History Main / TidallyLockedPlanet

->''"One side [of Daled IV] 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."''

[[https://en.wikipedia.org/wiki/Tidal_locking 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[[/note]]. 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.

* ''TabletopGame/{{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[[/note]].
* Classic ''TabletopGame/{{Traveller}}'' 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.

* 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”[[/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.

* An implied trope in ''Series/DoctorWho'' 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.

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.

* 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, at adequate distance of its star if it's dim enough you could have an [[http://www.space.com/20856-alien-planets-eyeball-earths.html eyeball Earth]], with [[http://www.orionsarm.com/im_store/eyeball.jpg an ocean in the subsolar point]][[note]]The point of the planet that always has the Sun in the zenith[[/note]], or [[https://getpocket.com/explore/item/forget-earth-like-we-ll-first-find-aliens-on-eyeball-planets a planet]] where the always illuminated side would be a desert, the one in perpetual darkness covered in ice, and most life and liquid water being concentrated in the terminator. This is 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.

* In the science-fiction short story [[Literature/ScienceFiction101 "Hothouse"]], the Moon's tidal locking has progressed further over millions of years, to the point that its orbit now perfectly keeps pace with Earth's day/night cycle[[note]]An alternative way of phrasing this is that the Earth and Moon are tidally locked to each other, which does actually happens in real life; Charon and Pluto are mutually tidally locked[[/note]]. As a result, the Moon floats over one sole area of Earth's surface, making travel to it much easier by "traversers", enormous spiders capable of passing through space on silk strands miles long connecting the Earth to the Moon.

* 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 gravitation influence from Bryyo's sun.

* 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 left the survivors as primal savages.

* ''Earthlock: Festival of Magic'' is set on such a planet, hence the game's name. Most of civilization exists on the terminal 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 [[ShiftingSandLand desert]] with the central area being so hot that [[ThirstyDesert the characters cannot withstand the heat for more than about 20 seconds]]; and the night side is [[SlippySlideyIceWorld 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.

* Supplementary material for ''Film/StarTrekNemesis'' indicates that the Remans evolved on the dark side of tidally locked Remus, explaining their photosensitivity.

* The two planets that ''Film/UpsideDown'' is set on are tidally locked to eachother to the point where a skyscraper bridges them with gravity flipping halfway. They seem to orbit eachother to generate a day/night cycle.

* ''TabletopGame/{{Pathfinder}}'' and ''TabletopGame/{{Starfinder}}'''s Golarion 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.

* Jinx in Creator/LarryNiven's ''Literature/KnownSpace'' series. It's 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.

** The 1956 sci-fi murder mystery "Literature/TheDyingNight" [[http://www.e-reading.co.uk/chapter.php/82002/10/Azimov_-_Asimovs_Mysteries.html]] used the [[ScienceMarchesOn then-current scientific knowledge]] that Mercury was tidally locked to the Sun as a major plot point. The killer had lived on Mercury's dark side and forgot that Earth had a normal night and day cycle. After astronomers found out that Mercury ''did'' have a conventional day and night (albeit very long), Asimov mentioned in the author's notes of later printings that he'd wanted to fix the problem but couldn't figure out how to do it without rewriting the whole plot, [[PlayedForLaughs and the scientists should've gotten it right to begin with]].
** Radole in the ''Literature/{{Foundation}}'' universe: most of it 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, as planets with such a narrow habitable strip are commonplace enough to have earned the nickname of "ribbon worlds".

* In the ''[[Literature/TheDracoTavern Draco Tavern]]'' 'verse, the Chirpsithra are probably the most powerful race out there, but aren't seen much because they only like tide-locked planets orbiting red dwarfs.

* 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, at adequate distance of its star if it's dim enough you could have an [[http://www.space.com/20856-alien-planets-eyeball-earths.html eyeball Earth]], with [[http://www.orionsarm.com/im_store/eyeball.jpg an ocean in the subsolar point]][[note]]The point of the planet that always has the Sun in the zenith[[/note]], or [[https://getpocket.com/explore/item/forget-earth-like-we-ll-first-find-aliens-on-eyeball-planets a planet where the always illuminated side would be a desert, the one in perpetual darkness covered in ice, and most life and liquid water being concentrated in the terminator. This is 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.

* 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 [[TimeAbyss 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 could end up tidally locked to the red giant Sun just before its destruction[[/note]].

* 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, at adequate distance of its star if it's dim enough you could have an [[http://www.space.com/20856-alien-planets-eyeball-earths.html eyeball Earth]], with [[http://www.orionsarm.com/im_store/eyeball.jpg an ocean in the subsolar point]][[note]]The point of the planet that always has the Sun in the zenith[[/note]]. This is 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.

->''"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."''

* Charlie Jane Anders' 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 1956 sci-fi murder mystery "Literature/TheDyingNight" [[http://www.e-reading.co.uk/chapter.php/82002/10/Azimov_-_Asimovs_Mysteries.html]] used the [[ScienceMarchesOn then-current scientific knowledge]] that Mercury was tidally locked to the Sun as a major plot point. The killer had lived on Mercury's dark side and forgot that Earth had a normal night and day cycle. After astronomers found out that Mercury ''did'' have a conventional day and night (albeit very long), Asimov mentioned in the author's notes of later printings that he'd wanted to fix the problem but couldn't figure out how to do it without rewriting the whole plot.

Because of this mechanism, a planet orbiting a star in this fashion will always be daytime on one side of the planet and always night on the other. 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.

* 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 [[TimeAbyss 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[[/note]].

* 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. (This shift caused, 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.

* In ''VideoGame/KerbalSpaceProgram'', 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.

* ''ComicBook/LegendsOfTheDeadEarth'': In ''ComicBook/DetectiveComics'' 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.

* 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 '65 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.)

* In the science-fiction short story [[Literature/ScienceFiction101 "Hothouse"]], the Moon's tidal locking has progressed further over millions of years, to the point 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 much easier by "traversers", enormous spiders capable of passing through space on silk strands miles long connecting the Earth to the Moon.