History Main / TidallyLockedPlanet

* 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.

[[https://en.wikipedia.org/wiki/Tidal_locking Tidal locking]] is the result of a body (a planet around a sun 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. 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.

* ''Franchise/MassEffect'' notes in the flavor text for numerous planets that they're tidally locked.
** Hagalaz, the location of the eponymous lair in the ''Lair of the Shadow Broker'' DLC, is one such planet. It features the convection mentioned in the introduction, with a twist: when warm and cold air collide in that kind of quantity the result is ''massive, never-ending thunderstorms''. The Lair is actually a CoolAirship which rides inside the storms to disguise itself.

** The phenomenon isn't strictly limited to moons, as 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.

* In the ''Franchise/StarWarsExpandedUniverse'' 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.

* In the ''Franchise/StarTrekExpandedUniverse'' novel ''[[Literature/StarTrekTwilightsEnd Twilight's 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.

* ''TabletopGame/{{Space 1889}}'' Mercury is tidally locked in the game, 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]].

* ''TabletopGame/{{Space 1889}}'' Mercury is tidally locked in the game, 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 is an actual thing with tide-locked bodies, including our own Moon[[/note]].

* In ''VideoGame/KerbalSpaceProgram'', Mun is tidally locked to Kerbin, Duna and Ike are mutually tidally locked, and Laythe, Vall, Tylo, Bop, and Pol are tidally locked to Jool.

* ''TabletopGame/{{Space 1889}}'' Mercury is tidally locked in the game giving rise to remarkable features and strange life-forms. A Challenge adventure adds that it is "nodding" a little, though.

* In ''TabletopGame/MageTheAscension'' the Euthanatos had a base on tidally locked Mercury. This was destroyed InUniverse when, out-of-universe, Mercury was revealed to not actually be tidally locked.

* In PeterFHamilton's ''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.

* In the ''Franchise/StarWarsExpandedUniverse'' 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 use exile to the sunward side as a form of capital punishment.

* ''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.

** Pluto and its largest moon, Charon, are an even more extreme case. Charon is tidally locked to Pluto as would be expected, but the size difference is so small compared to similar systems that Pluto is also tidally locked to Charon.

-> ''"What I meant, sir, is that Daled Four 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."''

[[caption-width-right:300:Gets a little boring looking at the same side of the Moon all the time. [-[[https://en.wikipedia.org/wiki/File:Synchronous_rotation.svg Source]]-] ]]

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 sunward 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.

** Radole in the ''Literature/{{Foundation}}'' universe.

* All large moons are 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.)
* 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.

* Creator/IsaacAsimov's 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.

* Planet Bryyo in ''VideoGame/MetroidPrime 3: Corruption'' has this characteristic. According to the lore data, 48% of the planet's surface is in perpetual daytime ([[LathalLavaLand 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]].