Appearance of the skies of other planets
The sky is generally referred to as the view that is presented to a viewer from the surface of a celestial body, in the original sense of the earth, of space . In the case of an extraterrestrial view of the sky , this differs from the earthly view for several reasons.
The most important factor in the appearance of a world's sky is its atmosphere . Depending on the density of the atmosphere , its transparency and the chemical composition , the sky can be represented in a different color spectrum. When the world is surrounded by clouds , they can appear in a variety of colors. However, many celestial objects have no or only a very thin atmosphere, which means that an observer has a clear view of space from there. In addition, it is particularly striking astronomical objects that determine the sky of a world, such as the sun , orbiting moons , neighboring planets or possibly existing planetary rings .
Extraterrestrial celestial views are a field of activity of astronomical phenomenology that is of interest to you through space travel , encompass the same problems and are determined celestial mechanically and with regard to atmospheric optics exactly as topocentric cases. Otherwise they are a central subject of Space Art , because they have so far hardly been accessible in real terms , also as popular scientific illustrations .
The planet Mercury , the closest planet to the Sun , has no atmosphere, so its sky is black and no different from the view of space offered by Earth's orbit. In principle, one could also see stars and planets on Mercury during the day. However, the nearby sun outshines its light very strongly, so that an observation would only be possible if the view of the sun and the glistening bright planetary surface were screened off.
On Mercury the apparent diameter of the sun is on average two and a half times larger than on Earth , whereby its brightness here is more than six times the value. Due to the eccentric orbit of the planet, the apparent size and brightness that the sun offers in the Mercury sky changes along its orbit. The sun's size in aphelion , the point furthest from the sun, is 2.2 times that of a comparable view of the earth, whereby its brightness here is 4.8 times, whereas in perihelion near the sun it increases to 3.2 times the diameter of the view of the earth and its brightness value is about 10.2 times larger.
Mercury orbits the sun in just 88 days with an orbital resonance of 3: 2. In other words, this means that the planet rotates three times around itself while it orbits the sun twice in the same direction. On Mercury a day related to the fixed stars - i.e. the time for a full rotation - lasts around 58.7 earth days, while Mercury day related to the sun is around 176 earth days as the time between two meridian passages of the sun.
A day of Mercury is therefore longer than the year of Mercury between the perihelion passages and lasts almost exactly twice as long as a full cycle. This leads to an unusual effect in which it appears as if the sun briefly reverses its usual east-west movement once within a Mercury year. This phenomenon can be observed with varying degrees of clarity, depending on where you are currently on Mercury. During this phase, an observer can see the sun rise at certain points on the planet's surface , where it only rises to about the middle of the horizon, here reversing its course until it finally sets again , before it again, still on the same day of Mercury can be seen ascending. This effect is due to the fact that from about four earth days before perihelion, the angular speed of Mercury's orbit is exactly the same as its rotation speed , so that the sun no longer seems to continue its apparent movement ; at perihelion orbital angular velocity of Mercury exceeds again its rotational speed, causing the sun apparently declined moved. Four earth days after perihelion, the sun will then continue its normal movement. Because of its orbital resonance, two points on the surface of Mercury in the perihelion are alternately facing the sun; one of these two points is the caloris planitia ("heat basin"). This plane was so named because an observer near its center sees the sun draw a loop around the zenith of Mercury once every day of Mercury and thus experiences a very hot day indeed.
View of other planets
After the sun, Venus is the second brightest object in the Mercury sky. Venus appears much brighter than to an observer on Earth. This is due to the fact that when Venus is closest to the earth, it stands in front of the sun and therefore an earth observer can only see its night side. Although Venus is the brightest object in the sky on earth (after the sun and moon), it is in fact the case that nothing more than a narrow sickle can be seen of it.
The earth and its moon are also very concise objects in the sky of the planet closest to the sun, as their apparent brightness values are approximately -5 m and −1.2 m , with the apparent distance between the earth and the moon being a maximum of about 15 arc minutes . All other planets achieve roughly the same visibility as from Earth, although they appear a little less bright in opposition. In contrast, the zodiacal light is likely to be more prominent than it is on earth.
The atmosphere of Venus has a completely closed, about 20 km thick cloud cover made of sulfuric acid and other components. As a result, their daytime sky is relatively dark despite their relative proximity to the sun, comparable to the brightness of an overcast day on earth. The atmosphere is extremely dense and hot on the surface. The Venera 8 lander of the Soviet Venera mission measured a visibility of about one kilometer.
The sun cannot be seen on Venus as a definable disk, and the stars remain completely invisible even at night. Color images from the Venera probes suggest that the sky on Venus appears orange-red and, according to another source, yellow-green. If the sun could be seen from the surface of Venus, the time from one sunrise to the next ( i.e. a solar day ) would be a total of 116.75 earth days. Since the rotation of Venus is retrograde, i.e. against the direction of rotation of the earth, the sun rises in the west and sets in the east.
The high atmosphere of Venus rotates differentially and near the equator around 60 times faster than Venus itself. An observer whose position is high above the clouds of Venus is rotated around the planet in just under four days and sees a sky in which , next to the sun, the earth and the earth's moon appear clear and bright (their brightness values are −6.6 m and −2.7 m ), especially in opposition when both planets are at a 180 ° angle to the sun and thus the To reach the point of maximum approximation. Mercury is also quite easy to discover in the Venusian sky because it is closer to the planet and is therefore up to a magnitude of −2.7 m brighter than it appears from Earth. In addition, its maximum angular distance to the sun is much larger here (40.5 °) than from an earthly observation point (28.3 °).
The earth moon
Since the moon , like Mercury, has no atmosphere, its sky is black throughout. However, even from the Earth's moon, the sun still appears so bright that it is impossible to see stars during the day unless the observer shields his view from both the glaring direct view of the sun and the dazzling bright surface. The moon has a southern pole star, δ Doradus, which has an apparent magnitude of 4.34 m . This is even better aligned than the Earth's pole star, but appears much weaker.
The sun disk is the same size on the moon as it is on earth. Because of the lack of atmosphere and the resulting lack of atmospheric scattering and absorption, however, it appears somewhat lighter and in a purer white. Apart from that, the solar radiation on the moon is identical to the radiation that can be measured in earth orbit.
A lunar day, i.e. the time from one highest point of the sun to the next, lasts a synodic month , about 29.5 days. Since the axis inclination of the moon is almost zero relative to its orbit around the sun, the sun moves almost always in the same orbit across the sky over the course of a year. As a result, the craters and gorges near the poles of the satellite never receive direct sunlight and on the other side there are some mountains and lunar craters whose peaks or crater edges are never in the shadow. (→ see Mountains of Eternal Light ).
One of the most outstanding features of the lunar sky is the view of the earth . Its visible diameter (1.9 °) is about four times as large as the diameter of the moon in the earth's firmament. Since the lunar orbit is eccentric, the apparent size of the earth in the sky fluctuates by about 5% (between 1.8 ° and 2.0 ° in diameter). The earth shows phases like the moon, but they are chronologically opposite to the moon phases : So when you see the full moon from the earth, the earth is in its dark phase for the moon observer and vice versa. The albedo of the earth is three times as high as that of the moon and together with the larger area that the full earth occupies in the lunar sky, it appears at the zenith over 50 times brighter than the full moon to the earthly observer.
As a result of the bound rotation that the moon has with respect to the earth, one side of the moon is constantly turned towards the earth, while the back of the moon is never visible from the earth. Conversely, this means that the earth can only be seen from this one side of the moon, while it always remains invisible on the other side.
If the rotation of the moon were absolutely synchronous, the earth would not make any noticeable movement in the moon's sky. In fact, when viewed from the moon, the earth performs a slow and complex tumbling motion ( libration ). Over a month, the earth describes an approximately oval movement with a diameter of 18 °. The exact shape and orientation of this oval depends on its location on the moon. For this reason, near the border region between the front and back of the moon, the earth is sometimes just below the horizon and sometimes just above it.
Solar and earth eclipses
From time to time the earth, sun and moon are on a direct line of sight, which means that a solar eclipse or an “ earth eclipse ” can then be experienced in the moon sky . During the solar eclipse on the moon, the sun is obscured by the earth. At the same time, a lunar eclipse can be observed on Earth , whereby the eclipse not only takes place in a narrow strip, but often over the entire front of the moon.
The course of the solar eclipse with its 4 contacts can be seen from the moon in the same way as from the earth. Since the apparent diameter of the earth is four times larger than that of the sun, the phase of the total solar eclipse can last over 2 hours, and the earth's atmosphere appears as a reddish ring during this phase. It was originally planned to photograph such a solar eclipse using the Apollo 15 lunar rover television cameras . Unfortunately, however, the camera and its power source were already defective immediately after the astronauts had started.
On the other hand, an "earth eclipse" is not a particularly spectacular event for a lunar observer, since the moon's shadow tapers significantly towards the earth. The only thing that remains for observers on the surface of the moon is to use a telescope to follow a small, round, dark spot that moves across the surface of the earth.
The Mars only has a very thin atmosphere . However, it is extremely dusty, so much of the light that reaches it is scattered. The sky thus appears unrestrictedly bright during the day and stars cannot be seen. The northern pole of Mars is aligned with the star Deneb , slightly offset towards Alderamin . Kappa Velorum can be described as the southern polar star of Mars .
Color of the marsh sky
Surprisingly, it turns out to be an extremely difficult task to produce accurate, true-color images of the surface of Mars. On the one hand because of the high aerosol content (which also varies greatly due to frequent sandstorms ), but also because of the Purkinje effect : According to this, how the human eye perceives a color depends, among other things, on the brightness of the ambient light. Because as soon as the brightness of the environment decreases, people's color perception quickly makes red objects appear dark, while the color effect of blue objects decreases less quickly. In addition, the images published in the past show great differences in the representation of the color of the sky. This is because many of the images used filters that maximize certain scientific aspects without focusing on the representation of true colors.
It is known that during the Martian day the sky turns a scarlet or bright orange-red color. During the sunrise and sunset phases, the sky turns pink , with it appearing blue near the setting sun, in contrast to the familiar sight on earth. At times the sky also takes on a purple color, which is caused by the fact that the light is scattered by very small water-ice particles in the clouds. After the sun has set and before it rises, there is a long period of twilight. This is caused by the fact that the specks of dust, which can be found high in the atmosphere of Mars, scatter the light of the sun behind the horizon and illuminate the sky.
On Mars, Rayleigh scattering , which is responsible, among other things, for the red color of dawn and sunset on Earth, usually has a very weak effect. Instead, the red color of the sky is caused by dust particles in the air enriched with ferric oxide .
Seen from Mars, the sun is only 5/8 of the size it is on the earth's sky (0.5 °). The planet receives 60% less light than our world, which roughly corresponds to the brightness of a slightly cloudy afternoon on earth.
A Martian day is almost the same length as a day on Earth: 24 hours, 39 minutes and 35.244 seconds. This period of time is also called Sol in English, for example by NASA in connection with Mars missions .
Two small satellites orbit Mars: Phobos and Deimos . Seen from the surface of Mars, Phobos has between a third and about half the angular diameter of the Sun, whereas Deimos is little more than a point with an angular diameter of only 2 '.
The apparent movement of Phobos is opposite to the movement of the sun due to its fast orbital speed, i.e. that is, it rises in the west and sets in the east. This results from the fact that it orbits the planet faster than it rotates around its own axis. Phobos is also in such a low equatorial orbit, so it is so close to the planet during its orbit that it can no longer be seen above latitude 70.4 ° north and below 70.4 ° south. An observer positioned just below or just above the 70.4 ° visibility limit will see a noticeably smaller Phobos because he is further away from him than an observer at the equator. The apparent size of the moon varies by up to 45%. For an equatorial observer, however, Phobos appears on the horizon with an angular diameter of about 0.14 ° and reaches a size of 0.20 ° at its maximum. It crosses the sky in about 4.24 hours and reappears every 11.11 hours.
Deimos rises, like a "normal" moon, in the east and sets in the west. Its appearance at an angular diameter between 1.8 'and 2.1' is more like that of a star. Its brightness varies between that of the star Vega and that of Venus as seen from Earth. Since it is also relatively close to Mars, Deimos can no longer be seen from Mars latitudes above or below 82.7 ° north or south latitude. At approx. 30.3 hours, the orbit time of Deimos slightly exceeds the Mars rotation of approx. 24.6 hours. For an equatorial observer, the moon appears every 5.5 days and then remains visible for 2.5 days before disappearing behind the horizon again. In summary, it can be said that Phobos moves almost 12 times over the Martian sky during one Deimos cycle.
Phobos and Deimos can both partially cover the sun as seen from Mars, but neither of the two moons is able to completely darken the solar disk. In fact, one can speak of a passage of the sun rather than an eclipse.
Earth view and Venus view
The earth and its moon act like a double star when viewed from Mars . The visible distance between the earth and the moon is at most, i.e. with the lower conjunction of the earth and the sun (for the earthly observer, this is the opposition of Mars and the sun) about 25 arc minutes , this corresponds approximately to the apparent diameter of the earth's moon from the earth seen. In the vicinity of the maximum elongation of 47.4 °, the earth and moon can be seen in the apparent magnitudes of −2.5 m and +0.9 m .
Seen from Mars, Venus shines near the maximum elongation to the sun of 31.7 ° with an apparent brightness of approx. −3.2 m . This corresponds roughly to the value that Mars reaches when viewed from Earth.
The sky of the Martian moons
From Phobos, Mars appears 6400 times larger and 2500 times brighter than the full moon to an earth observer, taking up a quarter of the entire hemisphere. In contrast, from Deimos, Mars appears 1000 times larger and 400 times brighter than our full moon and fills 1/11 of the hemisphere of the smaller satellite.
If you look closely, the asteroid belt is an area only sparsely occupied by objects. Occasionally “closer approximations” occur, but there are still huge gaps between the bodies. In contrast to other science fiction films , the film 2001: A Space Odyssey gives a realistic impression of the asteroid belt when the spaceship encounters a lonely asteroid in one scene.
Some of the asteroids that orbits of one or another planet tick , can approach a planet or another asteroid occasionally. Then it is possible that an observer from this asteroid can see the disk of a nearby object without the aid of binoculars or telescope. For example, in September 2004, the object (4179) Toutatis approached Earth to a distance four times the Earth-Moon distance. At the closest point to the earth, viewed from its surface, the earth could be seen at roughly the same apparent size as the moon seen from the earth. The moon was just as clearly visible as a small disk in the sky of Toutatis.
Some asteroids have such unusual orbits that they offer a lot of material for fiction. For example, the planetoid (3200) Phaethon has one of the most eccentric orbits of all known objects in the solar system. Its distance from the Sun varies between 0.14 and 2.4 AU during its orbit . In perihelion, the sun appears more than seven times larger in its sky than on earth, and its surface receives more than 50 times as much energy from it per unit area . In aphelion, on the other hand, the sun decreases to less than half of its apparent diameter, seen from the earth, and the object only reaches about one sixth of the light that the earth receives at the point closest to the sun .
87 Sylvia and her moons Romulus and Remus
The asteroid (87) Sylvia is one of the largest asteroids in the asteroid belt and the first in which two moons could be observed. The moons Romulus and Remus are approximately 18 km and 7 km in diameter, respectively. Romulus, the more distant, takes an apparent size of about 0.89 degrees in the sky of the asteroid , the smaller and closer Remus about 0.78 °. Because (87) Sylvia deviates strongly from the spherical shape, these values can vary by a little more than 10%, depending on the position of an observer on the surface. Since the two moons orbit the celestial body almost in the plane of its orbit, it happens every 2.2 days that one covers the other. Twice every 6.52 Sylvia years there is a solar eclipse, which, however, due to the smaller apparent size of the sun of 0.15 °, is a much less spectacular event than it is (0.53 °) on Earth. Viewed from Remus, (87) Sylvia appears huge in the night sky, as she occupies about 30 ° × 18 ° here, while the neighboring Romulus varies between the apparent sizes of 1.59 ° and 0.50 °. From Romulus, the asteroid has an angular diameter of 16 ° × 10 °, while Remus assumes values between 0.62 ° and 0.19 °.
Although pictures of the interior of Jupiter's atmosphere have never been taken, artistic representations typically assume that the sky of the gas planet appears blue. However, it can be assumed that it has a duller blue than the earth's sky, since sunlight, at least in the upper part of the atmosphere, is on average 27 times weaker than on earth. As seen from Jupiter, the sun is only 5 arc minutes in length, which is less than a fifth of what it is in the sky. The planet's nearby rings are likely faintly visible over the equator. Lower in the atmosphere, on the other hand, the sun appears through clouds and fog, presumably in different colors and mostly takes on blue, brown and red tones, which quickly darken the deeper one penetrates into the planet's atmosphere. Various theories are currently being debated about the cause of these colors, but there is still no exact answer to this question.
Apart from the sun, the four Galilean moons are the most prominent objects in Jupiter's sky. Io , which is closest to the planet, appears slightly larger than the full earth moon in the sky, but appears less bright. Despite the larger albedo of the second nearest moon, Europe , it does not manage to outshine Io due to its greater distance to Jupiter. In fact, the low solar constant (3.7% of that of the earth's surface) due to the distance between Jupiter and the sun ensures that none of the Galilean satellites appear as bright as the full moon on earth. From Io through Europe and Ganymede to Callisto , the apparent brightnesses of the moons are: −11.2 m , −9.7 m , −9.4 m and −7.0 m .
Ganymede, the largest and third closest of the great moons of Jupiter, is almost as bright as Europe, but appears only half the size of Io in the Jupiter sky. Callisto, which is still further away, is only a quarter of the apparent size of our full moon. All four Galilean moons have a higher orbital speed compared to the Earth's moon, which is why they move much faster across the Jupiter sky than the moon across the Earth's sky. Each of them is also large enough to fully cover the sun during a solar eclipse .
The smaller inner moons of Jupiter only appear as star-shaped points, while most of the outer moons would no longer be visible to the naked human eye at all.
The sky of the moons of Jupiter
None of Jupiter's moons possesses more than a trace of an atmosphere, so that their skies are completely or almost black. To an observer on one of the moons, the most striking feature of the sky, apart from the sun, is of course Jupiter. On Io , the large moon closest to the planet, Jupiter has an apparent diameter of about 20 °. This corresponds to 38 times the visible diameter of our moon, which means it occupies 1% of the sky of Io. For an observer on Metis , the innermost moon, the apparent diameter increases to a value of 68 ° or 130 times the apparent diameter of our Earth's moon, which means that 18% of the sky of Metis is taken up by the planet. A “full Jupiter” over Metis shines with about 4% the brightness of the sun, whereas our full moon is only 400,000 times weaker than sunlight.
Since the inner moons of Jupiter have a synchronous rotation to the planet, it always appears in almost the same place in the sky (Jupiter “ wobbles ” a little because of a small eccentricity). On the other hand, observers on the sides of the Galilean moons facing away from Jupiter never see the planet. From these Jupiter moons, the eclipses they themselves cause are very spectacular, as an observer can see the circular shadow of the respective moon slowly moving over the surface of Jupiter.
Saturn's atmosphere is likely to be blue in the upper layers, although the predominant color of the cloud cover suggests that it appears more yellowish further down. The rings of Saturn are certainly clearly visible here, at the upper edge of its atmosphere, although they are so thin that they remain almost invisible from a certain position at the equator of Saturn, since one only looks at the edge of the rings. From anywhere else on the planet, however, they can be seen as a spectacular arch spanning halfway across the celestial hemisphere of Saturn.
The moons of Saturn, however, do not look particularly impressive in its sky, as most are relatively small and the largest move their orbits far away from the planet. Titan, Saturn's largest moon, appears only half the size of the moon in the sky. The approximate angular diameters of the most important moons are given below (in comparison, the 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 pole star, δ Octantis, with a brightness value of 4.3 m . It is therefore significantly weaker than the pole star of the earth (α Ursae minoris).
The sky of Saturn's moons
Since the inner moons of Saturn are all in a bound rotation to the planet, it can always be seen in the same place in the sky, apart from certain fluctuations . Therefore, viewers on the sides of these satellites facing away from Saturn never see the planet.
Saturn is a decidedly dominant object in the sky view of the inner moons. Seen from Pan , Saturn, for example, has an apparent diameter of around 50 °. It appears 104 times larger than our moon and takes up 11% of the sky of Pan. Since Pan's orbit runs along Encke's division and thus within Saturn's rings, these can be seen from anywhere on Pan, even on the side facing away from Saturn.
The rings as seen from the moons of Saturn
In the sky of most of the moons, the rings of Saturn hardly represent a prominent appearance. This is because the rings are wide, but not very thick and the orbits of most of the moons are almost exactly (max. 1.5 ° deviation) in the ring plane of the planet. From the inner moons you can only see the edges of the rings, which means that they remain practically invisible. From the outer moons, in turn, beginning with Iapetus , one has an oblique view of the rings, whereby Saturn appears much smaller due to the greater distance to them. On the surface of Phoebe , the largest of the outer moons of Saturn, the planet therefore only reaches the size of the full moon in the earthly night sky. A calculation of the distances and angles of the individual moons to Saturn is difficult with the available values, but the results show that the best view of the rings can be expected from the inner moon Mimas , whose orbit is almost 1.5 ° from the Saturn's equatorial plane deviates, being in relative proximity to the rings. At the point at which Mimas reaches its greatest deviation in the equatorial plane of the planet, one can determine an apparent distance of 2.7 ° between the outer edge of ring B and the inner edge of ring A. The two moons Epimetheus and Janus , which are also in orbit, offer an impressive sight from the surface of the moon with maximum angles of between 1.5 ° and 2.9 °. The next best view is then to be expected on Tethys , which appears at almost half a degree in the Mima sky. Iapetus has an angular diameter of 0.20 °, which is more than can be reached from any of the outer moons.
Titan is the only moon in our solar system that is surrounded by a dense atmosphere. Images from the Huygens probe show that the titanium sky is bathed in an orange light. However, only a fuzzy, brownish / dark orange color can be perceived on the surface of titanium, since titanium only receives 1/3000 of the sunlight on our earth. Due to the dense atmosphere and the much greater distance to the sun, it is just as bright on Titan during the day as in the twilight of the earth. It seems likely that Saturn is permanently invisible behind the orange-colored smog and that even the sun is just a bright spot in the haze that can barely illuminate the surface of the celestial body covered by ice and methane lakes. In the upper atmosphere, on the other hand, the sky will probably appear blue and from here it is quite possible to take a look at Saturn.
In the sky of Enceladus , Saturn has a visible diameter of almost 30 ° and is therefore about 60 times larger than the moon in our night sky. Since the rotation of Enceladus is synchronous with its orbital motion around the planet, it always turns the same side towards Saturn. The planet therefore hardly moves at all in the sky of Enceladus, with the exception of slight variations resulting from the eccentricity of its orbit. In contrast, the planet can never be seen from the side facing away from Saturn.
The rings of Saturn can be viewed at a maximum angle of 0.019 °, which means that they are almost invisible. Only their shadows stand out clearly on the surface of Saturn. As with the Earth's moon, Saturn shows regular phases , which vary from a “full saturn” to a partial shade to a “new saturn”. Seen from Enceladus, the sun has a diameter of only 3.5 arc minutes, about a ninth of the diameter it is in the sky.
If an observer is on Enceladus, they can see from there how Mimas , the largest satellite in Enceladus orbit, passes in front of Saturn every 72 hours on average. In the best case, its apparent size is 26 minutes of arc, which means that it is roughly the same size as the moon in our sky. Pallene and Methone , on the other hand, appear almost star-like with a maximum of 30 arc seconds. Tethys is only visible from the side of Enceladus facing away from Saturn and reaches a maximum apparent size of approx. 64 arc minutes, which corresponds to about twice the value of the Earth's moon in our sky view.
Judging by the color of the atmosphere, it can be assumed that the sky of Uranus is likely to appear a light blue or more cyan . It is likely that the planet's rings cannot be seen from the surface because they are very thin and dark. Uranus has a northern pole star, Sabik (η Ophiuchi), with a magnitude of 2.4 m and also a southern pole star, 15 Orionis , with an apparent brightness value of 4.8 m . Both are weaker than the pole star on Earth, but Sabik is only slightly.
Uranus is unusual in that the inclination of its ecliptic , i.e. the angle of the axis inclination to its orbital plane, is a full 97.77 °. As a result of this inclination, the northern hemisphere and the southern hemisphere face the sun after every half orbit. On December 17, 2007, the sun passed the celestial equator of Uranus from the north and in 2029 its north pole will finally be aligned almost exactly with the sun.
The moons of Uranus don't look very large from the surface of the planet. Below are the angular diameters of the five large moons. (For comparison: on the Earth's moon it is 31 '): Miranda, 11–15'; Ariel, 18-22 '; Umbriel, 14-16 '; Titania, 11-13 '; Oberon, 8-9 '. The small inner moons appear as star-shaped dots, while the outer irregular moons are invisible to the naked eye.
Based on the color of the atmosphere, the sky of Neptune is likely to be azure or light blue , similar to that of Uranus. It is believed that the rings of the planet cannot be seen from the surface because they are very thin and dark.
Apart from the sun, its large moon Triton is the most striking object in the Neptune sky and is likely to be a little smaller than the moon on Earth. With an orbital period of 5.8 days, it moves much faster than our moon around its planet. This impression is reinforced by its retrograde direction of movement, with which it runs counter to the direction of Neptune's orbit. In contrast to Triton, the smaller moon Proteus can be seen as a disk about half the size of the full moon. Neptune's smaller inner moons and the large outer satellite Nereid can only be seen as star-shaped points, and its outermost satellites cannot be made out at all without a telescope.
Triton , Neptune's largest moon, has an atmosphere, but it is so thin that its sky appears practically completely black and, in the best case, some pale haze is visible on the horizon. Since Triton has a bound rotation to Neptune, one always sees the planet in the same position in its sky. Triton's axis of rotation is inclined by 157 ° to the equatorial plane of Neptune, which in turn is inclined by 30 ° to its orbit around the sun. Therefore, similar to Uranus , the poles of Triton are aimed directly at the sun twice per Neptune year. As a result, Triton's polar regions face the sun for 40 consecutive years before the other pole moves into the sunlight, which ultimately leads to a radical change in the seasons on the moon.
Neptune itself spans about 8 ° over the sky of Triton, although its maximum brightness is only roughly comparable to that of the full moon on earth, as it can hardly reflect more than 1/256 of the sunlight of the full earth moon per unit area . Due to its eccentric orbit, Nereid varies significantly in its brightness between the fifth and the first magnitude and its disk is much too small to be seen with the naked eye. With its 5–6 arc minutes, Proteus is also almost impossible to make out as a disc, but never appears weaker than 1st magnitude and can compete with Canopus at its closest distance .
Pluto and Charon
The dwarf planet Pluto , accompanied by its largest moon Charon , orbits the sun in an orbit that is far out of Neptune's orbit for most of the time. Due to the strongly eccentric orbit, however, Pluto is closer to the sun than Neptune for a period of about twenty years (the last time 1979–1999). From Pluto the sun can still be seen quite brightly, much brighter than the earth's moon appears from the earth. The brightness fluctuates due to the eccentricity of the orbit of Pluto, so that a human observer would notice a strong change in brightness in the course of the orbit around the sun.
Pluto and Charon have a double bonded rotation . This means that Pluto always faces the same side of Charon and Pluto always presents the same side to its moon as well. Observers on the other side of the Charon never get to see the dwarf planet, any more than do observers on the other side of Pluto get to see Charon. Every 124 years there is a period of mutual eclipse of the sun for several years when Pluto and Charon take each other's sunlight at intervals of 3.2 days.
From Pluto , Charon appears about 7 times larger than the full moon from Earth, i.e. with about 50 times the solid angle . Conversely, from Charon, Pluto appears again almost twice as large, with 180 times the solid angle of the full moon.
In contrast to other celestial bodies, the sky of a comet changes drastically as soon as it approaches the sun. As a comet approaches, the ice on the surface begins to sublime , that is, it changes from the solid to the gaseous state immediately. A tail of gas and dust forms, which creates a coma around the comet's body. An observer on a comet approaching the sun can therefore only see the stars through a slightly milky cloudiness, which presumably creates interesting halo effects around the sun and other bright objects.
Planets outside the solar system
For an observer on an extrasolar planet , the star constellations look very different from what we are used to from Earth. Our sun is only visible to the naked human eye up to a distance of 20-25 parsecs (65-80 light years ).
Looking at the Sun from the α-Centauri system , the star system closest to us , it appears as a bright star in the constellation Cassiopeia , almost as bright as the star Capella in our sky. From a hypothetical planet orbiting either α Centauri A or B, the other star in the system can be seen as a very bright second sun. For example, an Earth-like planet that is 1.25 astronomical units away from α Centauri A and has an orbital period of 1.34 years receives sun-like radiation from its primary star, while α Centauri B between 5.7 and 8.6 magnitudes less occupies in its sky (−21.0 m to −18.2 m ). This means that the second sun is 190 to 2700 times weaker than α Centauri A, but is still 2100 to 150 times brighter than the full moon. Conversely, an Earth-like planet at a distance of 0.71 AU from α Centauri B (and an orbital period of 0.63 years) receives the same solar illumination from its primary sun as the Earth, but α Centauri A appears 4.6 to 7.3 magnitudes less luminous (−22.1 m to −19.4 m ) and thus 70 to 840 times weaker than α Centauri B, but still between 5700 and 470 times brighter than the full moon. In both cases you can see the secondary sun moving in a circular path across the sky during the planet's orbit. Starting right next to the primary sun, it can be seen in the sky opposite it after half a period of revolution and thus represents a “midnight sun”. After another half period, it has completed the cycle. Incidentally, a similar celestial spectacle can be admired on other planets, which are only bound to one component of a binary star system.
Seen from a circumbinary planet , both suns stand together in the daytime sky, whereby their distance to one another changes during their mutual orbit.
From 40 Eridani , at a distance of 16 light years, the sun appears only as an average star in the constellation Snake , which has an apparent brightness of approx. 3.3 m . At this distance, most of the closest stars , including Alpha Centauri and Sirius , are in different positions than our sky.
From a planet orbiting Aldebaran , 65 light years away , the sun can be seen just above Antares in our constellation Scorpio . However, with a light size of 6.4 m, it can hardly be seen with the naked eye. Constellations that consist of bright, distant stars, such as B. the constellation Orion , are very similar to the view from our earth, but most of the other star constellations in our night sky hardly seem familiar to us in this place.
Calculation of the apparent brightness
The brightness of an object is inversely proportional to the square of the distance. The value of the apparent brightness can be assumed to be −2.5 times the decadic logarithm of the brightness ratio of two brightness classes . So if an object has an apparent brightness at a distance of from the observer and all other factors are equal, it can be of the order of magnitude of
- 1 arc minute corresponds to 1/60 ° = 0.0167 °, the full moon has a diameter of about 30 arc minutes when viewed from the earth.
- In order to calculate the values given, 1.1 solar masses were assumed for α Centauri A and 0.92 solar masses for α Centauri B, as well as brightnesses that are 1.57 and 0.51 times that of the sun (luminosity of the sun = −26.73 m ) based on orbits between 11.2 and 35.6 AU. For the minimum luminosity, the orbital radius of the planet was added to the maximum distance (conjunction) between α Centauri A and B, the maximum luminosity was again obtained by subtracting the orbit radius at the smallest distance between A and B (opposition).
- 3D Universe - How the universe looks in other places and times.
- JPL Solar System Simulator
- Phases of Charon as seen from Pluto
- Astronauts on other planets
- Windows planets-Mercury's atmosphere
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- This and other simulation images in this article were created using the Celestia Space Simulation software.
- Calculation of the apparent brightness ( Memento from June 22, 2012 in the Internet Archive )
- Pre-eclipse of the Sun by Callisto from the center of Jupiter . JPL Solar System Simulator . January 3, 2009. Retrieved June 4, 2008.
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- POV-Ray renderings from Huygens on Titan in descending order
- According to distance and diameter.