Angular momentum
The quantity of spin: mass times radius squared times spin rate. The total in a closed system never changes, so Concordia keeps its total at zero by turning two rotors each way. See counter-rotation.
Apparent weight
The force a floor or wall pushes back with, which is what a scale reads. In a spinning rotor it comes from the wall pressing you around a circle, and it changes with height and with how you move. See artificial gravity.
Artificial gravity
Weight made by rotation instead of by a planet's mass. At a 2,500 m radius, 0.6 turns a minute gives 1 g at the ground. See artificial gravity.
Artificial sky
A lit ceiling about 450 m above the ground of each rotor, built to look like open sky. It is a display membrane, not a window, and the air above it is the same air as below. See light and sky.
Ascender
A radial lift that carries about 150 people between the ground and the hub in roughly 10 minutes. Its cabin tilts to match the changing apparent vertical. See ascenders and sky.
Axis station
The station at the foot of an ascender tower, where passengers board the lift to the hub. Each rotor has four. See axis and spine.
Black start
Restarting a power grid from nothing, with no outside power. Concordia keeps two 250 MWe fission reactors on the aft rim for this. See power and heat.
Carbonaceous asteroid
A dark asteroid rich in carbon, water and nitrogen compounds, the elements the Moon lacks. Concordia's captured bodies run roughly 10 percent water. See materials.
Centripetal acceleration
The inward acceleration that keeps something moving in a circle, equal to the spin rate squared times the radius. In a rotor it is the weight you feel. See artificial gravity.
Cislunar space
The region between Earth and the Moon's orbit, including L5. It is where Concordia's customers, ships and cargo move. See getting there.
Closed-loop life support
Recycling air, water and nutrients so little is lost. Concordia aims for extremely high recovery, not a perfectly closed loop, and tracks every element. See air, water and food.
Common-mode failure
A single cause that takes out many parts at once, such as one bad software command, one shared coolant line or one blast that reaches every home. Concordia is built so that no single failure can do that. See safety.
Coriolis effect
The sideways push felt when you move inside a rotating frame. Walking is barely affected, but a train going with or against the spin changes weight by a third. See artificial gravity.
Delta-v
The change in speed a ship can make, the real budget of any trip. Concordia's Earth transfer spends it on a launch, a departure burn and a capture burn. See getting there.
End wall
The disc that closes each end of a rotor. Its inner face is an inhabited cliff 2,436 m tall. See structure.
Express
The train in the spine that runs the length of the station at 140 m/s in zero g. See axis and spine.
Galactic cosmic rays
Fast atomic nuclei from outside the solar system. They are hard to stop, so the station relies on sheer mass: metres of regolith, plus water near where people sleep. See radiation and debris.
Gravity assist
Using a planet or moon's gravity to bend or speed a path for free. Concordia's Earth transfer swings past the Moon on day 6. See getting there.
Gravity gradient
How much gravity changes across a distance. Across a standing person in a Concordia rotor it is 0.07 percent of 1 g, too small to feel. See artificial gravity.
Hoop stress
The tension in the wall of a pressurized or spinning ring, pulling it around the circle. Air pressure alone loads each rotor's tension bands with about 250 MN per metre of length. See structure.
HVDC
High-voltage direct current, used for low-loss power transmission. Concordia's main power line is sized for 40 GW at plus and minus 1.2 MV. See power and heat.
Hydroponics
Growing plants in nutrient water without soil. It is a main farm method in the upper city. See upper-city farms.
Aeroponics
Growing plants with their roots in a nutrient mist. It uses far less water than soil. See upper-city farms.
In-situ resource utilization
Making what you need from materials found where you are, such as oxygen from lunar rock. It is the principle behind Concordia's construction. See materials.
L5
One of the five Lagrange points of the Earth-Moon system. Concordia sits in the L5 region and is station-kept there. See getting there.
Lagrange point
A place where the gravity of two large bodies and the motion of the orbit balance, so a small object can stay. Concordia needs active control at L5. See getting there.
Land value capture
Paying for new infrastructure from the rise in value of the land it serves: the builder owns the land and leases or sells it. Concordia's builders own the ground and floor space they create. See who pays for Concordia.
Magnetic bearing
A bearing that holds a rotor with magnetic fields so nothing touches. Concordia's rotors turn on them at both ends. See counter-rotation.
Mass driver
An electromagnetic launcher that throws payloads at high speed. On the Moon it ships feedstock to L5. See materials.
Megaframe
The prebuilt lattice of ribs, rings and nodes that holds the elevated city, reaching 1,500 m up from the ground. See structure.
Microgravity
Near-weightlessness, as in orbit or at the station's axis. Concordia's shipyard and spine are in it. See artificial gravity.
Momentum ring
One of two heavy rings spinning in a stationary vacuum bay. Together they cancel the spin of Ehricke (the industrial rotor) and absorb the shifts that cargo and launches cause. See counter-rotation.
Near-Earth asteroid
An asteroid whose orbit brings it close to Earth's. The few that pass slowly enough can be steered into the Earth-Moon system for a small push. Concordia's first metal came from them. See mining.
O'Neill cylinder
A rotating-cylinder settlement proposed by Gerard O'Neill in the 1970s. Concordia is a mature descendant with four rotors. See five generations.
Perigee
The nearest point of an orbit to the planet. Earth capture happens at a 400 km perigee. See getting there.
Permanently shadowed region
A crater floor on the Moon that never sees sunlight and stays cold enough to hold ice. See materials.
Pressure district
One of the sealed sections a rotor is divided into, with pressure-rated barriers at its edges. A breach or fire is shut inside one district, and large buildings add refuge halls of their own. See safety.
Pressure vessel
A container built to hold air under pressure. Concordia's rotors are pressure vessels inside a larger unpressurized shell. See structure.
Prograde
Turning in the same sense as a reference spin. Bernal and O'Neill (Rotors 3 and 4) are prograde, seen from the forward end. See counter-rotation.
Radiator
A panel that rejects waste heat as infrared light into space. Concordia's radiators are sized for up to 40 GW. See power and heat.
Regolith
The loose rock and dust on the Moon's surface. It is mined, sintered into building stone and packed into shielding. See materials.
Retrograde
Turning opposite to a reference spin. Tsiolkovsky and Noordung (Rotors 1 and 2) are retrograde. See counter-rotation.
Rim speed
How fast the ground moves along the circle. In a Concordia rotor it is about 157 m/s. See artificial gravity.
Rotating frame
A point of view that turns with the rotor. Inside it, loose things drift sideways and feel weight. See artificial gravity.
Self-replicating factory
A factory whose main product is more of itself: machines that mine, refine and build the next machines. Its output grows by doubling, which is how two billion tonnes went up in about ten years. See who pays for Concordia.
Sintered regolith
Lunar dust heated until grains fuse into solid stone, without melting. It is Concordia's main building material. See materials.
Solar furnace
Mirrors that focus sunlight onto one small spot, hot enough to fuse or melt rock. On the Moon they turn loose dust into bricks and paved ground. See mining.
Solar particle event
A burst of energetic particles from the Sun. Concordia handles it with dedicated high-shielding shelters. See radiation and debris.
Specific impulse
How much thrust an engine gets from each unit of propellant, in seconds. Higher means fewer tonnes of fuel. See getting there.
Spin lock
The chambers at each hub where passengers are spun down to rest and latched to the stationary spine. A full cycle takes about 10 minutes. See axis and spine.
Spin pair
Two rotors turning the same way, locked in step and joined by bridges. Tsiolkovsky and Noordung (Rotors 1 and 2) form one pair, Bernal and O'Neill (Rotors 3 and 4) the other. See counter-rotation.
Spine
The stationary tube, 120 m across, that runs down the axis and carries transport, power and utilities. It never rotates. See axis and spine.
Stanford torus
A wheel-shaped settlement for 10,000 people designed in the 1975 NASA and Stanford summer study. Its radiation shield was a separate ring that did not rotate. See safety.
Station-keeping
Small, continuous corrections that hold a station on its intended path. Concordia's thrusters sit at the aft ring. See aft works.
Stefan-Boltzmann law
Heat radiated per square metre grows with the fourth power of temperature. It sets how big the radiators must be. See power and heat.
Transfer orbit
The path a ship follows to move from one orbit to another. Concordia's Earth transfer takes about 15 days. See getting there.
Volatiles
Light elements and compounds that boil off easily: water, carbon, nitrogen and hydrogen. They are scarce on the Moon and plentiful in carbonaceous asteroids. See materials.
Waste heat
The heat left over when power is used. Almost every watt Concordia makes ends as waste heat and must be radiated away. See power and heat.
Whipple shield
A thin outer bumper spaced away from the main wall, which breaks fast debris into a harmless cloud. See radiation and debris.