History Main / TidallyLockedPlanet

* This includes the Moon, which is locked to Earth. As a result, until the Space Age, nobody on Earth knew what the far side looked like. (It actually wobbles a tiny little bit on its axis, meaning we see slightly more than 50% of the surface, but not much. [[http://apod.nasa.gov/apod/ap120205.html Animation and explanation]]) Incidentally, this is what's usually meant by the phrase "the dark side of the Moon". It means the "invisible" side which doesn't move, not the ''unlit'' side which shifts to create the Moon's phases.
* Pluto's largest moon, Charon, is tidally locked to its planet as would be expected, but the size difference is so small compared to similar systems that Pluto is also tidally locked to Charon. In other words, Charon never moves in Pluto's sky, assuming you are on the part of Pluto where you can see Charon. For this reason, some astronomers argue that Pluto and Charon's tidal lock being mutual makes Charon not a moon of Pluto, but a dwarf co-planet '''with''' Pluto.

Mercury was thought to be a tidally locked planet until the 1960s, so works older than that [[ScienceMarchesOn may refer to it as one]]. Radar observations from Earth in the mid-1960s and by spacecraft in the 1970s showed that Mercury is actually a more complicated state where it rotates 3 times for every 2 times that it goes around the Sun; so that opposite hemispheres see the Sun on each orbit. Any given spot on Mercury sees one passage of the sun across the sky, one local day, every two local years.

Mercury was thought to be a tidally locked planet until the 1960s, so works older than that [[ScienceMarchesOn may refer to it as one]]. Observations from Earth and by spacecraft in the 1970s showed that Mercury is actually a more complicated state where it rotates 3 times for every 2 times that it goes around the Sun; so that opposite hemispheres see the Sun on each orbit.

This is thought to be an especially likely scenario for planets orbiting red dwarf stars. Since red dwarfs are much cooler and dimmer than other stars, any planets orbiting them would need to be very close to their sun to be habitable. Thus, while tidally-locked planets around other stars are generally too hot to host any kind of life, many or most livable worlds around red dwarfs would instead likely be locked in this manner.

* As noted in Literature, until 1965 Mercury was believed to be tidally locked to the Sun, and several major science fiction authors wrote stories featuring this. In 1965, radar measurements revealed that the planet actually rotated three times every two orbits. (The combination of motions from rotation and revolution means that an observer on Mercury would see one passage of the sun across the sky, one local day, every two local years.)
* Most, if not all, of the planets orbiting very close to their stars[[note]]An important fraction of those discovered by us[[/note]] are expected to be tidally locked. While that sounds like ''very'' bad news in terms of habitability, there are situations where a tidally locked planet could be habitable. [[https://getpocket.com/explore/item/forget-earth-like-we-ll-first-find-aliens-on-eyeball-planets The most obvious option]] has the always-illuminated side be a desert, the one in perpetual darkness covered in ice, and most life and liquid water being concentrated in the terminator between the two sides. A planet far from its star might be an [[https://en.wikipedia.org/wiki/Eyeball_planet eyeball planet]], covered in ice except for [[http://www.orionsarm.com/im_store/eyeball.jpg an ocean in the substellar point]][[note]]The point of the planet that always has the star in the zenith[[/note]]. A planet close to its star would [[https://arxiv.org/abs/1404.4992 instead]] develop thick clouds on its day side that reflect enough light to keep it relatively cool. These situations are particularly relevant for red dwarf stars: because they are dim, a planet has to be close to be in the "Goldilocks zone" (allowing liquid water) which in turn is close enough to expect tidal locking.

* ''Series/DoctorWho'': Implied when the Ninth Doctor says "Lots of planets have a north." The only planets that don't have a north are tidally locked and don't have a magnetic pole.

* All large moons are (or will eventually be, if they are young) tidally locked to the planet they orbit. This does not affect the surface conditions / climate in the same manner as a tidally locked planet, as the moon then experiences one day-night cycle each orbit. (Smaller, irregular moons may or may not be tidally locked, depending on their size and distance.)

->''"What I meant, sir, is that Daled IV rotates only once per revolution. Therefore one side is constantly dark, and the other side constantly light. One might surmise that the two hemispheres have developed disparate cultures, which is a major cause of most wars."''
-->-- '''Lt. Cdr. Data''', [[Recap/StarTrekTheNextGenerationS2E10TheDauphin "The Dauphin"]], ''Series/StarTrekTheNextGeneration''

* ''Series/StargateSG1'' had an example in a first-season episode called "The Broca Divide." A planet was tidally locked with its sun so one side was always light, the other always in darkness. The civilization lived in the light side near the terminator, where it was temperate. A plague that made humans devolve into Neandertalesque creatures had broken out, and the infected were banished to the dark side of the planet. Unrealistically, the border region was ''not'' in twilight, but had a sharp edge where day instantly turned into night. It also leads to a funny moment when the MALP can't see anything on the far side because its lamps broke while exiting the gate, which is on the dark side of the planet. SG-1 and SG-3 go through wearing night-vision goggles, and Jack sarcastically wonders [[LampshadeHanging "Why doesn't the MALP have a set of these?"]]

* Charlie Jane Anders' ''Literature/TheCityInTheMiddleOfTheNight'' takes place on a tidally locked planet. Humans live in the twilight band while aliens live in the night.
* In ''Franchise/StarWarsLegends'' the Twi'lek homeworld Ryloth has the sunward side an uninhabitable desert and the night side freezing cold. The Twi'leks mostly live on the terminator (and build their cities underground) and use exile to the sunward side as a form of capital punishment. The planet also is sometimes affected by "heat storms", cyclones of superheated air from the sunward side that can reach 300 degrees centigrade and wind speeds of 500 kilometers per hour.
* Adumbria in ''Literature/CiaphasCain: The Traitor's Hand'' is mostly inhabited in the twilight zone, and its inhabitants have [[LanguageEqualsThought 37 different words for degrees of twilight]]. (Amberley Vail cites a FictionalDocument titled ''Sablist in Skitterfall'' whose title derives from this. Witty wordplay to an Adumbrian, nonsensical to an offworlder[[note]]a close analogy would be something like “Midnight at Sunset” with 'Sunset' being the name of the location[[/note]].) Cain's Valhallan 597th are from an ice world and are assigned to the perpetual winter of the night side, while the Tallarn 229th, from a desert world, cover the sunward side.
* Creator/LarryNiven's ''Literature/KnownSpace'': Jinx is a moon tide-locked to its gas giant primary, and is so distorted by tidal forces that its prime meridian is buried under piled-up atmosphere and its near and far "poles" actually rise into space. It also has very high gravity. The colonists live in two bands midway between the two, put most of their heavy industry in the vacuum region, and send tank safaris into the meridian to hunt the giant bandersnatchi that live there.
* Creator/IsaacAsimov:
** "Literature/TheDyingNight": {{UsefulNotes/Mercury}} is tidally locked to the Sun, and this becomes a major plot point. The killer had lived on Mercury's northern pole for ten years, forgetting the normal night/day cycle of Earth. [[ScienceMarchesOn After astronomers found out Mercury wasn't tidally locked]], Dr Asimov said in an author's note that he'd wanted to fix it, but couldn't figure out how to do it without rewriting half the plot.
** ''Literature/FoundationSeries'' "Literature/TheMule": Radole is uninhabitable, apart from a few areas on the terminator. The capital city is in the largest such area, where conditions resemble a warm June morning on Earth. Possibly one of many such planets with a narrow habitable strip, because they are commonplace enough to have a nickname; "ribbon worlds". Radole hosts a meeting of Foundation citizens from the independent Trader worlds who wish to revolt against the tyrants of Terminus and the Four Kingdoms.
** ''Literature/LuckyStarrAndTheBigSunOfMercury'': In this story, {{UsefulNotes/Mercury}} is tidally locked with the sun, creating a day-side and night-side.
** "{{Literature/Runaround}}": Donovan and Powell are sent to {{UsefulNotes/Mercury}} to try reopening the "Sunside" mining operation. The story was premised on the conflict that the fields keeping the humans from dying would fail before sunset (since it was common scientific knowledge [[ScienceMarchesOn at the time]] that Mercury was tidally locked).

* In ''Literature/{{Proxima}}'', Per Ardua is tidally locked to Proxima Centauri.
* ''Franchise/StarTrekExpandedUniverse'':
** In ''Literature/StarTrekTwilightsEnd'', the planet Rimilla is one such world, until a daring plan comes up with a way to give it a standard rotation, thus enabling colonization over the entire planet.
** In ''Literature/TheRomulanWay'' the planet ch'Havran (a.k.a. Remus) is tidally locked to neighboring ch'Rihan (Romulus). Though oddly there's no indication that the reverse is true.
** In the novel ''Literature/Section31Rogue'', Chairos IV is tidally locked, giving rise to a species that is noted for being extremely robust.

* In ''Literature/TheNightsDawnTrilogy'', the Ly-cilph home planet is a hybrid case: it's a moon tidally locked to the planet it orbits; it thus experiences one solar day with respect to the primary star for every orbit around the planet. But it orbits a young, hot "super-Jupiter" (bordering on being a brown dwarf) which glows in the near infrared and red. This gives rise to a less extreme version of the climate duality experienced by planets that are tidally locked to their stars. The nearside biome is dominated by plants that exploit the always present red light of the planet; the farside has plants adapted to use just the yellow light of the primary star, with long nights.
* The Slan in ''Literature/StarCarrier: Deep Space'' evolved in caverns underneath the day side of a tidally locked world. They "see" by echolocation, with the closest thing they have to actual eyes being light-sensitive organs on stalks to keep them from accidentally wandering out onto the surface. Their version of capital punishment is to be "sent into the light", [[spoiler:which the humans use as a BadassBoast during the final confrontation, helping convince the Slan commander to turn tail in defiance of his orders from the Sh'daar]].
* Creator/ClarkAshtonSmith's short story "The Immortals of Mercury" describes the planet as tidally locked and inhabited by [[ScrewYouElves utterly insufferable]] SpaceElves in [[BeneathTheEarth vast underground colonies]]. By coincidence, Mercury's correct day-night cycle [[ScienceMarchesOn was discovered]] the year after the story was published.
* Earth is on its way to becoming one in the novel ''Literature/TheAgeOfMiracles'' by Karen Thompson Walker. An event called the Slowing begins, and days and nights become longer. At first, it's only barely noticeable, a few extra minutes. But then it goes to hours and then days and by the later part of the book, each day and night is lasting weeks. It's indicated that eventually, the rotation will stop altogether and Earth will effectively be tidally locked with the sun. Some people try to adapt to the new cycles, while others try to keep going on regular 24 hour cycles. The protagonist is sure by the end of the book that civilization is nearing its end and humanity will eventually die.
* Creator/ArthurCClarke's short story "The Wall of Darkness" is set on a planet whose one side is permanently oriented towards the sun while the other is in permanent darkness and seperated by a mysterious WallAroundTheWorld as well, with the planet's orbital inclination being the only thing that give its inhabitants a sense of time passage. The protagonist's lifelong ambition is to find out what secrets lie on the dark side.
* The world where the Wamphyri originate in the ''Literature/{{Necroscope}}'' series is such a world. Life exists almost entirely on the narrow habitable band.
* Victoria in ''Literature/Aeon14: Building Victoria'' is a super-Earth (a rocky planet larger than Earth) tidally locked to a red dwarf, Kapteyn's Star. The habitable zone of a red dwarf is quite close in, so that makes sense.

* In ''VideoGame/KerbalSpaceProgram'', the Mun is tide-locked to Kerbin, Duna and Ike are locked to each other, and all five of Jool's moons (Laythe, Vall, Tylo, Bop, and Pol) are locked to their primary. The only moons in stock KSP that ''aren't'' tidally locked to their primaries are Minmus (Kerbin's small, far-out second moon) and Gilly (a tiny moon in an eccentric orbit of Eve).
* Planet Bryyo in ''VideoGame/MetroidPrime3Corruption'' has this characteristic. According to the lore data, 48% of the planet's surface is in perpetual daytime ([[LethalLavaLand thus very hot]]), 48% is in perpetual nightime ([[SlippySlideyIceWorld thus very cold]]), and the remaining 4% has a temperate climate that allows the existence of [[DeathMountain cliffs]] and [[JungleJapes jungles]]. Interestingly, the tidal lock was a result of a [[TheMagicVersusTechnologyWar planet-shattering war]] that decimated the population and [[FormerlySapientSpecies left the survivors as primal savages]], as opposed to any gravitational influence from Bryyo's sun.
* ''Franchise/MassEffect''
** The Shepard Trilogy games note in the flavor text for numerous planets that they're tidally locked.
** Elaaden in ''VideoGame/MassEffectAndromeda'' is a large moon locked to both the gas giant it orbits and another nearby moon. As a result the sun never sets for the playable area, making it a barely habitable desert (except for the [[SuperToughness krogan]], who can live practically anywhere). As to why no-one goes to the twilight zone, it's because while the desert area is hellish, it's also where the only water supply on the planet is.
* In ''VideoGame/{{Battleborn}}'', Tempest the throneworld of the Jennerit Imperium is an artificially tidally-locked planet, set in orbit around the star, Solus.
* In ''VideoGame/{{Evolve}}'', the planet Shear was going to be one of these but was changed later in development as the writer felt it added nothing of value to the narrative.
* The setting of the ''{{VideoGame/Borderlands}}'' series, Pandora, is apparently tidally locked according to WordOfGod, but it's an odd case. For one thing, its habitable zone is not the twilight band, but the night side, and this zone does seem to shift very slowly (causing, for example, the area near T-Bone Junction to change from a vast ocean to a desert). Second, the planet experiences pseudo-seasons due to its very eccentric orbit - never mind that tidal locking and eccentric orbits are not compatible, as it requires too much variation in rotation speed. And finally, Pandora has a false day-night cycle caused by the rotation of the moon, which radiates intense heat from one side only, and its phases. While Pandora's moon Elpis is not locked and rotates quite rapidly, the Helios space station that hangs above Elpis is locked to a position that always faces Pandora, so it can always fire "moonshot" deliveries to the planet surface.

* The story of ''VideoGame/{{Prismata}}'' takes place on a tidally locked planet called Beacon. Humans live on the daylight side, while robots live on the night side.
* The [[WhatCouldHaveBeen original design document]] for ''VideoGame/{{Doom}}'', the "Doom Bible", mentions that the game was originally supposed to take place on Tei Tenga, a tidally-locked world of some variety (the doc is inconsistent, calling it both a moon and a planet at separate points, though no mention of a primary body and gravitational physics suggest it would have to be a planet with no particularly-large moons). Not much was written about it before the game was shifted to take place on Mars' moons, other than that the UAC has a presence on it to excavate a combustible rock-like substance from the poles called "Fire Dust".

* Some habitable planets in ''VideoGame/{{Stellaris}}'' may be found to be tidally locked after being surveyed by science ships. Although still habitable, these will have a lower cap on the total number of districts that can be built due to the limited area suitable to live in compared to similar planets.

* In ''WesternAnimation/Ben10UltimateAlien'', Mykdl'dy is a tidally locked planet with one side burning, and the other side frozen.
* A variant in ''WesternAnimation/{{Futurama}}''-- where the planet Thuban 9 gradually began to lose rotation, and thus became half-burning, half-freezing. The cat beings who inhabited the planet then selected Earth to siphon gravitational energy, building the Great Pyramid of Giza to do so. However, the technology was lost and the cats became domesticated, thus it took the leader of the cats stealing Amy's idea for a perpetual motion machine utilizing Earth's rotation-powered magnetic field for it to work.

* The world where the Wamphyri originate in the Necroscope series is such a world. Life exists almost entirely on the narrow habitable band.
* Victoria in ''Literature/{{Aeon 14}}: Building Victoria'' is a super-Earth (a rocky planet larger than Earth) tidally locked to a red dwarf, Kapteyn's Star. The habitable zone of a red dwarf is quite close in, so that makes sense.

* ''WebOriginal/OrionsArm'' classifies these planets as [[https://www.orionsarm.com/eg-article/4922e224a740d Vesperian]]. They are quite varied, for example Bullseye has a semi-permanent hurricane on its day side, while Yanqiu is covered in ice except for a patch of ocean on its day side.

* In Creator/PeterFHamilton's ''Literature/NightsDawnTrilogy'', the Ly-cilph home planet is a hybrid case: it's a moon tidally locked to the planet it orbits; it thus experiences one solar day with respect to the primary star for every orbit around the planet. But it orbits a young, hot "super-Jupiter" (bordering on being a brown dwarf) which glows in the near infrared and red. This gives rise to a less extreme version of the climate duality experienced by planets that are tidally locked to their stars. The nearside biome is dominated by plants that exploit the always present red light of the planet; the farside has plants adapted to use just the yellow light of the primary star, with long nights.

* Tand, the homeworld of the ǵnonw in ''Podcast/{{ROGUEMAKER}}'', is tidally locked. The aliens who live there explain that Day and Night are places to them, not times.

* In ''TabletopGame/TwentyThreeHundredAD'' several planets in human space are tidally-locked, probably most notably Aurore, which is featured in the introductory module.

* ''TabletopGame/{{Pathfinder}}'' and ''TabletopGame/{{Starfinder}}'''s setting has the planet Verces, where the habitable central zone is settled by a union of bio-augmented {{transhumanis|m}}ts, [[SpaceAmish predominantly pastoral]] Pure Ones, and [[ReligionIsMagic God-Vessels]]. The sun-blasted Fullbright and the ice flats of the Dark Side are largely avoided as {{Death World}}s, since most creatures that can survive the temperature extremes are serious bad news for any traveler.

* ''TabletopGame/{{Lancer}}'': Megacorp/HarrisonArmory's home world Ras Shamra is this. The thin temperate zone along the terminator has been turned into a gigantic arcology where the population lives. The extreme temperatures of the day and night side are used for combat training and weapon testing.

* ''Fanfic/{{Antipodes}}'': An unusual fantasy version. Equestria became the geocentric equivalent of this after Celestia and Luna, who had been responsible for moving the sun and moon, vanished ten thousand years before the story's events. This left the sun permanently stuck over one hemisphere and the moon over the other, causing one half of the world to become a barren desert hot enough to kill anything on the surface while the other froze over. A few communities survived in the twilight zone, while those stranded outside of it either hid themselves underground to escape the worst of the elements or perished.

* ''[[https://www.fimfiction.net/story/519465/a-long-night A Long Night]]'': A fantasy variant. After defeating her sister and taking over Equestria, Nightmare Moon begins TheNightThatNeverEnds by keeping the moon permanently over Equestria and the sun on the far side of the planet. As a result of this, Equestria's hemisphere promptly begins to freeze, while the opposite one experiences steadily rising temperatures. Nightmare Moon refuses to do anything about this at first, and by the time she wakes up the facts the enchantments that had made the sun and moon mobile at all have decayed into uselessness. Eventually, all other living things in Equestria either perish or make the long trip to the distant twilight zone, which is believed to still be habitable despite being scoured by strong winds, leaving Nightmare Moon to rule her empty kingdom.
* ''Fanfic/ThePalaververse'': In "[[https://www.fimfiction.net/story/240152/1/the-motion-of-the-stars/the-motion-of-the-stars The Motion of the Stars]]", the aftermath of the cataclysmic war between Equestria and the Capra-Corva alliance results in the death of the Princesses and the ceasing of the sun and moon's motion in the sky. Equestria is left in an eternal dusk, and the lands to its east in eternal night; alongside the devastation of the war itself, the death of most growing things and the frantic migrations towards the daylit regions result in a total collapse of civilization. By the story's time, Equestria has become a twilit wasteland, and only a trickle of survivors still comes now and again from the night in the east, [[spoiler:but by the end Rarity's sacrifice is able to restart the motion of the celestial bodies]].

[[quoteright:300:[[Wiki/{{Wikipedia}} https://static.tvtropes.org/pmwiki/pub/images/tidal_locking_wikipedia_5380.png]]]]

[[folder:Film]]

Because of this mechanism, a planet orbiting a star in this fashion will be AlwaysNight on one side while the other will have EndlessDaytime. Originally it was thought that the starward side would always be a blazing hot desert and the night side freezing cold. [[ScienceMarchesOn More recent computer models]] indicate that, assuming the planet ''has'' an atmosphere, convection currents will transfer hot air from the day side to the night side and bring cold air to the day side, alleviating the extremes somewhat. [[https://arxiv.org/abs/1404.4992 It's also been suggested]] that thick cloud cover would build up on the day side of tidally locked planets, reflecting much of the sunlight and keeping the day side relatively cool.

* Mercury in ''Comicbook/DCOneMillion'' has apparently been engineered to be tidally locked, just as people used to think it was. "The planet that's too busy to spin" is the hub of the 853rd century's all-important data network, with solar panels on the light side powering supercooled processors on the dark side.
* Another DC story states Krypton used to be tidally locked, with early civilisations from both light and dark sides warring over scarce resources from the habitable twilight zone. Eventually both decided to have one last battle with elected champions to determine who would have sole rights to the land. At one point both warriors became disarmed, their weapons of unique metals in contact with each other. They interacted to generate a force that rotated the boulder they rested upon. Duly inspired they reported this phenomenon back to their people, who then mined as much of if not all their respective metals as possible and dumped them together into a chasm. This had the desired effect of causing Krypton to rotate, shifting the ecosystem to something more hospitable for everyone.

* The planet Vinea in ''ComicBook/YokoTsuno'' is tidally locked to its sun after a catastrophe. Fortunately the surviving Vineans are very technologically advanced and can create new living spaces artificially.

* The Remans in ''Film/StarTrekNemesis'' evolved on the dark side of tidally-locked Remus, explaining their photosensitivity.
* A speculative documentary, ''Film/WhatIfTheEarthSTOPSSpinning'', examines what would befall human civilization if this trope began to be applied to Earth (answer: nothing good).
* The two planets that ''Film/UpsideDown'' is set on are tidally locked to each other to the point where a skyscraper bridges them with gravity flipping halfway. They seem to orbit each other to generate a day/night cycle.

This is thought to an especially likely scenario for planets orbiting red dwarf stars. Since red dwarfs are much cooler and dimmer than other stars, any planets orbiting them would need to be very close to their sun to be habitable. Thus, while tidally-locked planets around other stars are generally too hot to host any kind of life, many or most livable worlds around red dwarfs would instead likely be locked in this manner.

* ''Series/StargateSG1'' had an example in a first-season episode called "The Broca Divide." A planet was tidally locked with its sun so one side was always light, the other always in darkness. The civilization lived in the light side near the terminator, where it was temperate. A plague that made humans devolve into Neandertalesque creatures had broken out, and the infected were banished to the dark side of the planet. Unrealistically, the border region was ''not'' in twilight, but had a sharp edge where day instantly turned into night.

* ''TabletopGame/Lancer'': Megacorp/HarrisonArmory's home world Ras Shamra is this. The thin temperate zone along the terminator has been turned into a gigantic arcology where the population lives. The extreme temperatures of the day and night side are used for combat training and weapon testing.

* In ''Franchise/StarWarsLegends'' the Twi'lek homeworld Ryloth has the sunward side an uninhabitable desert and the night side freezing cold. The Twi'leks mostly live on the terminator (and build their cities underground) and use exile to the sunward side as a form of capital punishment.

* A speculative documentary, ''What If The Earth [=STOPS=] Spinning?'', examines what would befall human civilization if this trope began to be applied to Earth (answer: nothing good).

[[folder:Web Original]]
* ''WebVideo/LifeAroundARedDwarf'': Nusku, Mazu, and Ullr all have this in common, as they orbit around the red dwarf Roseus with one side always facing the star. The Season 1 Episode 3 [[https://www.youtube.com/watch?v=0q6jkDc4wmY video]], "Adaptations to Climate Zones", specifically deals with how the different light levels would influence the development of biomes on Nusku. Among other things, terrestrial plants first emerge in the "pupil", the region that gets the most direct sunlight.
* ''WebOriginal/OrionsArm'' classifies these planets as [[https://www.orionsarm.com/eg-article/4922e224a740d Vesperian]]. They are quite varied, for example Bullseye has a semi-permanent hurricane on its day side, while Yanqiu is covered in ice except for a patch of ocean on its day side.
[[/folder]]