Extraterrestrial skies
Encyclopedia
The 'sky' of a world refers to the view of outer space
from its surface. This view varies from world to world for many reasons. The most important factor in the appearance of a world's sky is the world's atmosphere
, or the lack thereof. Depending on the atmosphere
's density and chemical composition, a world's sky may be any number of colors. Clouds may or may not be present and they may also be noticeably colored. Another factor is the astronomical objects that may appear in a world's sky, such as the Sun
, star
s, moon
s, planet
s, and ring
s.
has no atmosphere, a view of the planet's skies would be no different from viewing space from orbit. Mercury has a southern pole star
, α Pictoris
, a magnitude 3.2 star. It is fainter than Earth's Polaris
(α Ursae Minoris).
(with total brightness 4.8 times as great), to 3.2 times at perihelion
(with total brightness 10.2 times as great).
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
transit
s of the Sun) lasts ~176 Earth days.
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 apparent motion ceases; at perihelion, Mercury's orbital angular velocity then exceeds the rotational velocity; thus, the Sun appears to be 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.
, 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 opposition to the Sun and is showing its full disk. The apparent magnitude
of Venus is as bright as −7.7.
The Earth and the Moon are also very prominent, their apparent magnitudes being about −5 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.
The zodiacal light
is probably more prominent than it is from Earth.
is so thick that the Sun is not distinguishable in the daytime sky, and the star
s are invisible at night. Color images taken by the Soviet Venera probes suggest that the sky on Venus is orange-red. 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 rotation
, the Sun would appear to rise in the west and set in the east.
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 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, and because its maximum elongation from the Sun is considerably larger (40.5°) than when observed from Earth (28.3°).
, a magnitude 4.34 star. It is better aligned than Earth's Polaris
(α Ursae Minoris), but much fainter.
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
).
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.
. 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 eclipse
s, 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.
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
).
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, the 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.
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 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
.
as seen from Mars (when near the maximum elongation from the Sun of 31.7°) would have an apparent magnitude of about −3.2.
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.
.
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.
is one of the largest main-belt asteroids and the first asteroid observed to have two moons. These moons, Romulus
and 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°.
'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.
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 magnitude
s 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 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 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.
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 diameter
s 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 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.
, 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.
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 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. 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.
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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.
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 diameter
s 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.
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.
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
, 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.
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.
s, the constellation
s would be quite different. The Sun would be visible to the naked human eye
only at distances below 20–25 parsec
s (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 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.
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.
at distance 
from the observer, then all other things being equal, it will have magnitude 
at distance 
.
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
from its surface. This view varies from world to world for many reasons. The most important factor in the appearance of a world's sky is the world's atmosphere
Extraterrestrial atmospheres
The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmospheres. These include all the gas giants, as well as Mars...
, or the lack thereof. Depending on the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
's density and chemical composition, a world's sky may be any number of colors. Clouds may or may not be present and they may also be noticeably colored. Another factor is the astronomical objects that may appear in a world's sky, such as the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s, moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
s, planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s, and ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
s.
Mercury
Because MercuryMercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
has no atmosphere, a view of the planet's skies would be no different from viewing space from orbit. Mercury has a southern pole star
Pole star
The term "Pole Star" usually refers to Polaris, which is the current northern pole star, also known as the North Star.In general, however, a pole star is a visible star, especially a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent...
, α Pictoris
Alpha Pictoris
Alpha Pictoris is the brightest star in the constellation Pictor with an apparent magnitude of 3.30. It is located about 99 light years from the Sun...
, a magnitude 3.2 star. It is fainter than Earth's Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
(α Ursae Minoris).
The Sun from Mercury
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 aphelionApsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...
(with total brightness 4.8 times as great), to 3.2 times at perihelion
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...
(with total brightness 10.2 times as great).
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
Meridian (astronomy)
This article is about the astronomical concept. For other uses of the word, see Meridian.In the sky, a meridian is an imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the zenith, through the south point on the...
transit
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
s of the Sun) lasts ~176 Earth days.
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 apparent motion ceases; at perihelion, Mercury's orbital angular velocity then exceeds the rotational velocity; thus, the Sun appears to be retrograde
Retrograde motion
Retrograde motion is motion in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such...
. 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.
Other planets seen from Mercury
After the Sun, the second brightest object in the Mercurian sky is VenusVenus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, 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 opposition to the Sun and is showing its full disk. The apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of Venus is as bright as −7.7.
The Earth and the Moon are also very prominent, their apparent magnitudes being about −5 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.
The zodiacal light
Zodiacal light
Zodiacal light is a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the sun along the ecliptic or zodiac. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders...
is probably more prominent than it is from Earth.
Venus
The atmosphere of VenusVenus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
is so thick that the Sun is not distinguishable in the daytime sky, and the star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s are invisible at night. Color images taken by the Soviet Venera probes suggest that the sky on Venus is orange-red. 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 rotation
Retrograde motion
Retrograde motion is motion in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such...
, the Sun would appear to rise in the west and set in the east.
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 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, and because its maximum elongation from the Sun is considerably larger (40.5°) than when observed from Earth (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 DoradusDelta Doradus
δ Doradus is a faint star in the Dorado constellation that has the distinction of being the Moon's south pole star. It is better aligned than Earth's Polaris , but much fainter.-External links:...
, a magnitude 4.34 star. It is better aligned than Earth's Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
(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
Peak of Eternal Light
Peak of Eternal Light describes a point on a body within the Solar System which is eternally bathed in sunlight. This is due to both the bodies' rotation and the point's altitude...
).
The Earth from the Moon
Planetary phase
Planetary phase is the term used to describe the appearance of the illuminated section of a planet. Like lunar phases, the planetary phase depends on the relative position of the sun, the planet and the observer....
, 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
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
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
Synchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
, one side of the Moon (the "near side
Near side of the Moon
The near side of the Moon is the lunar hemisphere that is permanently turned towards the Earth, whereas the opposite side is the far side of the Moon. Only one side of the Moon is visible from Earth because the Moon rotates about its spin axis at the same rate that the Moon orbits the Earth, a...
") 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
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar 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
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...
and sometimes above it.
Eclipses from the Moon
The Earth and the Sun sometimes meet in the lunar sky, causing an eclipseEclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
. On the Earth, one then sees a lunar eclipse
Lunar eclipse
A lunar eclipse occurs when the Moon passes behind the Earth so that the Earth blocks the Sun's rays from striking the Moon. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can only occur the night of a...
, 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
Solar eclipse
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...
. 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
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
would be visible as a reddish ring. An attempt was made to use the Apollo 15
Apollo 15
Apollo 15 was the ninth manned mission in the American Apollo space program, the fourth to land on the Moon and the eighth successful manned mission. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous...
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 eclipse
Solar eclipse
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...
s, 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
MarsMars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
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
Deneb
Deneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...
(although the actual pole is somewhat offset in the direction of Alpha Cephei
Alpha Cephei
Alpha Cephei is a second magnitude star in the constellation of Cepheus that is relatively close to Earth at only 49 light years...
).
The color of the Martian sky
Purkinje effect
The Purkinje effect is the tendency for the peak luminance sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.This effect introduces a difference in color contrast under different levels of...
: 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
Scarlet (color)
Scarlet is a bright red color with a hue that is somewhat toward the orange. It is redder than vermilion. It is a pure chroma on the color wheel one-fourth of the way between red and orange. Scarlet is sometimes used as the color of flame...
or bright orangeish-red color. Around sunset and sunrise, the sky is rose
Rose (color)
Rose is the color halfway between red and magenta on the HSV color wheel, also known as the RGB color wheel, on which it is at hue angle of 330 degrees.Rose is one of the tertiary colors on the HSV color wheel...
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
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
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 SunSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
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 (moon)
Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...
and Deimos
Deimos (moon)
Deimos is the smaller and outer of Mars's two moons . It is named after Deimos, a figure representing dread in Greek Mythology. Its systematic designation is '.-Discovery:Deimos was discovered by Asaph Hall, Sr...
. From the Martian surface, Phobos has one-third to one-half the angular diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
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
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...
(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
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
the Sun as seen from Mars, although neither can completely cover its disk and so the event is in fact a transit
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
, rather than an eclipse. For a detailed description of such events see the articles Transit of Phobos from Mars
Transit of Phobos from Mars
A transit of Phobos across the Sun as seen from Mars takes place when Phobos passes directly between the Sun and a point on the surface of Mars, obscuring a large part of the Sun's disc for an observer on Mars. During a transit, Phobos can be seen from Mars as a large black disc rapidly moving...
and Transit of Deimos from Mars
Transit of Deimos from Mars
A transit of Deimos across the Sun as seen from Mars takes place when Deimos passes directly between the Sun and a point on the surface of Mars, obscuring a small part of the Sun's disc for an observer on 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
VenusVenus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
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 Odyssey2001: A Space Odyssey (film)
2001: A Space Odyssey is a 1968 epic science fiction film produced and directed by Stanley Kubrick, and co-written by Kubrick and Arthur C. Clarke, partially inspired by Clarke's short story The Sentinel...
.
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
4179 Toutatis
4179 Toutatis/1989 AC is an Apollo, Alinda, and Mars-crosser asteroid with a chaotic orbit produced by a 3:1 resonance with the planet Jupiter, a 1:4 resonance with the planet Earth, and frequent close approaches to the terrestrial planets...
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
Extinct comet
Extinct comets are comets that have expelled most of their volatile ice and have little left to form a tail or coma. The volatile material contained in the comet nucleus evaporates away, and all that remains is inert rock or rubble that can resemble an asteroid. Comets may go through a transition...
) 3200 Phaethon
3200 Phaethon
3200 Phaethon is an asteroid with an unusual orbit that brings it closer to the Sun than any other named asteroid . For this reason, it was named after the Greek myth of Phaëton, son of the sun god Helios...
has one of the most eccentric orbits; its distance from the Sun varies between 0.14 and 2.4 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
. 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 and its moons Romulus and Remus
The asteroid 87 Sylvia87 Sylvia
87 Sylvia is one of the largest main-belt asteroids. It is a member of the Cybele group located beyond the core of the belt . Sylvia is remarkable for being the first asteroid known to possess more than one moon....
is one of the largest main-belt asteroids and the first asteroid observed to have two moons. These moons, Romulus
Romulus (moon)
Romulus is the outer and larger moon of the main-belt asteroid 87 Sylvia, not to be confused with the directly Sun-orbiting asteroid 10386 Romulus. It follows an almost-circular close-to-equatorial orbit around the parent asteroid. In this respect it is similar to the other moon Remus.Romulus was...
and Remus
Remus (moon)
Remus is the inner and smaller moon of the main-belt asteroid 87 Sylvia. It follows an almost-circular close-to-equatorial orbit around the parent asteroid. In this respect it is similar to the other moon Romulus....
, 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°.
Jupiter
Although no images from within JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
'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
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
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
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
, the nearest to the planet, would be slightly larger than the full Moon in Earth's sky, though less bright. The higher albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
of Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
would not overcome its greater distance from Jupiter, so it would not outshine Io. In fact, the low solar constant
Solar constant
The solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit...
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
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
s would be: −11.2, −9.7, −9.4, and −7.0.
Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...
, the largest moon and third from Jupiter, is almost as bright as Io and Europa, but appears only half the size of Io. Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...
, 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
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
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 IoIo (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
, 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
Metis (moon)
Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, 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
Synchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
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 eclipse
Solar eclipse
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...
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.
Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.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
Rings of Saturn
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about 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
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
, the largest moon of Saturn, appears only half the size of Earth's moon. Here are the approximate angular diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
s 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
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
(α Ursae Minoris).
The skies of Saturn's moons
Since the inner moons of Saturn are all in synchronous rotationSynchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
, 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
Pan (moon)
Pan is the innermost moon of Saturn. It is a walnut-shaped small moon about 35 kilometres across and 23 km high that orbits within the Encke Gap in Saturn's A Ring. Pan acts as a ring shepherd and is responsible for keeping the Encke Gap free of ring particles.It was discovered by Mark R...
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 IapetusIapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...
, a more oblique view of the rings would be available, although the greater distance would make Saturn appear smaller in the sky; from Phoebe
Phoebe (moon)
Phoebe is an irregular satellite of Saturn. It was discovered by William Henry Pickering on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart...
, 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
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....
, 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
Epimetheus (moon)
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus...
and Janus
Janus (moon)
Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus...
would also get a good view, with maximum opening angles ranging between 1.5 and 2.9°. Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...
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.
The sky of Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
is the only moon in the solar system to have a thick atmosphere. Images from the Huygens probe
Huygens probe
The Huygens probe was an atmospheric entry probe carried to Saturn's moon Titan as part of the Cassini–Huygens mission. The probe was supplied by the European Space Agency and named after the Dutch 17th century astronomer Christiaan Huygens....
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 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. With its thick atmosphere and methane rain, Titan is the only celestial body other than Earth upon which rainbow
Rainbow
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
s could form. However, given the extreme opacity of the atmosphere in visible light, the vast majority would be in the infrared.
The sky of Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...
, 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
Lunar phase
A lunar phase or phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun...
. 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
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....
(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
Pallene (moon)
Pallene is a very small natural satellite of Saturn. It is one of three small moons known as the Alkyonides that lie between the orbits of the larger Mimas and Enceladus. It is also designated as '.-Discovery:...
and Methone
Methone (moon)
Methone is a very small natural satellite of Saturn lying between the orbits of Mimas and Enceladus.It was first seen by the Cassini Imaging Team and given the temporary designation '. Methone is also named '....
would appear nearly star-like (maximum 30 seconds of arc). Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...
, 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 UranusUranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
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
15 Orionis
15 Orionis has the distinction of being the southern pole star of Uranus.It is an early F-type subgiant located 318 ly from the solar system....
, an unremarkable magnitude 4.8 star. Both are fainter than Earth's Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
(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
Antares
Antares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...
and its South Pole halfway between Betelgeuse
Betelgeuse
Betelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...
and Aldebaran
Aldebaran
Aldebaran is a red giant star located about 65 light years away in the zodiac constellation of Taurus. With an average apparent magnitude of 0.87 it is the brightest star in the constellation and is one of the brightest stars in the nighttime sky...
. 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 diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
s 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 NeptuneNeptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
is likely an azure or sky blue
Azure (color)
The color bleu de France is displayed at right.Bleu de France is a color that has been associated in heraldry with the Kings of France since the 12th century.-Brandeis blue:...
, similar to Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
'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
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
, 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
Proteus (moon)
Proteus , also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology...
would show a disk about half the size of the full Moon. Neptune's small inner moons, and its large outer satellite, Nereid
Nereid (moon)
Nereid , also known as Neptune II, is the third-largest moon of Neptune. It has a highly eccentric orbit. Nereid was discovered by Gerard Kuiper in 1949 and was the second moon of Neptune to be discovered.- Discovery and naming :...
, would appear as starlike points, and its irregular outer satellites would not be visible to the naked eye.
The sky of Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
, 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
Synchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
, 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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
twice per Neptunian year, much like Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
'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
Nereid (moon)
Nereid , also known as Neptune II, is the third-largest moon of Neptune. It has a highly eccentric orbit. Nereid was discovered by Gerard Kuiper in 1949 and was the second moon of Neptune to be discovered.- Discovery and naming :...
would vary considerably in brightness, from fifth to first magnitude; its disk would be far too small to see with the naked eye. Proteus
Proteus (moon)
Proteus , also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology...
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
Canopus
Canopus |Alpha]] Carinae) is the brightest star in the southern constellation of Carina and Argo Navis, and the second brightest star in the night-time sky, after Sirius. Canopus's visual magnitude is −0.72, and it has an absolute magnitude of −5.53.Canopus is a supergiant of spectral...
.
Pluto and Charon
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
, accompanied by its largest moon Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
, orbits the Sun at a distance usually outside the orbit of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
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
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
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 cometComet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the 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
Coma (cometary)
frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...
. An observer on a comet nearing the Sun might see the stars slightly obscured by a milky haze, which could create interesting halo
Halo (optical phenomenon)
A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...
effects around the Sun and other bright objects.
Extrasolar planets
For observers on extrasolar planetExtrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
s, the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
s would be quite different. The Sun would be visible to the naked human eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
only at distances below 20–25 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s (65–80 light years). The star β Comae Berenices
Coma Berenices
Coma Berenices is a traditional asterism that has since been defined as one of the 88 modern constellations. It is located near Leo, to which it formerly belonged, and accommodates the North Galactic Pole...
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
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...
system, the nearest star system
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...
to ours, it would appear to be a bright star in the constellation Cassiopeia
Cassiopeia (constellation)
Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...
. It would be almost as bright as Capella
Capella (star)
Capella is the brightest star in the constellation Auriga, the sixth brightest star in the night sky and the third brightest star in the northern celestial hemisphere, after Arcturus and Vega. Although it appears to be a single star to the naked eye, it is actually a star system of four stars in...
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 unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
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.
From 40 Eridani
40 Eridani
40 Eridani is a triple star system less than 16.5 light years away from Earth. It is in the constellation Eridanus. The primary star of the system, 40 Eridani A, is easily visible to the naked eye...
, 16 light years away, the Sun would be an average looking star of about apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
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
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...
and Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
.
From a planet orbiting Aldebaran
Aldebaran
Aldebaran is a red giant star located about 65 light years away in the zodiac constellation of Taurus. With an average apparent magnitude of 0.87 it is the brightest star in the constellation and is one of the brightest stars in the nighttime sky...
, 65 light years away, the Sun would appear slightly above Antares
Antares
Antares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...
of our constellation Scorpius
Scorpius
Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...
, 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
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...
), 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 at distance from the observer, then all other things being equal, it will have magnitude at distance .See also
- SkySkyThe sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...
- Large Magellanic Cloud: View from the LMC