# Astronomy glossary — AstroPilot 129 terms stargazers use, explained in plain words. Each links to a page with a longer explanation (add `.md` for Markdown). ## Deep sky Galaxies, nebulae and star clusters far beyond the Solar System. - [Asterism](https://astropilot.space/glossary/asterism.md): A recognisable pattern of stars that isn't an official constellation, like the Big Dipper or the Coathanger. - [Cluster + nebula](https://astropilot.space/glossary/cluster-nebula.md): A young star cluster still embedded in the cloud of gas it formed from, which its stars make glow. - [Dark nebula](https://astropilot.space/glossary/dark-nebula.md): A dense cloud of dust seen in silhouette against the stars or glowing gas behind it. - [Deep-sky object](https://astropilot.space/glossary/deep-sky-object.md): Anything in the night sky beyond the Solar System that isn't a single star: star clusters, nebulae and galaxies. - [Edge-on galaxy](https://astropilot.space/glossary/edge-on-galaxy.md): A spiral galaxy seen exactly side-on: a thin streak of light, often split by a dark lane of dust. - [Elliptical galaxy](https://astropilot.space/glossary/elliptical-galaxy.md): A smooth, featureless galaxy of mostly old stars, shaped like a ball or a squashed oval. - [Emission nebula](https://astropilot.space/glossary/emission-nebula.md): A cloud of gas made to glow by the ultraviolet light of hot young stars inside it — a stellar nursery. - [Galaxy](https://astropilot.space/glossary/galaxy.md): A vast system of billions of stars, gas and dust, held together by gravity — our own is the Milky Way. - [Galaxy group & cluster](https://astropilot.space/glossary/galaxy-group.md): Galaxies bound together by gravity: a few in a group, hundreds to thousands in a cluster. - [Globular cluster](https://astropilot.space/glossary/globular-cluster.md): A dense ball of hundreds of thousands of very old stars orbiting in a galaxy's halo. - [Hubble type](https://astropilot.space/glossary/hubble-type.md): A short code for a galaxy's shape: E for elliptical, S0 for lenticular, Sa–Sd for spirals (SB if barred), Irr for irregular. - [Magellanic Clouds](https://astropilot.space/glossary/magellanic-clouds.md): Two small galaxies orbiting the Milky Way, visible to the naked eye from the southern hemisphere. - [Nebula](https://astropilot.space/glossary/nebula.md): A cloud of gas and dust in space — glowing on its own, reflecting starlight, or blocking the light behind it. - [Open cluster](https://astropilot.space/glossary/open-cluster.md): A loose group of tens to thousands of young stars born together from the same cloud. - [Planetary nebula](https://astropilot.space/glossary/planetary-nebula.md): A glowing shell of gas thrown off by a dying Sun-like star — nothing to do with planets. - [Reflection nebula](https://astropilot.space/glossary/reflection-nebula.md): A cloud of dust that shines by reflecting the light of nearby stars, usually bluish. - [Spiral galaxy](https://astropilot.space/glossary/spiral-galaxy.md): A galaxy with a bright central bulge and a flat disk of stars, gas and dust wound into spiral arms. - [Star cloud](https://astropilot.space/glossary/star-cloud.md): A dense patch of the Milky Way where we look through a gap in the dust into thousands of distant stars. - [Supernova remnant](https://astropilot.space/glossary/supernova-remnant.md): The expanding debris of a star that exploded as a supernova. ## Stars Double, multiple and variable stars, colours and our own Milky Way. - [Double star](https://astropilot.space/glossary/double-star.md): Two stars that appear close together in the sky — either a true orbiting pair (binary) or a chance alignment. - [Milky Way](https://astropilot.space/glossary/milky-way.md): Our home galaxy — and the faint band of light across a dark sky, which is its disk seen from inside. - [Multiple star](https://astropilot.space/glossary/multiple-star.md): Three or more stars bound together or appearing together — like Epsilon Lyrae, the “Double Double”. - [Spectral type](https://astropilot.space/glossary/spectral-type.md): A star's class from its spectrum — O, B, A, F, G, K or M from hottest to coolest — with a Roman numeral for its size. The Sun is G2V. - [Star](https://astropilot.space/glossary/star.md): A sun: a huge ball of hot gas shining by fusing hydrogen in its core. Every point of light in the night sky, apart from the planets, is one. - [Star colour](https://astropilot.space/glossary/star-colour.md): Stars are coloured by their temperature: blue-white stars are the hottest, orange and red the coolest. - [Variable star](https://astropilot.space/glossary/variable-star.md): A star whose brightness changes — because it pulsates, erupts, or is eclipsed by a companion. ## Moon & planets The Moon, the planets, their moons and the things to look for on them. - [Crater](https://astropilot.space/glossary/crater.md): A bowl-shaped pit blasted out by an asteroid or comet impact, many with central peaks and terraced walls. - [Earthshine](https://astropilot.space/glossary/earthshine.md): The faint glow on the dark part of a crescent Moon — sunlight reflected off the Earth. - [Elongation](https://astropilot.space/glossary/elongation.md): How far a planet appears from the Sun; Mercury and Venus are easiest to see at their greatest elongation. - [Galilean moons](https://astropilot.space/glossary/galilean-moons.md): Jupiter's four large moons — Io, Europa, Ganymede and Callisto — discovered by Galileo in 1610. - [Great Red Spot](https://astropilot.space/glossary/great-red-spot.md): A giant storm on Jupiter, larger than Earth, that has raged for at least 190 years. - [Libration](https://astropilot.space/glossary/libration.md): A slow rocking of the Moon as seen from Earth, which over time lets us see 59% of its surface. - [Mare](https://astropilot.space/glossary/mare.md): One of the Moon's dark, smooth plains — ancient lava flows, once mistaken for seas. - [The Moon](https://astropilot.space/glossary/the-moon.md): Earth's only natural satellite, 3,474 km across and about 384,000 km away — the easiest telescope target of all. - [Opposition](https://astropilot.space/glossary/opposition.md): When a planet is opposite the Sun in our sky — at its closest and brightest, and up all night. - [Phase](https://astropilot.space/glossary/phase.md): How much of the Moon's (or a planet's) sunlit side we can see: crescent, quarter, gibbous or full. - [Planet](https://astropilot.space/glossary/planet.md): A world orbiting the Sun — Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune in our sky. - [Planetary moon](https://astropilot.space/glossary/planetary-moon.md): A natural satellite orbiting a planet — like Jupiter's Io or Saturn's Titan — seen as a star-like point next to it. - [Saturn's rings](https://astropilot.space/glossary/saturns-rings.md): Saturn's vast, thin rings of ice — with the Cassini Division, a dark gap between the two brightest. - [Terminator](https://astropilot.space/glossary/terminator.md): The line between day and night on the Moon or a planet — where shadows are longest and detail is sharpest. ## Sky events Eclipses, meteor showers, conjunctions, comets, aurorae and passes. - [Aurora](https://astropilot.space/glossary/aurora.md): Curtains of light in the sky near the poles, caused by solar particles hitting the upper atmosphere. - [Comet](https://astropilot.space/glossary/comet.md): A small icy body that grows a glowing head (coma) and tails when it nears the Sun. - [Conjunction](https://astropilot.space/glossary/conjunction.md): When two sky objects — planets, the Moon, bright stars — appear close together. - [Eclipse](https://astropilot.space/glossary/eclipse.md): When one body moves into another's shadow: the Moon into Earth's (lunar eclipse) or the Moon's shadow onto Earth (solar). - [Kp index](https://astropilot.space/glossary/kp-index.md): A 0–9 scale of how disturbed Earth's magnetic field is — the higher, the farther from the poles aurorae reach. - [Meteor shower](https://astropilot.space/glossary/meteor-shower.md): A night with many shooting stars, when Earth passes through the dusty trail of a comet. - [Occultation](https://astropilot.space/glossary/occultation.md): When one object passes in front of another and hides it — the Moon covering a star or planet, say. - [Space station pass](https://astropilot.space/glossary/iss-pass.md): A few minutes when the International Space Station (or China's Tiangong) crosses your sky, lit by the Sun. - [Transit](https://astropilot.space/glossary/transit.md): When a small body crosses the face of a larger one — a moon over Jupiter, or Mercury over the Sun. ## Finding your way Directions, coordinates and measuring the sky. - [Altitude](https://astropilot.space/glossary/altitude.md): How high an object is above the horizon, from 0° on the horizon to 90° straight overhead. - [Angular size](https://astropilot.space/glossary/angular-size.md): How big something looks, as an angle: degrees (°), arcminutes (′, 1/60°) and arcseconds (″, 1/60′). - [Azimuth](https://astropilot.space/glossary/azimuth.md): The compass direction of an object along the horizon: 0° north, 90° east, 180° south, 270° west. - [Celestial sphere](https://astropilot.space/glossary/celestial-sphere.md): The imaginary dome of the sky around the Earth, on which the stars appear fixed and turn as the Earth rotates. - [Circumpolar](https://astropilot.space/glossary/circumpolar.md): Never setting: close enough to the celestial pole that, from your latitude, it circles the pole all night. - [Constellation](https://astropilot.space/glossary/constellation.md): One of 88 official regions of the sky, most named after ancient star patterns. - [Declination](https://astropilot.space/glossary/declination.md): The sky's “latitude”: degrees north (+) or south (−) of the celestial equator. - [Ecliptic](https://astropilot.space/glossary/ecliptic.md): The Sun's yearly path across the sky — the planets and the Moon always stay close to it. - [Epoch (J2000)](https://astropilot.space/glossary/epoch.md): The date that coordinates refer to — usually J2000, the start of the year 2000 — because the sky's grid slowly shifts. - [Light-year](https://astropilot.space/glossary/light-year.md): The distance light travels in a year: about 9.5 trillion kilometres. A parsec is 3.26 light-years. - [Meridian](https://astropilot.space/glossary/meridian.md): The north–south line across the sky through the zenith; objects are highest when they cross it. - [Orientation](https://astropilot.space/glossary/orientation.md): Sky charts show north up and east left; telescopes rotate or mirror the view, so it may not match. - [Position angle](https://astropilot.space/glossary/position-angle.md): The direction of one object from another, measured from north (0°) through east (90°). - [Right ascension](https://astropilot.space/glossary/right-ascension.md): The sky's “longitude”, measured eastward in hours, minutes and seconds (0h to 24h). - [Separation](https://astropilot.space/glossary/separation.md): The angle between two objects — for a double star, the distance between its stars in arcseconds. - [Star hopping](https://astropilot.space/glossary/star-hopping.md): Finding a faint object by hopping from a bright star through recognisable patterns of fainter ones. - [Zenith](https://astropilot.space/glossary/zenith.md): The point directly overhead — altitude 90°. ## Brightness & dark skies Magnitudes, light pollution and how the eye sees faint light. - [Averted vision](https://astropilot.space/glossary/averted-vision.md): Looking slightly to one side of a faint object, so its light falls on the most sensitive part of your eye. - [Bortle scale](https://astropilot.space/glossary/bortle-scale.md): A 1–9 scale of night-sky darkness, from 1 (pristine, the Milky Way casts shadows) to 9 (inner city). - [Contrast](https://astropilot.space/glossary/contrast.md): How much brighter an object is than the sky around it — the key to seeing faint things. - [Dark adaptation](https://astropilot.space/glossary/dark-adaptation.md): Your eyes becoming far more sensitive in the dark — it takes 20–30 minutes and one white light ruins it. - [Extinction](https://astropilot.space/glossary/extinction.md): The dimming of starlight by the atmosphere, which grows quickly towards the horizon. - [Light pollution](https://astropilot.space/glossary/light-pollution.md): Artificial light scattered into the sky, which brightens it and hides faint stars, galaxies and nebulae. - [Limiting magnitude](https://astropilot.space/glossary/limiting-magnitude.md): The faintest magnitude you can see — with the naked eye (NELM) or through a given telescope. - [Magnitude](https://astropilot.space/glossary/magnitude.md): The brightness of a sky object on a reversed scale: smaller numbers are brighter, and each step is 2.5× fainter. - [Moonlight](https://astropilot.space/glossary/moonlight.md): Sunlight reflected by the Moon, which brightens the whole sky and hides faint deep-sky objects. - [SQM](https://astropilot.space/glossary/sqm.md): Sky brightness in magnitudes per square arcsecond — higher numbers are darker (22 is pristine, 18 is a city). - [Surface brightness](https://astropilot.space/glossary/surface-brightness.md): How bright an object is per patch of sky — what really decides whether a galaxy or nebula is easy to see. ## Weather & conditions What makes a night good: clouds, seeing, transparency, darkness. - [Aerosols](https://astropilot.space/glossary/aerosols.md): Tiny particles in the air — dust, smoke, sea salt, pollution — that dim stars and brighten the sky. - [Cloud cover](https://astropilot.space/glossary/cloud-cover.md): How much of the sky is covered by cloud, split into low, middle and high layers. - [Cooldown](https://astropilot.space/glossary/cooldown.md): Letting a telescope reach the outdoor temperature, so warm air inside it doesn't blur the view. - [Dew](https://astropilot.space/glossary/dew.md): Moisture condensing on optics when they cool below the dew point — it fogs eyepieces, finders and lenses. - [Jet stream](https://astropilot.space/glossary/jet-stream.md): A river of fast wind about 10 km up; when it's overhead, the seeing is usually poor. - [Seeing](https://astropilot.space/glossary/seeing.md): How steady the air is. Good seeing gives sharp, still views at high power; bad seeing makes stars boil and twinkle. - [Transparency](https://astropilot.space/glossary/transparency.md): How clear the air is: on a transparent night faint stars and galaxies show; haze, humidity and dust hide them. - [Twilight](https://astropilot.space/glossary/twilight.md): The glow after sunset and before sunrise, in three stages: civil, nautical and astronomical. ## Telescopes & optics Instruments, eyepieces, magnification and accessories. - [Airy disk](https://astropilot.space/glossary/airy-disk.md): The tiny disk, surrounded by faint rings, that a perfect telescope makes of a star — set by its aperture. - [Aperture](https://astropilot.space/glossary/aperture.md): The diameter of a telescope's main lens or mirror — the single most important number. - [Apparent field of view](https://astropilot.space/glossary/apparent-field.md): How wide an eyepiece's view looks to your eye, in degrees: about 50° for a Plössl, 82° or more for ultra-wides. - [Barlow lens](https://astropilot.space/glossary/barlow.md): A lens that goes before the eyepiece and multiplies its magnification, usually 2× or 3×. - [Binoculars](https://astropilot.space/glossary/binoculars.md): Two small telescopes side by side; “10×50” means 10× magnification and 50 mm lenses. - [Central obstruction](https://astropilot.space/glossary/central-obstruction.md): The secondary mirror blocking the middle of a reflector's or SCT's aperture, which slightly lowers contrast. - [Chromatic aberration](https://astropilot.space/glossary/chromatic-aberration.md): A purple or yellow fringe around bright objects in simple lens telescopes, because lenses bend colours differently. - [Collimation](https://astropilot.space/glossary/collimation.md): Aligning a telescope's mirrors so they share one axis — essential for sharp views in reflectors. - [Colour filter](https://astropilot.space/glossary/color-filter.md): A tinted filter (light blue, orange, red…) that raises the contrast of particular features on planets. - [Coma](https://astropilot.space/glossary/coma.md): A flaw of Newtonian mirrors that turns stars near the edge of the field into little comets; worse in fast telescopes. - [Coma corrector](https://astropilot.space/glossary/coma-corrector.md): A lens group that slips in the focuser of a fast Newtonian and makes stars sharp right to the edge of the field. - [Dobsonian](https://astropilot.space/glossary/dobsonian.md): A Newtonian reflector on a simple rocking-box mount — the easiest way to get a big aperture. - [Exit pupil](https://astropilot.space/glossary/exit-pupil.md): The width of the beam of light leaving the eyepiece: aperture ÷ magnification. It sets how bright and sharp the view looks. - [Eye relief](https://astropilot.space/glossary/eye-relief.md): How far your eye must be from an eyepiece to see the whole field; 15–20 mm is comfortable, and needed with glasses. - [Eyepiece](https://astropilot.space/glossary/eyepiece.md): The small lens you look through, which magnifies the telescope's image; its focal length sets the power. - [Finder](https://astropilot.space/glossary/finder.md): A small sighting device on the telescope for aiming: a red-dot or Telrad reflex sight, or a small magnifying finderscope. - [Focal length](https://astropilot.space/glossary/focal-length.md): The distance at which a telescope's lens or mirror brings light to a focus; with the eyepiece it sets the magnification. - [Focal ratio](https://astropilot.space/glossary/focal-ratio.md): Focal length divided by aperture (f/5, f/10…). “Fast” (low) ratios give wide fields; “slow” (high) ones give easy high power. - [Focal reducer](https://astropilot.space/glossary/focal-reducer.md): A lens that shortens a telescope's focal length — an f/6.3 reducer turns an f/10 SCT into a wider-field instrument. - [Light grasp](https://astropilot.space/glossary/light-grasp.md): How much more light a telescope collects than your eye: (aperture ÷ 7 mm)². An 8-inch gathers about 840×. - [Magnification](https://astropilot.space/glossary/magnification.md): How much bigger a telescope makes things look: its focal length divided by the eyepiece's focal length. - [Maksutov–Cassegrain](https://astropilot.space/glossary/maksutov.md): A compact catadioptric telescope with a thick curved front lens: sharp, slow (f/12–f/15), great for planets and the Moon. - [Maximum useful magnification](https://astropilot.space/glossary/max-magnification.md): About twice the aperture in millimetres (50× per inch) — but the air rarely allows more than 250–350×. - [Moon filter](https://astropilot.space/glossary/moon-filter.md): A neutral grey filter that dims the dazzling Moon for comfortable viewing. - [Mount](https://astropilot.space/glossary/mount.md): What holds and points the telescope: alt-azimuth (up-down, left-right) or equatorial (aligned with the sky's rotation). - [Nebula filter](https://astropilot.space/glossary/nebula-filter.md): A filter that passes the light emitted by nebulae and blocks most skyglow, making them stand out. - [Reflector](https://astropilot.space/glossary/reflector.md): A telescope that uses mirrors; the Newtonian, with the eyepiece near the top, gives the most aperture for the money. - [Refractor](https://astropilot.space/glossary/refractor.md): A telescope that uses lenses: sharp, contrasty, low-maintenance — with no central obstruction. - [Resolution](https://astropilot.space/glossary/resolution.md): The finest detail a telescope can show; the Dawes limit, 116 ÷ aperture in mm, is the closest equal double star it can split. - [Schmidt–Cassegrain](https://astropilot.space/glossary/schmidt-cassegrain.md): A compact telescope with a mirror and a front corrector plate; long focal length (f/10) in a short tube. - [Tracking](https://astropilot.space/glossary/tracking.md): Moving a telescope to follow the sky's rotation, so objects stay in view. - [True field of view](https://astropilot.space/glossary/true-field.md): How much sky you see through the eyepiece — roughly its apparent field divided by the magnification. ## Catalogs & names Where object names like M 31, NGC 7000 and α Lyrae come from. - [Caldwell catalog](https://astropilot.space/glossary/caldwell-catalog.md): Patrick Moore's 1995 list of 109 bright objects Messier missed, including the best of the southern sky. - [Messier catalog](https://astropilot.space/glossary/messier-catalog.md): Charles Messier's list of 110 bright nebulae and clusters, made in the 1700s — the classic first targets. - [NGC & IC](https://astropilot.space/glossary/ngc-ic.md): The New General Catalogue (1888) and its Index Catalogues: the standard numbers for about 13,000 galaxies, clusters and nebulae. - [Star names & designations](https://astropilot.space/glossary/bayer-designation.md): How stars are named: proper names (Vega), Greek letters by brightness (α Lyrae), or numbers (61 Cygni). - [Washington Double Star Catalog](https://astropilot.space/glossary/wds.md): The US Naval Observatory's master catalog of double stars, with more than 150,000 pairs. ## In AstroPilot The scores, ratings and views AstroPilot gives you. - [Best window](https://astropilot.space/glossary/best-window.md): The hours of the night best for observing: dark, clear and — for faint objects — without the Moon. - [Cloud map](https://astropilot.space/glossary/cloud-map.md): Tonight's map of clouds around you: Meteosat satellite images for the last hours, then a week of forecast. - [Difficulty rating](https://astropilot.space/glossary/difficulty.md): How hard an object is tonight — easy, moderate, challenging, very hard or out of reach — for your sky, instrument and Moon. - [Observing score](https://astropilot.space/glossary/observing-score.md): AstroPilot's 0–100 rating of the night for deep-sky observing, from cloud, seeing, transparency, darkness and moonlight. - [Showpiece](https://astropilot.space/glossary/showpiece.md): A highlight of the sky that impresses in almost any telescope — the objects to show friends first. - [Sky-survey photograph](https://astropilot.space/glossary/sky-survey-photo.md): The photo on each deep-sky object's page: a long-exposure sky survey image, north up, to scale. - [Sweet spots](https://astropilot.space/glossary/sweet-spot.md): The three exit-pupil bands a good eyepiece kit covers: low (4–6 mm), medium (2–3 mm) and high power (0.7–1.2 mm).