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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."

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.

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.

Also known as a Twilight Planet, in reference to the perpetual twilight experienced by the narrow band between the sun-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.

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.

Examples:

    open/close all folders 

    Film 
  • Supplementary material for Star Trek: Nemesis indicates that the Remans evolved on the dark side of tidally locked Remus, explaining their photosensitivity.

    Literature 
  • In the Star Wars Expanded Universe 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.
  • Adumbria in Ciaphas Cain: The Traitor's Hand 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.) 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.
  • Jinx in Larry Niven's Known Space series. The colonists live along a narrow band encompassing the prime meridian and have completely black skin from the radiation. It also has very high gravity.
  • Isaac Asimov:
    • The 1956 sci-fi murder mystery "The Dying Night" [1] used the 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.
    • Radole in the 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.
  • In the 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.
  • In Proxima, Per Ardua is tidally locked to Proxima Centauri.
  • In the Star Trek Expanded Universe novel 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.
  • In the science-fiction short story "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.
  • In Peter F. Hamilton's 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.
  • The Slan in Star Carrier: 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", 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.

    Live-Action TV 
  • 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.
  • Stargate SG-1 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.

    Tabletop Games 
  • 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.
  • Classic 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.
  • In Mage: The Ascension the Euthanatos had a base on tidally locked Mercury. This was destroyed In-Universe when, out-of-universe, Mercury was revealed to not actually be tidally locked.

    Video Games 
  • In Kerbal Space Program, 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.
  • Planet Bryyo in Metroid Prime 3: Corruption 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.

    Web Original 
  • Inverted in RWBY, where the moon's phases (and shape) change because it rotates (instead of being tidally locked, as it is on Earth).

    Western Animation 
  • In Ben 10: Ultimate Alien, Mykdl'dy is a tidally locked planet with one side burning, and the other side frozen.

    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.)
  • 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. Animation and explanation)
  • Pluto's 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.
  • 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.)

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