Astronomy glossary

129 words stargazers use, explained in plain language. AstroPilot's app adds interactive demos: slide along the magnitude scale, dim the city lights on the Bortle scale, steady the air, split a double star.

Deep sky

Galaxies, nebulae and star clusters far beyond the Solar System.

Asterism: A recognisable pattern of stars that isn't an official constellation, like the Big Dipper or the Coathanger.
Cluster + nebula: A young star cluster still embedded in the cloud of gas it formed from, which its stars make glow.
Dark nebula: A dense cloud of dust seen in silhouette against the stars or glowing gas behind it.
Deep-sky object: Anything in the night sky beyond the Solar System that isn't a single star: star clusters, nebulae and galaxies.
Edge-on galaxy: A spiral galaxy seen exactly side-on: a thin streak of light, often split by a dark lane of dust.
Elliptical galaxy: A smooth, featureless galaxy of mostly old stars, shaped like a ball or a squashed oval.
Emission nebula: A cloud of gas made to glow by the ultraviolet light of hot young stars inside it — a stellar nursery.
Galaxy: A vast system of billions of stars, gas and dust, held together by gravity — our own is the Milky Way.
Galaxy group & cluster: Galaxies bound together by gravity: a few in a group, hundreds to thousands in a cluster.
Globular cluster: A dense ball of hundreds of thousands of very old stars orbiting in a galaxy's halo.
Hubble type: 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: Two small galaxies orbiting the Milky Way, visible to the naked eye from the southern hemisphere.
Nebula: A cloud of gas and dust in space — glowing on its own, reflecting starlight, or blocking the light behind it.
Open cluster: A loose group of tens to thousands of young stars born together from the same cloud.
Planetary nebula: A glowing shell of gas thrown off by a dying Sun-like star — nothing to do with planets.
Reflection nebula: A cloud of dust that shines by reflecting the light of nearby stars, usually bluish.
Spiral galaxy: A galaxy with a bright central bulge and a flat disk of stars, gas and dust wound into spiral arms.
Star cloud: A dense patch of the Milky Way where we look through a gap in the dust into thousands of distant stars.
Supernova remnant: 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: Two stars that appear close together in the sky — either a true orbiting pair (binary) or a chance alignment.
Milky Way: Our home galaxy — and the faint band of light across a dark sky, which is its disk seen from inside.
Multiple star: Three or more stars bound together or appearing together — like Epsilon Lyrae, the “Double Double”.
Spectral type: 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: 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: Stars are coloured by their temperature: blue-white stars are the hottest, orange and red the coolest.
Variable star: 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: A bowl-shaped pit blasted out by an asteroid or comet impact, many with central peaks and terraced walls.
Earthshine: The faint glow on the dark part of a crescent Moon — sunlight reflected off the Earth.
Elongation: How far a planet appears from the Sun; Mercury and Venus are easiest to see at their greatest elongation.
Galilean moons: Jupiter's four large moons — Io, Europa, Ganymede and Callisto — discovered by Galileo in 1610.
Great Red Spot: A giant storm on Jupiter, larger than Earth, that has raged for at least 190 years.
Libration: A slow rocking of the Moon as seen from Earth, which over time lets us see 59% of its surface.
Mare: One of the Moon's dark, smooth plains — ancient lava flows, once mistaken for seas.
The Moon: Earth's only natural satellite, 3,474 km across and about 384,000 km away — the easiest telescope target of all.
Opposition: When a planet is opposite the Sun in our sky — at its closest and brightest, and up all night.
Phase: How much of the Moon's (or a planet's) sunlit side we can see: crescent, quarter, gibbous or full.
Planet: A world orbiting the Sun — Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune in our sky.
Planetary moon: 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: Saturn's vast, thin rings of ice — with the Cassini Division, a dark gap between the two brightest.
Terminator: 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: Curtains of light in the sky near the poles, caused by solar particles hitting the upper atmosphere.
Comet: A small icy body that grows a glowing head (coma) and tails when it nears the Sun.
Conjunction: When two sky objects — planets, the Moon, bright stars — appear close together.
Eclipse: 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: A 0–9 scale of how disturbed Earth's magnetic field is — the higher, the farther from the poles aurorae reach.
Meteor shower: A night with many shooting stars, when Earth passes through the dusty trail of a comet.
Occultation: When one object passes in front of another and hides it — the Moon covering a star or planet, say.
Space station pass: A few minutes when the International Space Station (or China's Tiangong) crosses your sky, lit by the Sun.
Transit: 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: How high an object is above the horizon, from 0° on the horizon to 90° straight overhead.
Angular size: How big something looks, as an angle: degrees (°), arcminutes (′, 1/60°) and arcseconds (″, 1/60′).
Azimuth: The compass direction of an object along the horizon: 0° north, 90° east, 180° south, 270° west.
Celestial sphere: The imaginary dome of the sky around the Earth, on which the stars appear fixed and turn as the Earth rotates.
Circumpolar: Never setting: close enough to the celestial pole that, from your latitude, it circles the pole all night.
Constellation: One of 88 official regions of the sky, most named after ancient star patterns.
Declination: The sky's “latitude”: degrees north (+) or south (−) of the celestial equator.
Ecliptic: The Sun's yearly path across the sky — the planets and the Moon always stay close to it.
Epoch (J2000): The date that coordinates refer to — usually J2000, the start of the year 2000 — because the sky's grid slowly shifts.
Light-year: The distance light travels in a year: about 9.5 trillion kilometres. A parsec is 3.26 light-years.
Meridian: The north–south line across the sky through the zenith; objects are highest when they cross it.
Orientation: Sky charts show north up and east left; telescopes rotate or mirror the view, so it may not match.
Position angle: The direction of one object from another, measured from north (0°) through east (90°).
Right ascension: The sky's “longitude”, measured eastward in hours, minutes and seconds (0h to 24h).
Separation: The angle between two objects — for a double star, the distance between its stars in arcseconds.
Star hopping: Finding a faint object by hopping from a bright star through recognisable patterns of fainter ones.
Zenith: The point directly overhead — altitude 90°.

Brightness & dark skies

Magnitudes, light pollution and how the eye sees faint light.

Averted vision: Looking slightly to one side of a faint object, so its light falls on the most sensitive part of your eye.
Bortle scale: A 1–9 scale of night-sky darkness, from 1 (pristine, the Milky Way casts shadows) to 9 (inner city).
Contrast: How much brighter an object is than the sky around it — the key to seeing faint things.
Dark adaptation: Your eyes becoming far more sensitive in the dark — it takes 20–30 minutes and one white light ruins it.
Extinction: The dimming of starlight by the atmosphere, which grows quickly towards the horizon.
Light pollution: Artificial light scattered into the sky, which brightens it and hides faint stars, galaxies and nebulae.
Limiting magnitude: The faintest magnitude you can see — with the naked eye (NELM) or through a given telescope.
Magnitude: The brightness of a sky object on a reversed scale: smaller numbers are brighter, and each step is 2.5× fainter.
Moonlight: Sunlight reflected by the Moon, which brightens the whole sky and hides faint deep-sky objects.
SQM: Sky brightness in magnitudes per square arcsecond — higher numbers are darker (22 is pristine, 18 is a city).
Surface brightness: 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: Tiny particles in the air — dust, smoke, sea salt, pollution — that dim stars and brighten the sky.
Cloud cover: How much of the sky is covered by cloud, split into low, middle and high layers.
Cooldown: Letting a telescope reach the outdoor temperature, so warm air inside it doesn't blur the view.
Dew: Moisture condensing on optics when they cool below the dew point — it fogs eyepieces, finders and lenses.
Jet stream: A river of fast wind about 10 km up; when it's overhead, the seeing is usually poor.
Seeing: How steady the air is. Good seeing gives sharp, still views at high power; bad seeing makes stars boil and twinkle.
Transparency: How clear the air is: on a transparent night faint stars and galaxies show; haze, humidity and dust hide them.
Twilight: The glow after sunset and before sunrise, in three stages: civil, nautical and astronomical.

Telescopes & optics

Instruments, eyepieces, magnification and accessories.

Airy disk: The tiny disk, surrounded by faint rings, that a perfect telescope makes of a star — set by its aperture.
Aperture: The diameter of a telescope's main lens or mirror — the single most important number.
Apparent field of view: 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: A lens that goes before the eyepiece and multiplies its magnification, usually 2× or 3×.
Binoculars: Two small telescopes side by side; “10×50” means 10× magnification and 50 mm lenses.
Central obstruction: The secondary mirror blocking the middle of a reflector's or SCT's aperture, which slightly lowers contrast.
Chromatic aberration: A purple or yellow fringe around bright objects in simple lens telescopes, because lenses bend colours differently.
Collimation: Aligning a telescope's mirrors so they share one axis — essential for sharp views in reflectors.
Colour filter: A tinted filter (light blue, orange, red…) that raises the contrast of particular features on planets.
Coma: A flaw of Newtonian mirrors that turns stars near the edge of the field into little comets; worse in fast telescopes.
Coma corrector: A lens group that slips in the focuser of a fast Newtonian and makes stars sharp right to the edge of the field.
Dobsonian: A Newtonian reflector on a simple rocking-box mount — the easiest way to get a big aperture.
Exit pupil: The width of the beam of light leaving the eyepiece: aperture ÷ magnification. It sets how bright and sharp the view looks.
Eye relief: How far your eye must be from an eyepiece to see the whole field; 15–20 mm is comfortable, and needed with glasses.
Eyepiece: The small lens you look through, which magnifies the telescope's image; its focal length sets the power.
Finder: A small sighting device on the telescope for aiming: a red-dot or Telrad reflex sight, or a small magnifying finderscope.
Focal length: The distance at which a telescope's lens or mirror brings light to a focus; with the eyepiece it sets the magnification.
Focal ratio: Focal length divided by aperture (f/5, f/10…). “Fast” (low) ratios give wide fields; “slow” (high) ones give easy high power.
Focal reducer: 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: How much more light a telescope collects than your eye: (aperture ÷ 7 mm)². An 8-inch gathers about 840×.
Magnification: How much bigger a telescope makes things look: its focal length divided by the eyepiece's focal length.
Maksutov–Cassegrain: 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: About twice the aperture in millimetres (50× per inch) — but the air rarely allows more than 250–350×.
Moon filter: A neutral grey filter that dims the dazzling Moon for comfortable viewing.
Mount: What holds and points the telescope: alt-azimuth (up-down, left-right) or equatorial (aligned with the sky's rotation).
Nebula filter: A filter that passes the light emitted by nebulae and blocks most skyglow, making them stand out.
Reflector: A telescope that uses mirrors; the Newtonian, with the eyepiece near the top, gives the most aperture for the money.
Refractor: A telescope that uses lenses: sharp, contrasty, low-maintenance — with no central obstruction.
Resolution: 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: A compact telescope with a mirror and a front corrector plate; long focal length (f/10) in a short tube.
Tracking: Moving a telescope to follow the sky's rotation, so objects stay in view.
True field of view: 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: Patrick Moore's 1995 list of 109 bright objects Messier missed, including the best of the southern sky.
Messier catalog: Charles Messier's list of 110 bright nebulae and clusters, made in the 1700s — the classic first targets.
NGC & IC: The New General Catalogue (1888) and its Index Catalogues: the standard numbers for about 13,000 galaxies, clusters and nebulae.
Star names & designations: How stars are named: proper names (Vega), Greek letters by brightness (α Lyrae), or numbers (61 Cygni).
Washington Double Star Catalog: 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: The hours of the night best for observing: dark, clear and — for faint objects — without the Moon.
Cloud map: Tonight's map of clouds around you: Meteosat satellite images for the last hours, then a week of forecast.
Difficulty rating: How hard an object is tonight — easy, moderate, challenging, very hard or out of reach — for your sky, instrument and Moon.
Observing score: AstroPilot's 0–100 rating of the night for deep-sky observing, from cloud, seeing, transparency, darkness and moonlight.
Showpiece: A highlight of the sky that impresses in almost any telescope — the objects to show friends first.
Sky-survey photograph: The photo on each deep-sky object's page: a long-exposure sky survey image, north up, to scale.
Sweet spots: 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).