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
Characteristics of extraterrestrial sky appear to vary substantially due to a number of factors. An
bodies.Luminosity and angular diameter of the Sun
The
Where "distance" can be in
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
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[nb 1]
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
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 :[2]
- .
Horizon
On
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 (α Ursae Minoris).[3] Omicron Draconis is its north star.[4]
Other planets seen from Mercury
After the Sun, the second-brightest object in the Mercurian sky is
The Earth and the Moon are also very prominent, their apparent magnitudes being about −5[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
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 (about magnitudes −6.6
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 impossible to see stars during the lunar daytime, unless the observer is well shielded from sunlight (direct or reflected from the ground).
The Moon has a southern polar star, δ Doradus, a magnitude 4.34 star. It is better aligned than Earth's Polaris (α Ursae Minoris), but much fainter. Its north pole star is Omicron Draconis.[nb 2]
-
Zodiacal light viewed from the Moon, during Apollo 15
-
The thin lunar atmosphere is visible on the Moon's surface at sunrise and sunset with the lunar horizon glow[11] and lunar twilight rays, like Earth's crepuscular rays. This Apollo 17 sketch depicts the glow and rays[12] among the general zodiacal light.[13][14]
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
The famous Earthrise image by Apollo 8 though is an instance where the astronauts moved around the Moon, making the Earth to rise above the Moon because of that motion.
Eclipses from the Moon
When sometimes the Moon, Earth and the Sun align exactly in a straight line (a syzygy), 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 (which is observable as a Lunar eclipse facing 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.
When Earth moves into the Moon's shadow a
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A painting by Lucien Rudaux showing how a solar eclipse might appear when viewed from the lunar surface.[17]
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A simulation of the start and end of the August 28, 2007 lunar eclipse, viewed from the center of the Moon.[18]
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From space, the Moon's shadow during the solar eclipse of March 9, 2016 appears as a dark spot moving across the Earth.
Mars
Mars has only a thin atmosphere; however, it is extremely dusty and there is much light that is scattered about. The sky is thus rather bright during the daytime and stars are not visible. The Martian northern pole star is Deneb,[19] although the actual pole is somewhat offset in the direction of Alpha Cephei; it is more accurate to state that the top two stars of the Northern Cross, Sadr and Deneb, point to the north Celestial pole of Mars.[20] Kappa Velorum is only a couple of degrees from the south Celestial pole of Mars.[20]
The color of the Martian sky
Generating accurate true-color images from Mars' surface is surprisingly complicated.[21] 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 butterscotch color.[22] Around sunset and sunrise, the sky is rose in color, but in the vicinity of the setting Sun it is blue. This is the opposite of the situation on Earth. Twilight lasts a long time after the Sun has set and before it rises because of the dust high in Mars's atmosphere.
On Mars,
-
Sunset,Gale crater– photo sequence by Curiosity rover, April 15, 2015
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Mars sky at noon, as imaged by Mars Pathfinder (June 1999)
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Mars sky at sunset, as imaged by Mars Pathfinder (June 1999)
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Mars sky at sunset, as imaged by theSpirit rover(May 2005)
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Mars sky atCuriosity rover(February 2013; Sun simulated by artist)
The Sun from Mars
The Sun as seen from Mars appears to be 5⁄8 the angular diameter as seen from Earth (0.35°), and sends 40% of the light, approximately the brightness of a slightly cloudy afternoon on Earth.
On June 3, 2014, the
Earth and Moon from Mars
The Earth is visible from Mars as a double star; the Moon would be visible alongside it as a fainter companion. The difference in brightness between the two would be greatest around
Year | Event | Image | Refs |
---|---|---|---|
2003 | Earth and Moon, imaged by UTC . South America is visible.
|
[25][26] | |
2014 | Curiosity's first view of the Earth and the Moon from the surface of Mars (January 31, 2014). | [27] | |
2016 | Earth and the Moon as viewed from orbit around Mars (MRO; HiRISE; November 20, 2016) |
[28] |
Venus from Mars
Venus as seen from Mars (when near the maximum elongation from the Sun of 31.7°) would have an apparent magnitude of about −3.2.[5]
Jupiter
Although no images from within
From Jupiter, the Sun appears to cover only 5 arcminutes, less than a quarter of its size as seen from Earth. The north pole of Jupiter is a little over two degrees away from Zeta Draconis, while its south pole is about two degrees north of Delta Doradus.
Jupiter's moons as seen from Jupiter
Aside from the Sun, the most prominent objects in Jupiter's sky are the four
All four Galilean moons stand out because of the swiftness of their motion, compared to the Moon. They are all also large enough to fully eclipse the Sun.[31] Because Jupiter's axial tilt is minimal, and the Galilean moons all orbit in the plane of Jupiter's equator, solar eclipses are quite common.
The skies of Jupiter's moons
None of Jupiter's moons have more than traces of atmosphere, so their skies are very nearly black. For an observer on one of the moons, the most prominent feature of the sky by far would be Jupiter. For an observer on Io, the closest large moon to the planet, Jupiter's apparent diameter would be about 20° (38 times the visible diameter of the Moon, covering 5% 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 the Moon, covering 18% of Metis's sky). A "full Jupiter" over Metis shines with about 4% of the Sun's brightness (light on Earth from a full moon is 400,000 times dimmer than sunlight).
Because the inner moons of Jupiter are in
From the moons of Jupiter, solar eclipses caused by the Galilean satellites would be spectacular, because an observer would see the circular shadow of the eclipsing moon travel across Jupiter's face.[32]
Saturn
The sky in the upper reaches of Saturn's atmosphere is blue (from imagery of the Cassini mission at the time of its September 2017 demise), but the predominant color of its cloud decks suggests that it may be yellowish further down. Observations from spacecraft show that seasonal smog develops in Saturn's southern hemisphere at its perihelion due to its axial tilt. This could cause the sky to become yellowish at times. As the northern hemisphere is pointed towards the Sun only at aphelion, the sky there would likely remain blue. 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. However, from anywhere else on the planet, they could be seen as a spectacular arc stretching across half the celestial hemisphere.[29]
Delta Octantis is the south pole star of Saturn. Its north pole is in the far northern region of Cepheus, about six degrees from Polaris.
The sky of Titan
Uranus
From a vantage above the clouds on Uranus, the sky would probably appear dark blue.[
Neptune
The north pole of Neptune points to a spot midway between Gamma and Delta Cygni. Its south pole star is Gamma Velorum.
Judging by the color of its atmosphere, the sky of Neptune is probably an azure or sky blue, similar to Uranus's. As in the case of Uranus, it is unlikely that the planet's rings can be seen from the upper atmosphere, as they are very thin and dark.
Aside from the Sun, the most notable object in Neptune's sky is its large moon
As with Uranus, the low light levels cause the major moons to appear very dim. The brightness of Triton at full phase is only −7.11, despite the fact that Triton is more than four times as intrinsically bright as Earth's moon and orbits much closer to Neptune.
The sky of Triton
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 1⁄256 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.
Trans-Neptunian objects
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater average distance (semi-major axis) than Neptune, 30 astronomical units (AU).
Pluto and Charon
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 point-like to human eyes, but still very bright, giving roughly 150 to 450 times the light of the full Moon from Earth (the variability being due to the fact that Pluto's orbit is highly elliptical, stretching from just 4.4 billion km to over 7.3 billion km from the Sun).[35] Nonetheless, human observers would find a large decrease in available light: the solar illuminance at Pluto's average distance is about 85 lx, which is equivalent to the lighting of an office building's hallway or restroom.
Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide gases, all of which are derived from the ices of these substances on its surface. When Pluto is close to the Sun, the temperature of Pluto's solid surface increases, causing these ices to sublimate into gases. This atmosphere also produces a noticeable blue haze that is visible at sunset and possibly other times of the Plutonian day.[36]
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, during which Pluto and Charon each alternately eclipse the Sun for the other at intervals of 3.2 days. Charon, as seen from Pluto's surface at the sub-Charon point, has an angular diameter of about 3.8°, nearly eight times the Moon's angular diameter as seen from Earth and about 56 times the area. It would be a very large object in the night sky, shining about 8%[nb 3] as bright as the Moon (it would appear darker than the Moon because its lesser illumination comes from a larger disc). Charon's illuminance would be about 14 mlx (for comparison, a moonless clear night sky is 2 mlx while a full Moon is between 300 and 50 mlx).
-
View from Pluto of Charon and the Sun (artist concept).
-
Pluto by moonlight
(artist concept).
Extrasolar planets
This section possibly contains original research. (April 2013) |
For observers on
In May 2017, glints of light from
The position of stars in extrasolar skies differs the least to the positions in Earth's sky at the closest stars to Earth, with nearby stars shifting position the most. The Sun would appear as a bright star only at the closest stars. At the
Planets of the TRAPPIST-1 system orbit extremely close together, enough so that each planet of the system would provide a detailed view of the other six. Planets of the TRAPPIST-1 system would appear in the sky with angular diameters comparable to the moon as viewed from Earth. Under clear viewing conditions, details such as phases and surface features would be easily visible to the naked eye.[43]
From the Large Magellanic Cloud
From a viewpoint in the LMC, the Milky Way's total
See also
Notes
- ^ This can be derived using the formula for the length of a cord found at http://mathworld.wolfram.com/CircularSegment.html
- ^ note: due to axial precession, the lunar pole describes a small circle on the celestial sphere every 18.6 years. Patrick Moore (1983), The Guinness Book of Astronomy Facts & Feats, p. 29,
In 1968 the north pole star of the Moon was Omega Draconis; by 1977 it was 36 Draconis. The south pole star is Delta Doradus.
- ^ Charon's radius is 606 km to the Moon's 1737 (surface ratio 0.12), its albedo is 0.35 to the Moon's 0.14 (2.6 ratio), its semi-major axis is 19591 km to the Moon's 384,400 (385 ratio of illumination), and the Sun is 39.482 times farther (0.00064 ratio of solar illumination). Multiplying these together yields an illuminance 0.077 times the Moon's.
- ^ These numbers are extrapolated from the fact that the human eye can generally see up to magnitude 6.5 on Earth, while it can see up to magnitude 7 under the best possible conditions.
- ^ Some of the figures in the "View" section were extrapolated from data in the Appendix of Chaisson and McMillan's Astronomy Today (Englewood Cliffs: Prentice-Hall, Inc., 1993).
References
- ^ Coffey, Donavyn (July 4, 2020). "What color is the sunset on other planets?". Live Science. Retrieved July 4, 2020.
- ^ "Archived copy" (PDF). Archived from the original (PDF) on February 18, 2015. Retrieved August 5, 2017.
{{cite web}}
: CS1 maint: archived copy as title (link) - ^ "Thin Atmosphere of Mercury, Formation and Composition – Windows to the Universe". www.windows.ucar.edu. Archived from the original on March 27, 2010. Retrieved September 5, 2008.
- ISBN 978-0-07-333666-4. www.starrynight.com
- ^ ISBN 978-0-89875-056-0.
- ^ "Possible Venus twin discovered around dim star". SETI Institute. April 6, 2017.
- ^ "The Planets: The Definitive Visual Guide to Our Solar System". Dorling Kindersley Limited. September 1, 2014. Retrieved October 6, 2023 – via Google Books.
- ^ "Venera 13 – Missions – NASA Solar System Exploration". NASA Solar System Exploration. Archived from the original on March 6, 2016. Retrieved December 22, 2015.
- ^ "The Terrestrial Planets". The Planetary Society. Archived from the original on July 28, 2011. Retrieved August 3, 2007.
- S2CID 189842666.
- ^ "Lunar horizon glow from Surveyor 7". The Planetary Society. May 6, 2016. Retrieved August 8, 2022.
- ^ "NASA Mission To Study Mysterious Lunar Twilight Rays". Science Mission Directorate. September 3, 2013. Retrieved August 8, 2022.
- . Retrieved August 8, 2022.
- ^ Deborah Byrd (April 24, 2014). "The zodiacal light, seen from the moon". EarthSky. Retrieved August 8, 2022.
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- ^ "Return to Orbit". www.hq.nasa.gov.
- ^ The Moon's surface appears red because the only sunlight available is refracted through Earth's atmosphere on the edges of Earth, as shown in the sky in this painting.
- ^ Earth would be at new phase completely dark, except for sunlight refracted through Earth's atmosphere, visible as a reddish ring of light.
- S2CID 121342862.
- ^ a b Mars Archived October 28, 2011, at the Wayback Machine
- ^ Plait, Phil. "Phil Plait's Bad Astronomy: Misconceptions: What Color is Mars?". Bad Astronomy.
- ^ "Why isn't the Martian sky blue like the Earth's". Archived from the original on August 10, 2004. Retrieved April 23, 2005.
- ^ Webster, Guy (June 10, 2014). "Mercury Passes in Front of the Sun, as Seen From Mars". NASA. Retrieved June 10, 2014.
- ^ "Earth and Moon as Viewed from Mars". Earth Observatory. May 8, 2003. Archived from the original on August 6, 2003. Retrieved June 3, 2008. (JPL Horizons shows: 0.9304AU from Earth; Phase 43%; Sun Elongation 43°)
- ^ "Milestones in Space Photography". National Geographic Society. November 11, 2010. Archived from the original on April 1, 2020.
- ^ "Viewing The Earth From Space Celebrates 70 Years". Forbes. August 22, 2016. Archived from the original on August 23, 2016.
- ^ Revkin, Andrew C. (February 6, 2014). "Martian View of Our Pale Dot". The New York Times. Retrieved February 9, 2014.
- ^ St. Fleur, Nicholas (January 9, 2017). "Looking at Your Home Planet from Mars". The New York Times. Retrieved January 9, 2017.
- ^ a b c Bagenal, Fran (2005). "Class 17 – Giant Planets". Laboratory for Atmospheric and Space Physics. Retrieved September 5, 2008.
- ^ Cook, Jia-Rui C.; Gutro, Rob; Brown, Dwayne; Harrington, J. D.; Fohn, Joe (December 12, 2013). "Hubble Sees Evidence of Water Vapor at Jupiter Moon". NASA. Retrieved December 12, 2013.
- ^ "Pre-eclipse of the Sun by Callisto from the center of Jupiter". JPL Solar System Simulator. June 3, 2009. Archived from the original on December 13, 2012. Retrieved June 4, 2008.
- ^ Thommes, Jim. "Jupiter Moon Shadow Transit". Jim Thommes Astrophotography. Retrieved September 3, 2008.
- ^ Pascal, René. "POV-Ray renderings of Huygens descending to Titan". www.beugungsbild.de.
- ^ "Rainbows on Titan". NASA. February 25, 2005. Archived from the original on October 21, 2011. Retrieved October 8, 2011.
- ^ Plait, Phil (March 15, 2012). "BAFact math: How bright is the Sun from Pluto?". Discover Magazine. Archived from the original on July 7, 2015. Retrieved July 6, 2015.
- ^ "New Horizons shows Pluto sporting blue skies and red water ice". New Scientist. Retrieved October 6, 2023.
- ^ "Vmag<6.5". SIMBAD Astronomical Database. Retrieved June 26, 2010.
- ^ Bortle, John E. (February 2001). "The Bortle Dark-Sky Scale". Sky & Telescope. Archived from the original on March 23, 2009. Retrieved November 18, 2009.
- ^ St. Fleur, Nicholas (May 19, 2017). "Spotting Mysterious Twinkles on Earth From a Million Miles Away". The New York Times. Retrieved May 20, 2017.
- S2CID 109930589.
- ^ "Alien Skies: The View from Alpha Centauri". Drew Ex Machina. August 28, 2020. Retrieved February 22, 2023.
- ^ "See the Sun from Other Stars". Sky & Telescope. February 2, 2022. Retrieved February 22, 2023.
- ^ "10 Things: All About TRAPPIST-1". solarsystem.nasa.gov. February 20, 2018. Retrieved April 29, 2023.
Further reading
- Carroll, Michael (2007). Space art : how to draw and paint planets, moons, and landscapes of alien worlds. New York: Watson-Guptil Publications. ISBN 9780823048762.
- Carroll, Michael (2010). Drifting on Alien Winds Exploring the Skies and Weather of Other Worlds. New York, NY: Springer Science+Business Media, LLC. ISBN 9781441969170.
- Barnett, Lincoln (December 20, 1954). "The Starry Universe". Life. pp. 44–64. Retrieved April 12, 2013.
- Perelman, Yakov (2000). "Lunar heavens". Astronomy for entertainment. Honolulu: University Press of the Pacific. pp. 78–84. ISBN 9780898750560.
External links
- Astronauts on the planets
- Essay on the possible sky colours of alien worlds.
- JPL Solar System Simulator
- Phases of Charon as seen from Pluto
- The Starry Universe – Life magazine (December 20, 1954).
- Sunsets simulated on other planets (NASA; June 22, 2020)
- Video (1:06) on YouTube
- Video (2:59) on YouTube
- Video (1:06) on