173 terms · 8 categories

Glossary

Each entry carries both names, a one-line definition and one number worth remembering.

Orbits and mechanics · 19

Orbit — The path an object follows while falling around another body.
Low Earth orbit (LEO) — Orbit between roughly 160 and 2000 km, where most stations and constellations live.
Geostationary orbit (GEO) — Orbit at 35,786 km whose period equals one sidereal day.
Geostationary transfer orbit (GTO) — An elliptical transfer orbit with a low perigee and an apogee near GEO.
Sun-synchronous orbit (SSO) — An orbit whose plane keeps a nearly fixed angle to the Sun.
Orbital inclination — The angle between the orbital plane and the equator.
Apogee — The farthest point of an orbit from Earth.
Perigee — The closest point of an orbit to Earth.
Two-line element set (TLE) — A 69-character-per-line format that encodes an orbit.
Earth's shadow — The stretch where a satellite passes behind Earth and loses sunlight.
Delta-v — The velocity change needed to alter an orbit.
Escape velocity — The minimum speed needed to leave a body's gravity.
Atmospheric reentry — A spacecraft coming back into the atmosphere.
Hohmann transfer — A two-burn manoeuvre that moves between circular orbits on minimum fuel.
Orbital plane — The plane an orbit lies in, tilted by its inclination.
Ascending node — Where the orbit crosses the equator going north.
Precession — The slow rotation of an orbital plane.
Atmospheric drag — Thin air slowing a satellite down.
Orbital decay — Altitude loss until reentry.

Observing basics · 32

Twilight — The interval after sunset when the sky still glows.
Astronomical darkness — Sun more than 18° below the horizon, when twilight is gone.
Seeing — How much the atmosphere smears a star's image.
Transparency — How well the sky passes starlight, set by water vapour and dust.
Light pollution — Artificial light brightening the night sky and hiding faint objects.
Bortle scale — A 1–9 scale describing how dark a night sky is.
Magnitude — A logarithmic brightness scale: smaller numbers are brighter.
Limiting magnitude — The faintest magnitude visible under current conditions.
Apparent magnitude — Brightness as seen from Earth, not intrinsic output.
Absolute magnitude — The magnitude an object would have at 10 parsecs.
Altitude — Angle above the horizon: 0° at the horizon, 90° at zenith.
Azimuth — Horizontal angle measured clockwise from north.
Zenith — The point straight overhead, altitude 90°.
Meridian — The great circle from north through zenith to south.
Conjunction — Two bodies appearing close together on the sky.
Opposition — A superior planet opposite the Sun, visible all night.
Elongation — Angular distance from the Sun for an inner planet.
Meteor shower — Many meteors when Earth crosses a comet's dust trail.
ZHR — The theoretical hourly rate of a shower under perfect skies.
Sidereal day — One rotation relative to the stars, about 4 minutes short of a solar day.
Solar eclipse — The Moon covering the Sun.
Lunar eclipse — The Moon passing into Earth's shadow.
Zodiacal light — Sunlight scattered by interplanetary dust, a faint cone.
Air mass — Relative air path; 1 at zenith.
Extinction — Atmosphere dimming starlight.
Dark adaptation — Eyes adjusting to darkness over 20–40 minutes.
Averted vision — Looking slightly off-target to use rod-rich retina.
Red light — Reading charts under red light to keep dark adaptation.
Pickering scale — A 1–10 scale for how steady a star looks.
Transit — An inner planet crossing the Sun's disc.
Occultation — A nearer body hiding a farther one.
Appulse — The moment two bodies look closest.

Celestial bodies · 11

Moon phase — The shape of the Moon's lit half as seen from Earth.
Planet — A large body orbiting a star and not shining by itself.
Venus — The brightest planet, reaching about 47° elongation.
Jupiter — The largest planet, around −2.5 magnitude.
Saturn — The ringed planet; separating rings from disc needs about 30×.
Mars — The red planet, about −2.9 magnitude at opposition.
Mercury — The innermost planet, never more than 28° from the Sun.
The Moon — Earth's only natural satellite, 3474 km across.
Comet — An icy body that grows a tail near the Sun.
Asteroid — Small rocky bodies, mostly between Mars and Jupiter.
Milky Way — Our own galaxy seen edge-on as a band of light.

Spacecraft · 26

International Space Station — The largest object in orbit, at 51.6° inclination.
Hubble Space Telescope — An optical telescope launched in 1990, still operating.
Starlink — SpaceX's low-orbit communications constellation, thousands strong.
Satellite — Anything, natural or artificial, orbiting a body.
Payload — Whatever a rocket carries: a satellite, a crew vehicle, an experiment.
Payload fairing — The nose shell protecting the payload, jettisoned above the atmosphere.
Stage — A self-contained propulsion section, dropped when spent.
Reusable — A launcher that flies again after recovery.
Booster — Strap-on or parallel stages giving extra thrust at liftoff.
Thrust — Force from an engine, given in kilonewtons or tonnes-force.
Specific impulse — Impulse per unit of propellant — an efficiency measure.
LEO capacity — How much mass a launcher can put in low Earth orbit.
CubeSat — A small-satellite standard built from 10×10×11.35 cm units.
Cryogenic propellant — Propellants kept very cold, such as liquid hydrogen and oxygen.
Methalox — Liquid oxygen with liquid methane.
Solid rocket motor — A motor whose fuel and oxidiser are solid.
Hypergolic propellant — Two fluids that ignite on contact.
Reaction control system — Small thrusters that point a spacecraft.
Docking — Two spacecraft joining in orbit.
Spacewalk — A crew member working outside in vacuum.
Life support — The system keeping air, water and temperature livable.
Heat shield — The layer protecting a vehicle during reentry.
Parachute — Canopies used to slow a landing.
Landing burn — The final deceleration burn before touchdown.
Drone ship — An uncrewed barge that catches first stages.
Constellation — A group of satellites working together.

Launch and sites · 21

Payload integration — Bolting the satellite onto the rocket and checking it.
NET (No Earlier Than) — The earliest possible launch time — it usually moves.
T-0 — The reference instant of liftoff; the count counts from it.
Launch window — The interval in which a launch may occur, set by the target orbit.
Scrub — Cancelling a launch attempt, usually for weather or hardware.
Polar launch — Launching north or south to reach a polar orbit.
Low-latitude site — A site near the equator, borrowing more of Earth's rotation.
Sea launch — Launching from a sea platform, free to pick a latitude.
Cape Canaveral — The Florida launch complex at about 28.5°N.
Baikonur Cosmodrome — A Russian-leased site in Kazakhstan.
Kourou (Guiana Space Centre) — Europe's site in French Guiana, about 5°N.
Launch cost — The price of a flight, per payload or per rocket.
Cost per kilogram — Price divided by mass delivered.
Rideshare — Several small satellites sharing one rocket.
Fairing size — The diameter and usable volume of the fairing.
Pad turnaround — Time to reset a pad between flights.
Weather criteria — The weather rules a launch must satisfy.
Range — The ground corridor the rocket overflies, kept clear.
Downrange — The area ahead along the flight path.
Telemetry — The rocket streaming its state to the ground.
Flight termination system — The system that ends a flight if it goes wrong.

Data and algorithms · 25

SGP4 — A simplified perturbation model that propagates a TLE.
Launch Library 2 — The open launch database maintained by The Space Devs.
Celestrak — A public source of TLE orbital data.
astronomy-engine — An MIT-licensed astronomy library that runs in your browser.
Ephemeris — A table or algorithm giving positions at given times.
UTC — The global time standard; it ignores daylight saving.
ISO 8601 — The international date-time format, e.g. 2026-10-30T18:21:00Z.
Snapshot — The copy of the data taken at build time.
CAGR — A multi-year growth rate expressed as a yearly average.
Hexagonal grid — An indexing scheme that tiles Earth in equal-area hexagons.
Delta T — The mismatch between atomic and Earth-rotation time.
Sidereal time — Time measured against the vernal equinox.
Light-year — Distance light travels in a year, 9.46 trillion km.
Astronomical unit — Earth–Sun mean distance, 149.6 million km.
Parsec — Distance at one arcsecond parallax, 3.26 light-years.
Declination — The celestial equivalent of latitude.
Right ascension — The celestial equivalent of longitude, in hours.
Epoch — The reference time for coordinates or elements.
Ground station — A facility talking to satellites.
S-band — The 2–4 GHz band, common for tracking and control.
X-band — The 8–12 GHz band, used for high-rate data.
Downlink — The satellite-to-ground data path.
Orbit determination — Deriving an orbit from observations.
TLE format — Two fixed-width 69-character lines describing an orbit.
TLE checksum — A modulo-10 check digit at the end of each line.

Equipment · 18

Aperture — The diameter of the objective — it sets light grasp and resolution.
Focal ratio — Focal length divided by aperture; f/5 is faster than f/10.
Eyepiece — The interchangeable lens that sets magnification.
Barlow lens — A lens before the eyepiece that doubles the effective focal length.
Dobsonian — A large reflector on a simple alt-az rocker box.
GoTo mount — A mount that slews to a target and tracks it.
Tripod — The base holding a telescope or camera.
Binoculars — Two short scopes side by side: wide field, easy to aim.
Exposure — How long a camera sensor collects light.
Tracking — Moving the gear with the sky so stars stay round.
Collimation — Aligning a reflector's mirrors.
Cool-down — A scope reaching ambient temperature.
Polar alignment — Pointing an equatorial mount at the celestial pole.
Refractor vs reflector — Lens-based versus mirror-based telescopes.
Star diagonal — The 90° eyepiece adapter on a refractor.
Focal length — Distance from objective to focus.
Field of view — How much sky you see at once, in degrees.
Exit pupil — Aperture divided by magnification.

Chinese spaceflight · 21

Tiangong space station — China's crewed station, inclined about 41.5°.
Jiuquan Satellite Launch Center — China's first launch site, in Inner Mongolia's Ejin Banner.
Xichang Satellite Launch Center — In Sichuan, mainly for geostationary missions.
Taiyuan Satellite Launch Center — In Shanxi, mostly Sun-synchronous launches.
Wenchang Space Launch Site — China's southernmost site, on Hainan island.
Hainan commercial spaceport — A commercial site next to Wenchang, opened in 2024.
Long March — China's main launch vehicle family, designated CZ.
Long March 5 — China's most powerful active rocket, about 25 t to LEO.
Long March 2F — The crew-rated rocket that launches Shenzhou.
Shenzhou — China's crew vehicle, seating three.
Chang'e program — China's lunar program: orbit, land, return.
Tianwen — China's planetary exploration series.
BeiDou — China's navigation constellation, completed globally in 2020.
Tianhe core module — The core module of Tiangong, launched in 2021.
Wentian lab module — One of Tiangong's two lab modules, docked in 2022.
Mengtian lab module — Tiangong's second lab module, docked in 2022.
LandSpace — A Chinese private launcher; Zhuque-2 is its methalox rocket.
i-Space — A Chinese private launcher behind the Hyperbola series.
Galactic Energy — A Chinese private launcher behind Ceres-1.
CAS Space — A Chinese private launcher behind the Kinetica series.
Orienspace — A Chinese private launcher behind the Gravity series.