pluto/chphases.html Phases of Charon as seen from Pluto]
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| *[http://www.hudsonfla.com/spaceview.htm Astronauts on the planets]
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| {{DEFAULTSORT:Skies, extraterrestrial}}
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| [[Category:Observational astronomy]]
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| [[Category:Planetary science]]
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