A Mercurial Base is a large installation on a planet extremely close to a star — so close that direct sunlight would be lethal. As a result, the base is continually moving, to stay in either perpetual twilight or perpetual night.
More plausible versions (relatively speaking — this isn't a particularly plausible idea) often put the Mercurial Base on train tracks, and use the thermal expansion and contraction of the tracks to move the base; this only works at sunrise or sunset, which can result in added drama. Less plausible versions might move it in just about any way the author thinks is cool, and are probably more likely to try to stay near midnight, or just after sunset, to give themselves a larger safety margin.
Because of the Rule of Drama, a Mercurial Base inevitably comes along with one of two events. Either:
- some major character will get stranded on the surface, and needs to get back to the base before the sun rises, or
- the base itself will break down.
At least as applied to Mercury itself, the Mercurial Base is of necessity a relatively new idea. Astronomers used to think Mercury was tidally locked to the sun, with the day side constantly searing hot and the night side constantly near absolute zero. But in 1965 it was discovered that the planet is only sort-of "locked"note , in a 3:2 orbital resonance that gives it three days every two years. Thus came the idea that a base would have to move, slowly, in order to stay out of the sun.
- ElfQuest: The future arc has Cauldron City on the Mercury-like planet Cauldron; it runs on rails that go around the polar region of the planet, and the main characters have to save it from sabotage.
- Star Wars (Marvel 1977) features a way to have a base INSIDE a star. Taking a stroll is not an option.
- The Transformers: Windblade: The planet Velocitron is dangerously close to its sun, so to avoid the daytime side's temperatures and radiation, all their cities are on massive wheels and move continuously ahead of the dawn.
- 400 Graden In De Schaduw (400 Degrees in the Shade) features multiple "walking cities" on Mercury, used to house vat-grown miners.
- The Inverted World: A city moves slowly along on rails, which the inhabitants of the city are constantly building ahead of it and dismantling behind it, and rather than having motors driving wheels it uses winches and cables to slide along. The city is forced to keep moving because the geometry of space is distorted, with the world "ahead" and "behind" them stretched into uninhabitable proportions and the safe zone of "normal" space was gradually moving relative to the surface of the planet. The city has to keep up. In the end, it's revealed that the planet the story takes place on is a future Earth, the space distortion is purely an effect of the city's power source, and its nomadic life runs into a serious problem when it finds a river much too wide to ford — the Atlantic Ocean.
- Kim Stanley Robinson:
- Blue Mars has one of these on Mercury, of the "more plausible" variety. The city of Terminator (so called because it always rests on the planet's terminator) is mounted on a pair of planet-encircling tracks. The substantial thermal expansion due to direct solar exposure causes the tracks to continuously "push" the city away from the sunrise. The viewpoint character accidentally gets trapped outside.
- A short story, a murder mystery, is set on a Mercury with a city on tracks.
- 2312 starts on Mercury, with a city on tracks.
- The Night Land: In the backstory, as the Earth's rotation gradually began to slow, humanity built immense World Roads that encircled the planet. These served as tracks for great mobile arcologies, which made endless circuits of the planet to stay in the light. Eventually, as the Earth's slowing caused the oceans to slowly pool at the poles,note the majority of the human population came to live in vast caravans of mobile cities traveling the World Roads, alongside walker tribes and vast fleets making the same circuits at higher latitudes. This period lasted until the Earth became tidally locked and a massive eruption broke a vast crack into the Earth, bringing the age of mobile cities to an end.
- Revelation Space Series: In Absolution Gap, the theocratic society on the ice moon of Hela relies on travelling Cathedrals, which keep pace with the moon's rotation not for environmental but for religious reasons: to keep the vanishing gas giant within sight of their zealots.
- The Periodic Table Of Science Fiction: Mercury's entry features Quicksilver City, which must constantly drive across Mercury's surface so as to stay at a human-habitable temperature. The road has (mostly) been prepared and seeded with supplies to pick up, by machines that can tolerate extreme heat and cold, but only at the border between night and day can people survive.
- Saturn's Children by Charles Stross also has a city on tracks, following the terminator; in this case, the protagonist's enemies tie her to the tracks and leave her for dead.
- In The Thrawn Trilogy of the Star Wars Legends, there's the mining colony of Nomad City on the planet Nkllon. The city is made of huge platforms and grounded ships, sitting on top of 40 AT-AT walkers, and it uses gigantic umbrella-like ships to protect visiting ships on the way to the planet. Without the shade of an umbrella ship, any but the toughest spacecraft would soon be melted to slag. In the first novel, Thrawn does manage to use an Imperial Star Destroyer in direct sunlight there, but it's heavily implied to take a great deal of effort to prepare the ISD for this operation, and it needs a great deal of repair afterwards.
Inevitably, the walkers are damaged (by an Imperial attack in the third novel, which uses smaller Dreadnaught cruisers capturing the umbrella ships), threatening to leave the city stranded until dawn catches it. Contrary to most versions of the trope, however, the threatened destruction of the base is a D-plot at best. Everyone aboard is easily evacuated after a minimum of drama (though Lando does have a few hours to worry about it before a New Republic Dreadnaught arrives behind a freshly-repaired umbrella ship), it's just Lando's latest ill-starred business venture that's in danger. A later book revealed that the city was successfully rebuilt. And then Lando lost it in a sabacc game.
- To the Stars: Wheelworld features a planet with very extreme seasons where the entire population of the colony escapes the brutal summers twice each (longer-than-Earth-normal) year by picking up and moving from one of the planet's poles to the other. This is done by jacking up the colony's main buildings on wheels, forming them up behind the colony's nuclear power plants (now transformed into enormous locomotives) and making the 12,000-mile trek to the other side of the planet.
- In Buck Rogers XXVC there are cities on the surface of Mercury which keep on the move to stay at the edge between day and night where the temperature is tolerable.
- Mass Effect 2: In the Lair of the Shadow Broker DLC, the Shadow Broker's center of operations is on a ship constantly traversing a slowly rotating planet at the terminus between the day side that is blazing hot, and the night side which freezes solid ten minutes after sunset. Tthe terminus itself is not safe either as it's wracked by a never-ending lightning storm. This is actually a case of showing their work, because this is exactly what would happen along the boundary of such an extreme temperature differential.
- Starcraft II: There's a mission where the local sun suddenly increases its output by 500% as a prelude to going supernova. Playing the Terrans, the player has to move his base and his units to avoid the light of day which destroys units and buildings pretty quickly.