Utente:Bukkia/sandbox V

Con il termine cielo di un pianeta ci si riferisce alla vista dello spazio esterno dalla sua superficie. Queta vista varia di pianeta in pianeta per svariate ragioni. Il fattore più importante riguardo l'apparenza del cielo di un dato pianeta è la presenza o la mancanza di un'atmosfera. A seconda della densità e della composizione chimica dell'atmosfera il cielo può assumere diversi tipi di colore. Possono essere presenti o assenti le nubi e anche queste possono assumere diverse colorazioni. Un altro fattore sono gli oggetti astronomici che possono apparire nel cielo, quali il Sole, le stelle, i satelliti naturali, i pianeti, e gli anelli.

Mercurio

Dato che Mercurio non possiede un'atmosfera la vista del cielo del pianeta non dovrebbe essere differente da quella in orbita. Mercurio ha una stella polare meridionale, α Pictoris, una stella di magnitudine 3,2. È più debole della stella polare terrestre (α Ursae Minoris).

Il Sole da Mercurio

In media il diametro visibile del Sole da Mercurio è 2,5 volte più largo di come appare dalla Terra, ed è 6 volte più luminoso. A causa dell'orbita eccentrica del pianeta, la dimensione apparente del Sole nel cielo dovrebbe variare da 2,2 volte più grande della dimensione apparente sulla Terra all'afelio (e 4,8 più luminoso) a 3,2 volte più grande al perielio (e 10,2 volte più luminoso).

Mercurio presenta una risonanza orbitale 3:2 con il Sole. Ciò significa che nonostante un giorno siderale (il periodo di rotazione) duri all'incirca ~58.7 giorni terrestri, un giorno solare (il lasso di tempo tra due transiti del Sole attraverso il meridiano) è di circa ~176 giorni terrestri.

La risonanza orbitale di Mercurio genera un effetto inconsueto nel quale sembra che il Sole inverta brevemente il suo moto consueto da est verso ovest una volta durante l'anno mercuriano. L'effetto è visibile da ogni punto di Mercurio, ma ci sono alcuni punti della superficie di Mercurio dove un osservatore sarebbe in grado di vedere il Sole sorgere a metà, fermarsi, invertire il proprio moto, tramontare e poi sorgere di nuovo, tutto durante lo stesso giorno mercuriano. Ciò succede perchè all'incirca quattro giorni prima del perielio, la velocità angolare della rivoluzione del pianeta diventa uguale a quella di rotazione, così che il moto apparente del Sole si interrompe. Durante il perielio, la velocità orbitale di Mercurio supera la velocità di rotazione; perciò il Sole sembra avere un moto retrogrado. Quattro giorni dopo il perielio, il moto apparente normale del Sole riprende il suo corso. A causa della risonanza orbitale, Mercurio presenta due punti della superficie che alternativamente hanno il Sole al proprio zenit al perielio; uno di questi punti subsolari è la Caloris Planitia ("Piana del Calore"), chiamata in modo appropriato perché un osservatore al suo centro potrebbe vedere il Sole fare un giro completo intorno allo zenit ogni giorno mercuriano, con un conseguente aumento del calore.

Altri pianeti osservati da Mercurio

Dopo il sole il secondo oggetto più luminoso del cielo di Mercurio è Venere, che appare molto più luminoso che nel cielo terrestre. La ragione di ciò sta nel fatt che quando Venere è nel punto più vicino alla Terra, si trova tra la Terra stessa ed il Sole e perciò ne vediamo il lato notturno non illuminato. Infatti anche nel picco di luminosità di Venere nel cielo terrestre si può vedere solo una falce. Per un osservatore da Mercurio, d'altra parte, Venere si ritrova più vicino quando è in opposizione al sole e mostra l'intero disco. La magnitudine apparente di Venere in quel momento è di −7,7.

Inoltre la Terra e la Luna sono molto evidenti, con una magnitudine apparente di −5 e di −1,2 rispettivamente. La massima distanza apparente tra la Terra e la Luna è di circa 15′. Gli altri pianeti rimangono visibili come dalla Terra, tuttavia appena meno luminosi all'opposizione.

La luce zodiacale è probabilmente più evidente di quella terrestre.

Venere

L'atmosfera di Venere è così spessa che il sole non risulta distinguibile nel cielo diurno, e le stelle sono invisibili di notte. Le immagini a colori riprese dalle sonde sovietiche Venera suggeriscono che il cielo diurno venusiano abbia un colore arancione o rossastro. Se fosse possibile osservare direttamente il Sole dalla superficie venusiana, il lasso di tempo tra un'alba ed un'altra (un giorno solare) sarebbe di 116.75 giorni terrestri. A causa della rotazione retrogada, il Sole sorgerebbe ad ovest e tramonterebbe ad est.

Un osservatore in volo al di sopra delle nubi di Venere, invece, girerebbe intorno al pianeta in circa quattro giorni e osserverebbe un cielo in cui la Terra e la Luna splenderebbero in modo molto luminoso (con magnitudini di circa −6.6 e −2.7 rispettivamente) dato che l'avvicinamento massimo avviene in opposizione. Anche Mercurio sarebbe più facile da osservare, dato che è più vicino e brillanete fino ad una magnitudine di −2.7, e perchè la sua elongazione massima rispetto al Sole è molto maggiore (40.5°) che di quella osservata dalla Terra (28.3°).

The Moon

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, δ Doradus, a magnitude 4.34 star. It is better aligned than Earth's Polaris (α Ursae Minoris), but much fainter.

The Sun from the Moon

The Sun looks the same from the Moon as it does from Earth orbit, somewhat brighter than it does from the Earth's surface, and colored pure white, due to the lack of atmospheric scattering and absorption.

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 mountains and hilltops that are never in shadow (see peak of eternal light).

The Earth from the Moon

Apollo 17 commander Eugene Cernan on the Moon, with Earth visible in the sky

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 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.

As a result of the Moon's synchronous rotation, one side of the Moon (the "near side") is permanently turned towards Earth, and the other side, the "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.

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.

Eclipses from the Moon

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 TV camera to view such an eclipse, but the camera or its power source failed after the astronauts left for Earth.

Terrestrial solar eclipses, on the other hand, would not be spectacular for lunar observers, because the Moon's shadow nearly tapers out at the Earth's surface. Lunar observers with telescopes might simply see a small darkened spot travel across the full Earth's disk.

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.

Mars

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 (although the actual pole is somewhat offset in the direction of Alpha Cephei).

The color of the Martian sky

Il cielo di Marte colorato di viola dalle nubi di ghiaccio d'acqua

Il cielo marziano a mezzogiorno, fotografia del Mars Pathfinder

Il cielo marziano al tramonto, fotografia del Mars Pathfinder

Dettaglio del cielo al tramonto, con una maggiore variazione dei colori, fotografia del Mars Pathfinder

Generating accurate true-color images from Mars' surface is surprisingly complicated. 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.

It is now known that during the Martian day, the sky is a scarlet, or bright orangeish-red color. Around sunset and sunrise, sky is rose in colour, 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. Twilight lasts a long time after the Sun has set and before it rises, because of the dust high in Mars' atmosphere.

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.

The Sun from Mars

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.

Mars' moons as seen from Mars

Phobos transits the Sun, as seen by Mars Rover Opportunity on March 10, 2004

Mars has two small moons: Phobos and Deimos. From the Martian surface, Phobos has one-third to one-half the angular diameter of the Sun, but Deimos is barely more than a dot (only 2' angular diameter). The apparent motion of Phobos is in reverse, due to its fast orbital motion: 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' apparent size, the additional distance being non-negligible. Phobos' apparent size varies by up to 45% as it passes overhead, due to its proximity to Mars' 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.

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' 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. Thus Phobos crosses the Martian skies nearly 12 times whilst Deimos crosses them just once.

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 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.

Earth from Mars

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 inferior conjunction of the Earth and the Sun (for the terrestrial observer, this is the opposition of Mars and the Sun). Near maximum elongation (47.4°), the Earth and Moon would shine at apparent magnitudes −2.5 and +0.9, respectively.

Venus from Mars

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.

The skies of Mars' moons

From Phobos, Mars appears 6,400 times larger and 2,500 times brighter than the full Moon as seen from Earth, taking up a quarter of the width of a celestial hemisphere.

From Deimos, Mars appears 1,000 times larger and 400 times brighter than the full Moon as seen from Earth, taking up an eleventh of the width of a celestial hemisphere.

Asteroids

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.

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 that the Moon appears from Earth. The Moon would also be easily visible as a small shape in Toutatis' sky at that time.

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 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.

87 Sylvia ed i suoi satelliti Romulus e Remus

L'asteroide 87 Sylvia è uno degli asteroidi più grandi della fascia principale, ed il primo asteroide di cui si conoscano due satelliti. Dalla superficie di Sylvia, i suoi due satelliti, Romulus e Remus, avrebbero approssimativamente la stessa dimensione. Romulus, la luna più esterna, avrebbe un diametro apparente di circa 0,89°, poco più grande di Remus (più vicino ma anche più piccolo) che avrebbe un diametro di circa 0,78°. Poiché Sylvia non ha una forma sferica, questi valori possono variare di poco più del 10%, a seconda di dove si trovi l'osservatore sulla superficie del corpo principale. Poiché le due lune asteroidali sembrano orbitare sullo stesso piano (secondo quanto noto finora), si occulterebbero a vicenda ogni 2,2 giorni. Quando la stagione è giusta, per due volte durante il periodo orbitale Sylvia (6,52 anni), eclisserebbero il Sole, il quale, con un diametro angolare di 0,15°, è molto più piccolo di quando è osservato dalla Terra (0,53°). Visto da Remus, la luna interna, Sylvia appare enorme, grande approssimativamente 30°×18°, mentre la visuale di Romulus varia da 1,59 a 0,50° di diametro. Visto da Romulus, Sylvia ha un diametro pari a 16°×10°, mentre quello di Remus varia fra 0,62° e 0,19°..

Jupiter

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 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 clear answer.

From Jupiter, the Sun appears to cover only 5 arc minutes, less than a quarter of its size as seen from Earth.

Jupiter's moons as seen from Jupiter

Simulated view of Io, Europa, and the rings of Jupiter seen from their parent planet

Aside from the Sun, the most prominent objects in Jupiter's sky are the four Galilean moons. Io, the nearest to the planet, would be slightly larger than the full Moon in Earth's sky, though less bright. The higher albedo of 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 magnitudes would be: −11.2, −9.7, −9.4, and −7.0.

Ganymede, the largest moon and third from Jupiter, is almost as bright as Io and Europa, but appears only half the size of Io. Callisto, still further out, is only a quarter the size of the full Moon. All four Galilean moons also 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.

Jupiter's small inner moons appear only as starlike points, and most of the outer moons would be invisible to the naked eye.

The skies of Jupiter's moons

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 would, of course, be Jupiter. For an observer on 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, the innermost moon, would see Jupiter's apparent diameter increased to 68° (130 times the visible diameter of our Moon, covering 18% of Metis' 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).

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.

From the moons of Jupiter, solar eclipses 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.

Saturn

Simulated view of Saturn's rings seen from its equator

Simulated view of Saturn's rings seen from a latitude above its equator

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. 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.

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, the largest moon of Saturn, appears only half the size of Earth's moon. Here are the approximate angular diameters of the main moons (for comparison, Earth's moon has an angular diameter of 31'): Mimas: 5–10', Enceladus: 5–9', Tethys: 8–12', Dione: 8–12', Rhea: 8–11', Titan: 14–15', Iapetus: 1'.

Saturn has a southern polar star, δ Octantis, a magnitude 4.3 star. It is much fainter than Earth's Polaris (α Ursae Minoris).

The skies of Saturn's moons

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.

In the skies of Saturn's inner moons, Saturn is an enormous object. For instance, Saturn seen from 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.

The rings from Saturn's moons

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, a more oblique view of the rings would be available, although the greater distance would make Saturn appear smaller in the sky; from 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, 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 and Janus would also get a good view, with maximum opening angles ranging between 1.5 and 2.9°. 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.

Il cielo di Titano

Immagine della superficie di Titano scattata dalla sonda Huygens

Titano è l'unico satellite del sistema solare ad avere un'atmosfera densa. Le immagini scattate della sonda Huygens mostrano che il cielo di Titano è di un colore arancio mandarino chiaro. Ad ogni modo un astronauta che si trovasse sulla superficie di Titano vedrebbe un cielo velato di foschia tra il marrone e l'arancione scuro. Titano riceve 1/3000 della luce solare che raggiunge la Terra, in modo che la densa atmosfera e la maggiore distanza dal Sole rendono il giorno su Titano luminoso come il tramonto sulla Terra. È probabile che Saturno rimanga permanentemente invisibile, nascosto dallo smog arancione e perfino il Sole risulterebbe solo una macchia più chiara nella foschia, illuminando a malapena la superficie di ghiacchi e di laghi di metano. Ad ogni modo nell'alta atmosfera il cielo dovrebbe avere un colore azzurro e Saturno dovrebbe essere visibile.

The sky of Enceladus

An artist's view of Enceladus' sky.

Seen from 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' sky (albeit with slight variations coming from the orbit's eccentricity), and cannot be seen from the far side of the satellite.

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 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.

An observer located on Enceladus could also observe Mimas (the biggest satellite located inside Enceladus' 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 and Methone would appear nearly star-like (maximum 30 seconds of arc). Tethys, visible from Enceladus' anti-Saturnian side, would reach a maximum apparent size of about 64 minutes of arc, about twice the Moon as seen from the Earth.

Uranus

Judging by the colour of its atmosphere, the sky of Uranus is probably a light blue, i.e. cyan color. It is probable that the planet's rings can't 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 is only slightly fainter.

Uranus is unusual in that the obliquity of its ecliptic is 82° (angle between the orbital and rotational poles). The North Pole of Uranus points to somewhere near 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.

Uranus's moons would not look very large from the surface of their parent planet. The angular diameters of the five large moons are as follows (for comparison, Earth's moon measures 31' for terrestrial observers): Miranda, 11–15'; Ariel, 18–22'; Umbriel, 14–16'; Titania, 11–13'; Oberon, 8–9'. The small inner moons would appear as starlike points, and the outer irregular moons would not be visible to the naked eye.

Neptune

Judging by the color of its atmosphere, the sky of Neptune is likely an azure or sky blue, similar to Uranus's. It is probable that the planet's rings can't be seen from its surface, as they are very thin and dark.

Aside from the Sun, the most impressive object in Neptune's sky is its large 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 would show a disk about half the size of the full Moon. Neptune's small inner moons, and its large outer satellite, Nereid, would appear as starlike points, and its irregular outer satellites would not be visible to the naked eye.

The sky of Triton

Simulated view of Neptune in the sky of Triton

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.

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 would vary considerably in brightness, from fifth to first magnitude; its disk would be far too small to see with the naked eye. 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.

Pluto and Charon

Artist's concept of the surface of Pluto's small satellite Hydra. Pluto & Charon (right) & Nix (bright dot on left).

Pluto, accompanied by its largest 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 still very bright, giving roughly 150 to 450 times the light of the full Moon from Earth (the variability being due to the eccentricity of Pluto's orbit). Nonetheless, human observers would find a large decrease in available light.

Pluto and Charon are 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.

Comets

The sky of a comet changes dramatically as it nears the Sun. During perihelion, a comet's ices begin to sublime from its surface, forming tails of gas and dust, and a coma. An observer on a comet nearing the Sun might see the stars slightly obscured by a milky haze, which could create interesting halo effects around the Sun and other bright objects.

Extrasolar planets

For observers on extrasolar planets, the constellations would be quite different. The Sun would be visible to the naked human eye only at distances below 20–25 parsecs (65–80 light years). The star 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.

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. It would be almost as bright as Capella is in our sky.

A hypothetical 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 units 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.

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 stars nearest to us would be in different locations than in our sky, including Alpha Centauri and Sirius.

From a planet orbiting Aldebaran, 65 light years away, the Sun would appear slightly above Antares of 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 very similar (such as Orion), but much of the night sky would seem unfamiliar to someone from Earth.

A note on calculating apparent magnitudes

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) log of the brightness. Thus if an object has apparent magnitude $m_{1}$ at distance $d_{1}$ from the observer, then all other things being equal, it will have magnitude $m_{2}=m_{1}-2.5log(d_{1}^{2}/d_{2}^{2})=m_{1}+5log(d_{2}/d_{1})$ at distance $d_{2}$.

Collegamenti esterni

• 3D Universe - The Universe as seen from other places and other times.
• Essay on the possible sky colours of alien worlds.
• JPL Solar System Simulator
• Phases of Charon as seen from Pluto
• Astronauts on the planets