Skies of other planets
From open-encyclopedia.com - the free encyclopedia.
This article deals with what an observer placed on the surface of a planet would see overhead. Much of this material is duplicated from the entries on individual planets and moons, but is assembled here for those interested in the general subject of extraterrestrial skies.
| Contents |
Mercury
Mercury has no atmosphere, like the Moon, so its sky would always be black.
The Sun from Mercury
Because of Mercury's 3:2 spin-orbit resonance, although a sidereal day (the period of rotation) lasts ~58.7 Earth days, a solar day (the length between two meridian transits of the Sun) lasts ~176 Earth days.
The visible diameter of the Sun on Mercury would be 2.5 times larger than on Earth, and more than 6 times brighter. Mercury's closeness to the Sun also causes an unusual effect in which the Sun would appear to reverse direction during its traverse across the sky. At certain points on Mercury's surface, an observer standing upon one of the tidal bulges would be able to see the Sun rise about halfway, then reverse and set, then rise again, all within the same Mercurian day. This is because approximately four days prior to perihelion, Mercury's orbital velocity exactly equals its rotational velocity, so that the Sun's apparent motion ceases; at perihelion, Mercury's orbital velocity then exceeds the rotational velocity; thus, the Sun appears to be retrograde. Four days after perihelion, the Sun's normal apparent motion resumes.
Other planets seen from Mercury
The second brightest object in the Mercurian sky would be Venus, which would be 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 dark 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 would then be approximately −8.
The Earth and the Moon would also be very visible, their apparent magnitude being about −6 and −1 respectively. The maximum apparent distance between the Earth and the Moon would be about 15′. All other planets would be visible just as they are on Earth, but somewhat less bright in opposition.
The zodiacal light would probably be more prominent than it is from Earth.
Venus
The atmosphere of Venus is so thick that the Sun would not be distinguishable in the daytime sky, and the stars would be invisible at night. Colour images taken by the Soviet Venera probes suggest that the sky on Venus is yellow-orange.
The Moon
The Moon has no atmosphere, so its sky is always black. However, the Sun would be so bright that it would be impossible to see stars during the daytime, unless the observer was well shielded from sunlight (direct or reflected from the ground).
The Sun from the Moon
The Sun would look the same from the Moon as it does from Earth, except that it would be somewhat brighter (and colored pure white) due to the lack of atmospheric scattering and absorption.
The lunar day lasts a month. The side of the Moon that is invisible from Earth is sometimes called the "dark side", but in this case "dark" means "unknown and hidden" and not "lacking light"; in fact, the far side receives as much sunlight as the near side, but at opposite times.
The Earth from the Moon
The most interesting feature of the Moon's sky would, of course, be the Earth, spinning against the black backdrop of space. Its visible diameter (1.9°) would be four times the diameter of the Moon as seen from the Earth, although because the Moon's orbit is eccentric, the Earth's apparent size in the sky would vary slightly, by about 5% either way (between 1.8° and 2.0°). The Earth would show phases, just like the Moon for the terrestrial observer, but they would be opposite: when the terrestrial observer sees the full Moon, the lunar observer sees a "new Earth", and vice versa. The Earth's albedo is three times as high as that of the Moon. The full Earth would glow over 50 times brighter than the full Moon at zenith does for the terrestrial observer.
The Moon has synchronous rotation. As a result, 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 the Earth can only be seen on from the near side of the Moon, and would always be invisible from the far side.
Seen from the Moon's near side, the Earth would not move, but would be almost perfectly still. However, there would be some small movement: due to librations, it would perform a complex wobbling movement. Near the boundary of the near and far sides of the Moon, the Earth would sometimes be below the horizon, and sometimes above it.
Eclipses from the Moon
The Earth and the Sun would sometimes meet in the lunar sky, causing an eclipse. On the Earth, one would then see 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 hide behind the Earth for hours. The Earth's atmosphere would be visible as a reddish ring.
Terrestrial solar eclipses, on the other hand, would not be spectacular for lunar observers, because the Moon's shadow nearly tapers out at the Earth's surface. Lunar observers with telescopes might simply see a small darkened spot travel across the full Earth's disk.
In summary, whenever an eclipse of some sort is occurring on the Earth, an eclipse of another sort is occurring on the Moon. Eclipses occur for both Earth and Lunar observers whenever the two bodies and the Sun align in a straight line.
Mars
Mars has only a thin atmosphere; however, it is extremely dusty and lots of light is scattered. The sky is thus quite bright during the daytime and stars are not visible.
The colour of the Martian sky
Generating accurate true-color images from Mars' surface is surprisingly complicated. [1] There is much variation in the color of the sky as reproduced in published images, since many of those images are using filters to maximize their science 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 yellow-brown "butterscotch" colour. Around sunset and sunrise, sky is pinkish-red in color, but in the vicinity of the setting Sun it is blue. This is the exact opposite of the situation on Earth. At times, the sky takes on a violet color, due to the scattering of light by very small water ice particles in clouds.[2]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; it is believed that the color of the sky is caused by the presence of 1% by volume of magnetite in the airborne dust particles.
The Sun from Mars
The Sun as seen from Mars is seen 1.6 times smaller (0.35°) than on Earth, and sends 2.5 times less light. A detailed analysis of the Sun's movements as seen from Mars can be found in the article on time and date and astronomy on Mars.
Mars' moons as seen from Mars
Mars has two small moons: Phobos and Deimos. From the Martian surface, Phobos would have one-third to half the angular diameter of the Sun, but Deimos would be barely more than a dot (only 2' angular diameter). The apparent motion of Phobos would be in reverse, due to its fast orbital motion: it would rise in the west and set 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 would vary by up to 45% as it passed overhead, due to its proximity to Mars' surface. For an equatorial observer, for example, Phobos would be about 0.14° upon rising and swell to 0.20° by the time it reaches the zenith.
Deimos rises in the east and sets in the west, like a "normal" moon, although its appearance would be star-like. 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 30.5 hours exceeds the Martian solar day of about 24.5 hours by such a small amount that it would take 2.7 days between rising and setting for an equatorial observer.
Phobos and Deimos can both eclipse the Sun as seen from Mars, although neither can completely cover its disk and so the event is in fact a transit, rather than an eclipse. For a detailed description of such events see the articles Transit of Phobos from Mars and Transit of Deimos from Mars.
Earth from Mars
The Earth would be visible from Mars as a double star; the Moon would be visible alongside it as a fainter companion. The maximum visible distance between the Earth and the Moon would be about 25′ at conjunction of the Earth and the Sun — for the terrestrial observer it will be opposition of Mars and the Sun.
The skies of Mars' moons
From Phobos, Mars would appear 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 would appear 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
It is a common misconception, fuelled by movies such as The Empire Strikes Back, that the asteroid belt is so closely packed that an asteroid's sky would be filled with hurtling lumps of rock. In fact, 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.
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 than the Moon does. At its 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 from its sky at that time.
Jupiter
Although no images from within Jupiter's atmosphere have ever been taken, artistic representations typically assume that the planet's sky is blue, 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 colours.
From Jupiter, the sun would appear to cover only 5 arc minutes, less than a quarter of its size as seen from Earth.
Jupiter's moons as seen from Jupiter
Aside from the sun, the most prominent objects in Jupiter's sky would be the four Galilean moons. Io, the nearest to the planet, would appear as large as the full Moon in Earth's sky. However, it would be slightly outdone in brightness by Europa, even though that moon is smaller and further away; this is because Europa is composed of highly reflective water ice, unlike the reddish Io.
Ganymede, the largest moon and third from Jupiter, would be almost as bright as Io and Europa, but would appear only half the size of Io. Callisto, still further out, would be only a quarter the size of the full Moon.
Jupiter's small inner moons would appear only as starlike points, and most of the outer moons would be invisible to the naked eye.
Each of the Galilean satellites would appear large enough to fully eclipse the sun when it passed in front of it.
The skies of Jupiter's moons
None of Jupiter's moons have atmospheres, so their skies would be black. For an observer on one of the moons, the most prominent feature of the sky would, of course, be Jupiter. For an observer on Io, the closest large moon to the planet, Jupiter's apparent diameter would be about 20° (40 times the visible diameter of our Moon).
Since the four Galilean moons of Jupiter are in synchronous rotation around Jupiter, the planet would always appear in the same spot in their skies. Observers on the sides of the Galilean satellites facing away from the planet would never see Jupiter.
From the moons of Jupiter, solar eclipses caused by the Galilean satellites would be spectacular, as an observer would see the circular shadow of the eclipsing moon travel across the Jupiter's face.
Saturn
The sky in the upper reaches of Saturn's atmosphere is probably blue, but the predominant colour of its cloud decks suggests that it may be yellowish further down. The rings of Saturn would almost certainly be 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, would appear only half the size of Earth's moon. Here are the approximate angular diameters of the main moons (for comparison, Earth's moon has an angular diameter of 31'): Mimas: 5-10', Enceladus: 5-9', Tethys: 8-12', Dione: 8-12', Rhea: 8-11', Titan: 14-15', Iapetus: 1'.
The skies of Saturn's moons
Since the large moons of Saturn are all in synchronous rotation, the planet would always appear in the same spot in their skies. Observers on the sides of the satellites facing away from the planet would never see Saturn.
In the skies of Saturn's inner moons, Saturn would be an enormous object. However, the rings would not be prominent on most of the moons. This is because the rings, though wide, are not very thick, and most of the moons orbit almost exactly in the planet's ring plane. Thus, the rings would be practically invisible except from the moons Titan and Iapetus, which orbit in slightly inclined orbits and thus offer a more oblique view of the rings.
Saturn's small outer moons would also have excellent views of the rings. The planet would appear smaller than from Iapetus, but still quite spectacular: from Phoebe, the largest of the outer moons, Saturn would appear as big as the full moon does from Earth.
The sky of Titan
Titan is the only moon in the solar system to have a thick atmosphere. Although no images have yet been taken from its surface, it is presumed that the Titanian sky is orange-brown owing to the colour of its atmosphere. If Saturn could be seen from its surface, an observer would have a splendid view of the planet's rings as Titan has a slightly inclined orbit. However, it is possible that the atmosphere is so thick that Saturn would be permanently invisible behind smoggy clouds.
Uranus
Judging by the colour of its atmosphere, it is likely that the sky of Uranus is greenish-blue. It is unlikely that the planet's rings can be seen from its surface, as they are very thin and very dark.
Uranus' moons would not look very large from the surface of their parent planet. The angular diameters of the five large moons are as follows (for comparison, Earth's moon measures 31' for terrestrial observers): Miranda: 11-15', Ariel: 18-22', Umbriel: 14-16', Titania: 11-13', Oberon: 8-9'. The small inner moons would appear as starlike points, and the outer irregular moons would not be visible to the naked eye.
Neptune
Judging by the colour of its atmosphere, it is likely that the sky of Neptune is blue. It is unlikely that the planet's rings can be seen from its surface, as they are very thin and very dark.
Aside from the sun, the most impressive object in Neptune's sky would be its large moon Triton, which would appear just a little smaller than a full moon on Earth. The smaller moon Proteus would show a disk about half the size of the full moon. Neptune's small inner moons, and its large outer satellite, Nereid would appear as starlike points, and its irregular outer satellites would not be visible to the naked eye.
The sky of Triton
Triton, Neptune's largest moon, has an atmosphere, but it is so thin that the moon's sky would still be black, perhaps with some pale haze at the horizon. Because Triton orbits with synchronous rotation, Neptune would always appear in the same position in its sky.
Pluto and Charon
Pluto, accompanied by its large moon Charon, is the most distant planet from the Sun except during a twenty-year period in each orbit when it becomes closer than Neptune.
From Pluto, the sun would still be very bright, having a magnitude 1,500 times that of the full Moon from Earth. Nonetheless, human observers would find a noticeable decrease in available light.
Pluto and Charon are unusual among planets in that they 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 planet; observers on the far side of Pluto from Charon would never see the moon.
Comets
The sky of a comet would change dramatically as it neared the Sun. During perihelion, a comet's ices begin to sublime from its surface, forming tails of gas and dust, and a coma. An observer on a comet nearing the Sun might see the stars slightly obscured by a milky haze, which could create interesting halo effects around the Sun and other bright objects.
Extrasolar planets
For observers on extrasolar planets, the constellations would be quite different. The Sun would be visible to the naked human eye only at distances below 20–25 parsecs.
