Tidally Locked Planet

Honestly, it gets a little boring looking at the same side of the Moon all the time. Source

"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, Star Trek: The Next Generation, "The Dauphin"

Tidal locking is the result of a body (a planet around a star or a moon around a planet) being close enough to its parent that the pull of gravity on the satellite is stronger on the facing side than on the other^note This is actually always the case, but it's particularly noticeable if the satellite is very close to the primary or the primary's gravitational field is very intense. Over astronomical timescales, the parent body's gravity will slow the satellite's rotation until one side always faces the parent and the other always faces away.

Because of this mechanism, a planet orbiting a star in this fashion will have The Night That Never Ends on one side while the other will have Endless Daytime. Originally it was thought that the starward side would always be a blazing hot desert and the night side freezing cold. 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. 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.

Also known as a Twilight Planet, in reference to the perpetual twilight experienced by the narrow band between the starward side and dark-side. It is guessed that this narrow band may be capable of supporting life, and is a popular way to make a planet unique. In science fiction most of the population of a tidally locked world will inhabit this region, where the climate is fairly temperate.

Mercury was thought to be a tidally locked planet until the 1960s, so works older than that may refer to it as one. Radar observations from Earth in the mid-1960s and spacecraft images in the 1970s showed that Mercury is actually in 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.

Tidal locking 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.

A variant of this trope concerns non-tidally locked planets that become such in a second time, due to their rotation being altered for whatever reason. This will become a serious problem for their natives, who will suddenly find large portions of their world becoming uninhabitable wastelands. The result is usually an apocalypse on the scale of a full mass extinction; assuming that the problem cannot be addressed at its root, the only options for people caught in such an event are a mass migration to the twilight zone — which still won't be able to support to original population of what was once a fully habitable planet — or, if possible, an exodus offworld.

Compare Single-Biome Planet. The main difference is that a tidally locked world tends to have single biomes over vast stretches of its surface, but not the whole thing. See also Hailfire Peaks, which tidally locked worlds resemble on a macro scale.

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Examples:

Anime and Manga

• Macross Frontier: The planet Galia IV is tidally locked, with a habitable strip around the twilight zone. A human colony was established there, but was later destroyed by the Vajra.

Comic Books

• The DCU:
  • DC One Million: Mercury 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.
  • One story states that 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.
• White Sand's planet Taldain is tidally locked between two stars, probably as a Trojan planet in a Lagrangian point: a blue-white supergiant shines on the Dayside, while a white dwarf within a particle ring bathes the Darkside in ultraviolet. The arrangement is implied to have been designed by someone, presumably the planet's resident Physical God.
• Legends of the Dead Earth: In Detective Comics Annual #9, the planet Typhon revolves around the Binary Suns Osiris and Iris in the Greater Amun system. It is held in a roughly elliptical orbit by the conflicting gravitational forces of the two suns. As such, one side, Sybaris, is in perpetual daylight whereas the other side, Crotona, is in perpetual darkness.
• Yoko Tsuno: The planet Vinea is tidally locked to its sun after a catastrophe. Fortunately the surviving Vineans are very technologically advanced and can create new living spaces artificially.

Fan Works

• A common trope in My Little Pony: Friendship Is Magic fan works is a geocentric take on this trope. The setting's sun and moon rotate around its world under the direction of powerful magical beings, and it's established that, should something happen to the people responsible for the motion of the orbs, they would simply stop moving. Thus, many fanfics explore the question of what would happen if one of these interruptions became permanent.
  • Antipodes: 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.
  • The Frozen West: The story starts in a griffon civilization living along the day/night terminator of this sort of world, bounded at the east by a vast desert and at the west by eternal glaciers, with winds blowing steadily west to east. The main character, originally the son of ice harvesters, becomes an explorer and takes part in an airship expedition to the heart of the unknown western hemisphere. As the ship flies east, it passes through a ring of eternal blizzards, flies over vast fields of ice and under strange lights in the dark sky that no one had seen in the thousand years of recorded civilization, and eventually reaches areas so cold that carbon dioxide begins to condense out of the atmosphere and fall as a snow of dry ice above dead, frozen forests and ruined cities that by conventional wisdom should not even exist...
  • A Long Night: After defeating her sister and taking over Equestria, Nightmare Moon begins The Night That Never Ends 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.
  • The Palaververse: In "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, but by the end Rarity's sacrifice is able to restart the motion of the celestial bodies.
• Heat: Inalap'akis finds the heat of Tatooine a little comforting, being from tidally-locked Ryloth. Her family lived on the sunny side of the terminator, and Khem Val mentions having visited another such planet when he served Tulak Hord.

Films — Live-Action

• In the 1953 Film Serial Commando Cody: Sky Marshal of the Universe, one of the Evil Plans of the Ruler is to turn Earth into this. In "S.O.S. Ice Age", his minions connect a Tractor Beam from Earth's North Pole to Saturn to pull our planet off its axis, freezing one side and boiling the other.
• Star Trek: Nemesis: The Remans evolved on the dark side of tidally-locked Remus, explaining their photosensitivity.
• Upside Down: The two planets that the movie 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.
• What If The Earth STOPS Spinning examines what would befall human civilization if this trope began to be applied to Earth (answer: nothing good).

Literature

• Aeon 14: Building Victoria: 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.
• The Age Of Miracles: Earth is on its way to becoming one. 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.
• Ciaphas Cain: The Traitor's Hand: Adumbria is mostly inhabited in the twilight zone, and its inhabitants have 37 different words for degrees of twilight. (Amberley Vail cites a Fictional Document 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.) 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.
• The City in the Middle of the Night takes place on a tidally locked planet. Humans live in the twilight band while aliens live in the night.
• The Draco Tavern: Chirpsithra are probably the most powerful race out there, but aren't seen much because they only like tide-locked planets orbiting red dwarfs.
• Hothouse:
  • The Earth is locked to the Sun, and is divided between a constant dayside covered in riotous plant growth and a dark, lifeless Nightside.
    Above them, paralyzing half the heavens, burned a great sun. It burnt without cease, always fixed and still at one point in the sky, and so would burn until that day — now no longer impossibly distant — when it burnt itself out.
  • The Moon's tidal locking has itself progressed further over millions of years, so that its orbit now perfectly keeps pace with Earth's day/night cycle. As a result, the Moon floats over one sole area of Earth's surface, making travel to it possible by "traversers", enormous spider-like plants capable of passing through space on silk strands miles long connecting the Earth to the Moon.
• "The Immortals of Mercury", a Clark Ashton Smith short story, describes the planet as tidally locked and inhabited by utterly insufferable Space Elves in vast underground colonies. By coincidence, Mercury's correct day-night cycle was discovered the year after the story was published.
• Isaac Asimov:
  • "The Dying Night": 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. 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.
  • Foundation Series "The Mule": 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.
  • Lucky Starr and the Big Sun of Mercury: Mercury is tidally locked with the sun, creating a day-side and night-side.
  • "Runaround": Donovan and Powell are sent to 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 at the time that Mercury was tidally locked).
• Hal Clement:
  • Fossil, a book in the Isaac's Universe series, is set on a tidally locked ice-covered planet with a circular liquid surface ocean in the middle of the dayside and an ice cap extending over the terminator from the night side. It is called Eyeball, in an early use of the descriptive label.
  • Abyormen in Cycle of Fire is more complicated: The planet is tidally locked to the red dwarf star it orbits, but the orbit is eccentric and the planet's spin axis is inclined. This produces parts of the planet where the red dwarf does not set, parts where it rises and sets, and parts where it never rises. The red dwarf is in turn in an eccentric orbit around a blue giant star, further complicating Abyormen's climate.
• In The Heaven Chronicles by Joan D Vinge, the planet Morningside is tidally locked with most of the humans living along the terminator band.
• Known Space: 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.
• Necroscope: The world where the Wamphyri originate is such a world. Life exists almost entirely on the narrow habitable band.
• The Night's Dawn Trilogy: 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.
• Star Carrier: Deep Space: The Slan 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", which the humans use as a Badass Boast during the final confrontation, helping convince the Slan commander to turn tail in defiance of his orders from the Sh'daar.
• Proxima: Per Ardua is tidally locked to Proxima Centauri.
• Star Trek Expanded Universe:
  • The Romulan Way: 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.
  • Section 31: Rogue: Chairos IV is tidally locked, giving rise to a species that is noted for being extremely robust.
  • Star Trek Twilights End: 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.
• Star Wars Legends: 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.
• One of the novels by Janusz Zajdel has aliens who evolved on the day hemisphere of one of these. Consequently, they cannot survive without light and need protective suits with optical fibre cables connecting them to the central light-producing thing when they explore Earth.
• "The Wall of Darkness", a Arthur C. Clarke short story, 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 Wall Around the World 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.

Live-Action TV

• Alien Worlds (2020): Janus closely orbits a red dwarf, and the star's gravity thus has a strong hold on the planet and keeps one side of it always facing the star. Janus's day side is thus a parched, torrid desert, and its night side a frozen wasteland. Between the two is a thin temperate strip scoured by endless winds blowing between the two extremes. The planet's native pentapods mostly live in the twilight area, where they use the canyons carved by meltwater rivers flowing from the night side to shelter from the high winds, but also exploit the winds to scatter their airborne larvae. Life on the day side is adapted for high temperatures and mostly hides in the shade of rocks, while that on the night side relies on geothermal energy for survival and tends to be bioluminescent.
• The Ark (2023): Proxima b in the series is portrayed this way, although if the real planet is tidally locked or in a more complicated resonance is unknown. William Trust developed a device to spin up the planet. It did not work as planned .
• Extraterrestrial (2005): Aurelia orbits a small, dim red dwarf star, and due to its physical proximity to it — a red dwarf's dim heat and light can only support life on planets very close to it — it's permanently locked onto its sun, always showing it the same side. Its dark side is cold, dark and covered in vast glaciers, while the day side is covered in a permanent storm endlessly lashing it with torrential rains. Between them is a temperate, humid twilight band home to flourishing swamp environments fed by rivers pouring from the storm zone.
• Star Trek: The Next Generation never uses the actual term, but based on its descriptions of two Planets of the Week the trope applies.
  • Dytallix B in "Conspiracy" was a world inhabited only by the Dytallix Mining Company. Due to the temperature extremes on either facing of the planet the company placed its facilities in the twilight region.
  • "The Dauphin" had one distinct culture develop on the day side of Daled IV, and a different one on the night side. Their differences led to a world war that the Enterprise is trying to put an end to.
• The Stargate SG-1 episode "The Broca Divide" has a planet that is tidally locked with its sun so that one side is always light, the other always in darkness. The civilization lives in the light side near the terminator, where it's temperate. A plague that makes humans devolve into Neanderthalesque creatures has broken out, and the infected are banished to the dark side of the planet. Unrealistically, the border region is not in twilight, but has a sharp edge where day instantly turns 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 "Why doesn't the MALP have a set of these?"

Podcasts

• ROGUEMAKER: Tand, the homeworld of the ǵnonw, is tidally locked. The aliens who live there explain that Day and Night are places to them, not times.

Tabletop Games

• 2300 AD: Several planets in human space are tidally-locked, probably most notably Aurore, which is featured in the introductory module.
• Godforsaken: Korak-Mar orbits a white dwarf star in a close orbit, and as such is tidally locked. The region that the players can access trough the gateway to Bontherre is around the terminator line, and as such experiences a perennial dim twilight.
• Lancer: Harrison Armory'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.
• Pathfinder and Starfinder's setting has the planet Verces, where the habitable central zone is settled by a union of bio-augmented transhumanists, predominantly pastoral Pure Ones, and God-Vessels. The sun-blasted Fullbright and the ice flats of the Dark Side are largely avoided as Death Worlds, since most creatures that can survive the temperature extremes are serious bad news for any traveler.
• Space 1889: Mercury is tidally locked, in keeping with the game's theme of "the planets are as they were once believed to be", giving rise to remarkable features and strange life-forms. A Challenge adventure adds that it is "nodding" a little, though^note this is called "libration", and our Moon does it in real life — see below.
• Traveller: In Double Adventure 2 "Across the Bright Face", the planet Dinom is an interesting variation on this. Its north pole points toward its star, so it looks like it's on its side. Its northern continent is always in sunlight (up to 260 degrees Celsius) and its southern continent in darkness (and goes almost as low as absolute zero). Between the north and south continents there's a temperate zone where life can exist.
• Warhammer 40,000: Mordia is a hive world and a death world, where the sun side is only inhabited by mutants and chaos cultists, while the rest of population lives on the night side of the planet. It's the homeworld of the Imperial Guard's Mordian Iron Guard regiments.

Video Games

• Battleborn: Tempest the throneworld of the Jennerit Imperium is an artificially tidally-locked planet, set in orbit around the star, Solus.
• Body Blows: The home planet of two characters introduced in Body Blows Galactic named Inferno and Warra.
• Borderlands: The setting of the series, Pandora, is apparently tidally locked according to Word of God, 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.
• Doom: The original design document, 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".
• Earthlock: Festival of Magic is set on such a planet, hence the game's name. Most of civilization exists on the terminator region, between the day and night sides, where the temperature is better suited for humans and other races to live in. Nearly all plant life is in this area too. Otherwise, the day side is a desert with the central area being so hot that the characters cannot withstand the heat for more than about 20 seconds; and the night side is extremely cold where characters run a similar risk of freezing to death. The planet was not always like this—it's one of the big mysteries of the game as to what happened, though it's been tidally locked for long enough for everyone alive during the events of the game to take it for granted and not even question it.
• 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.
• Kerbal Space Program: 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).
• Mass Effect
  • The Shepard Trilogy games note in the flavor text for numerous planets that they're tidally locked.
  • Mass Effect: Andromeda: Elaaden 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 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.
• Metroid Prime 3: Corruption: Planet Bryyo has this characteristic. According to the lore data, 48% of the planet's surface is in perpetual daytime (thus very hot), 48% is in perpetual nightime (thus very cold), and the remaining 4% has a temperate climate that allows the existence of cliffs and jungles. Interestingly, the tidal lock was a result of a planet-shattering war that decimated the population and left the survivors as primal savages, as opposed to any gravitational influence from Bryyo's sun.
• Prismata takes place on a tidally locked planet called Beacon. Humans live on the daylight side, while robots live on the night side.
• Stellaris: Some habitable planets 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.
• Tales of Arise: The planets Dahna and Rena, being a dual-planet system, are tidally locked to each other as expected, never moving or rotating in each other's skies. The fact that people on Dahna and the artificial satellite Lenegis can only see one face of Rena prevents anyone from discovering that the planet was torn apart long ago.

Web Original

• Life Around a Red Dwarf: 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 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.
• Orion's Arm classifies these planets as 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.
• RWBY has a notable subversion in that Remnant's moon is not tidally locked, which is noticeable because one half of it is shattered. How much of the shattering is visible depends on the time, with a fully shattered moon being considered "full moon".

Western Animation

• Ben 10: Ultimate Alien: Mykdl'dy is a tidally locked planet with one side burning, and the other side frozen.
• Futurama: A variant — 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, and 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.
• My Little Pony: Friendship Is Magic: The unnamed[[note:Commonly called Equus by the fanfic community]] planet Equestria is on has a day/night cycle that is artifically controlled by Celestia and Luna and doesn't change on its own. It's later revealed that this isn't an ability of them specifically but something regular unicorns can learn, although little is actually known.

Real Life

• 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.) Our solar system has nineteen natural satellites massive enough to become round under their own gravity, and every single one of them is tidally locked to its primary.^note Earth has the Moon. Jupiter has Io, Europa, Ganymede, and Callisto. Saturn has Titan, Rhea, Enceladus, Mimas, Tethys, Iapetus, and Dione. Uranus has Titania, Oberon, Ariel, Umbriel, and Miranda. Neptune has Triton. Pluto has Charon.
• 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. (Because the Moon's orbit is eccentric, it actually wobbles a tiny little bit on its axis. This means we can see about 59% of the surface from Earth at one time or another. 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.
• Tidal friction applies to all bodies orbiting each other (the universe doesn't care about our definition of "planet"). However, with planets usually being both more massive and further away from their primary, this takes a very, very, very long time. Nobody knows yet how long it will take for the Earth, but several times the current age of the universe is a bare minimum^note Since it's likely the Earth will be destroyed by the Sun when it goes red giant-mode, this may be moot after all. However some have proposed Earth and Mars could end up tidally locked to the red giant Sun just before it reached its maximum size and destroyed our planet.
• Pluto's largest moon, Charon, is tidally locked to it 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.
• Most planets orbiting very close to their stars^note An important fraction of those discovered by us 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. 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 eyeball planet, covered in ice except for an ocean in the substellar point◊^note The point of the planet that always has the star in the zenith. A planet close to its star would 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.

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