Extraterrestrial sky
In astronomy, an extraterrestrial sky is a view of outer space from the surface of an astronomical body other than Earth.
The only extraterrestrial sky that has been directly observed and photographed by astronauts is that of the Moon. The skies of Venus, Mars and Titan have been observed by space probes designed to land on the surface and transmit images back to Earth.
Characteristics of extraterrestrial sky appear to vary substantially due to a number of factors. An extraterrestrial atmosphere, if present, has a large bearing on visible characteristics. The atmosphere's density and chemical composition can contribute to differences in color, opacity and the presence of clouds. Astronomical objects may also be visible and can include natural satellites, rings, star systems and nebulas and other planetary system bodies.
Luminosity and angular diameter of the Sun
The Sun's apparent magnitude changes according to the inverse square law, therefore, the difference in magnitude as a result of greater or lesser distances from different celestial bodies can be predicted by the following formula:Where "distance" can be in km, AU, or any other appropriate unit.
To illustrate, since Pluto is 40 AU away from the Sun on average, it follows that the parent star would appear to be times as bright as it is on Earth.
Though a terrestrial observer would find a dramatic decrease in available sunlight in these environments, the Sun would still be bright enough to cast shadows even as far as the hypothetical Planet Nine, possibly located 1,200 AU away, and by analogy would still outshine the full Moon as seen from Earth.
The change in angular diameter of the Sun with distance is illustrated in the diagram below:
The angular diameter of a circle whose plane is perpendicular to the displacement vector between the point of view and the centre of said circle can be calculated using the formula
in which is the angular diameter, and and are the actual diameter of and the distance to the object. When, we have, and the result obtained is in radians.
For a spherical object whose actual diameter equals and where is the distance to the centre of the sphere, the angular diameter can be found by the formula
The difference is due to the fact that the apparent edges of a sphere are its tangent points, which are closer to the observer than the centre of the sphere. For practical use, the distinction is significant only for spherical objects that are relatively close, since the small-angle approximation holds for :
Horizon
On terrestrial planets and other solid celestial bodies with negligible atmospheric effects, the distance to the horizon for a "standard observer" varies as the square root of the planet's radius. Thus, the horizon on Mercury is 62% as far away from the observer as it is on Earth, on Mars the figure is 73%, on the Moon the figure is 52%, on Mimas the figure is 18%, and so on. The observer's height must be taken into account when calculating the distance to the horizon.Mercury
Because Mercury has little 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. Omicron Draconis is its north star.Other planets seen from Mercury
After the Sun, the second-brightest object in the Mercurian sky is Venus, which is much brighter there 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 with the difference being most considerable for Mars.
The zodiacal light is probably more prominent than it is from Earth.
Venus
The atmosphere of Venus is so thick that the Sun is not distinguishable in the daytime sky, and the stars are not visible at night. Being closer to the Sun, Venus receives about 1.9 times more sunlight than Earth, but due to the thick atmosphere, only about 20% of the light reaches the surface. Color images taken by the Soviet Venera probes suggest that the sky on Venus is orange. If the Sun could be seen from Venus's surface, the time from one sunrise to the next 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 circumnavigate the planet in about four Earth days and see a sky in which Earth and the Moon shine brightly at opposition. The maximum angular separation between the Moon and Earth from the perspective of Venus is 0.612°, or approximately the same separation of one centimetre of separation at a distance of one metre and coincidentally, about the apparent size of the Moon as seen from Earth. 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 than when observed from Earth.
42 Draconis is the closest star to the north pole of Venus. Eta¹ Doradus is the closest to its south pole.
The Moon
The Moon's atmosphere is negligibly thin, essentially vacuum, so its sky is black, as in the case of Mercury. At lunar twilight astronauts have though observed some crepuscular rays and lunar horizon glow of the illuminated atmosphere, beside interplanetary light phenomenons like zodiacal light.Furthermore, the Sun is so bright that it is still very difficult to see stars during the lunar daytime, except possibly very bright ones like Sirius, unless the observer is well shielded from sunlight, while the brightest planets are observable. On the lunar near side during lunar night this is similarly the case due to the brightness of the Earth reflecting sunlight, producing the so-called earthshine, creating about 43 times and up to 55 times brighter surface conditions on the Moon than a full moon on Earth.
The Moon has a southern polar star, δ Doradus, a magnitude 4.34 star. It is better aligned than Earth's Polaris, but much fainter. Its north pole star is Omicron Draconis.
Sun and Earth in the lunar sky
While the Sun moves across the Moon's sky within fourteen days, the daytime of a lunar day or the lunar month, Earth is only visible on the Moon's near side and moves around a central point in the near side's sky.This is due to the Moon always facing the Earth with the same side, a result of the Moon's rotation being tidally locked to Earth. That said, the Earth does move around slightly around a central point in the Moon's sky, because of monthly libration.
Therefore rising or setting of Earth at the horizon on the Moon occurs only at few lunar locations and only to a small degree, at the border of the near side of the Moon to the far side, and takes much longer than a sunrise or sunset on Earth due to the Moon's slow monthly rotation.
The famous Earthrise image by Apollo 8 though is an instance where the astronauts moved around the Moon, causing the Earth to rise above the Moon because of that motion.
Earthshine (near side night)
Earthshine is the light reflected from the Sun reflected by Earth.Earthshine is in the lunar night about 43 times brighter, and sometimes even 55 times brighter than a night on Earth illuminated by the light of the full moon.
Only on the far side and during lunar eclipses on the near side does the night become dark. No person has yet been on the Moon during its night and experienced earthshine.
Eclipses from the Moon
When sometimes the Moon, Earth and the Sun align exactly in a straight line, the Moon or Earth move through the other's shadow, producing an eclipse for an observer on the surface in the shadow.When the Moon moves into Earth's shadow a Solar eclipse occurs on the near side of the Moon.
Since the apparent diameter of the Earth is four times larger than that of the Sun, the Sun would be hidden behind the Earth for hours. Earth's atmosphere would be visible as a reddish ring. During the Apollo 15 mission, an attempt was made to use the Lunar Roving Vehicle's TV camera to view such an eclipse, but the camera or its power source failed after the astronauts left for Earth.
When Earth moves into the Moon's shadow a Solar eclipse occurs on Earth where the Moon's shadow passes, and is visible facing Earth as a tapered out lunar shadow on Earth's surface traveling across the full Earth's disk. The effect would be comparable to the shadow of a golf ball cast by sunlight on an object away. Lunar observers with telescopes might be able to discern the umbral shadow as a black spot at the center of a less dark region. It would look essentially the same as it does to the Deep Space Climate Observatory, which orbits Earth at the L1 Lagrangian point in the Sun-Earth system, from Earth.