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- A-type asteroidA-type asteroids are relatively uncommon inner-belt asteroids that have a strong, broad 1
- A-type main-sequence starStellar classification
- A. E. DouglassAmerican astronomer
- A0620-00A0620-00 (abbreviated from 1A 0620-00) is a binary star system in the constellation of Mon
- AB AurigaeStar in the constellation Auriga
- AB DoradusPre-main sequence star system in the constellation Dorado
- Abell 1689Large galaxy cluster in the constellation Virgo
- Abell 2061Abell 2061 is a galaxy cluster in the constellation of Corona Borealis. On a larger scale
- Abell 2065Abell 2065 is a highly concentrated galaxy cluster in the constellation of Corona Borealis
- Abell 2067Abell 2067 is a galaxy cluster in the constellation of Corona Borealis. On a larger scale,
- Abell 2142Galaxy cluster in the constellation Corona Borealis
- Abell 2218Galaxy cluster in the constellation Draco
- Abell 2667Galaxy cluster in the constellation Sculptor
- Abell 370Galaxy cluster in the constellation Cetus
- Abell 39Nebula in the constellation Hercules
- Abell 520Galaxy cluster in the constellation of Orion
- Abell 63Planetary nebula in the constellation of Sagitta
- Abell Catalog of Planetary NebulaeAstronomical catalog
- Abell catalogueAstronomical catalogue of galaxy clusters
- AblationRemoval of material from an object's surface
- Abraham SharpAbraham Sharp (c. 1653 – 18 July 1742) was an English mathematician and astronomer.
- Absolute horizonIn general relativity, an absolute horizon is a boundary in spacetime, defined with respec
- Absolute magnitudeMeasure of the luminosity of celestial objects
- Absorbed doseAmount of energy deposited in matter by ionizing radiation
- Abundance of the chemical elementsThe abundance of the chemical elements is a measure of the occurrences of the chemical ele
- Accelerating expansion of the universeCosmological phenomenon
- Accidental deathUnnatural death caused by accident
- Accretion (astrophysics)Accumulation of particles into a massive object by gravitationally attracting more matter
- Accretion diskStructure formed by diffuse material in orbital motion around a massive central body
- AchernarBrightest star in the constellation Eridanus
- Acidalia PlanitiaPlanitia on Mars
- AcruxSeptuple star system in the constellation Crux
- Active asteroidBodies orbiting within the main asteroid belt which have shown cometary activity
- Active galactic nucleusCompact region at a galaxy's center with abnormally high luminosity
- Active regionTemporary region on the Sun
- AD LeonisM type star in the constellation Leo
- Adam BurrowsAmerican astrophysicist
- Adelaide AmesAmerican astronomer (1900–1932)
- Adriaan BlaauwDutch astronomer (1914–2010)
- Adrien AuzoutFrench astronomer (1622–1691)
- ADRON-RMNeutron spectrometer on mission to Mars
- ADS 9731ADS 9731 is a star system that consists of six stars, located in the constellation of Coro
- Advanced Camera for SurveysThe Advanced Camera for Surveys (ACS) is a third-generation axial instrument aboard the Hu
- Advanced Satellite for Cosmology and AstrophysicsThe Advanced Satellite for Cosmology and Astrophysics (ASCA, formerly named ASTRO-D) was t
- Adverbial genitiveIn grammar, an adverbial genitive is a noun declined in the genitive case that functions a
- AE Ursae MajorisVariable star in the constellation Ursa Major
- Aeolis PalusPalus on Mars
- Aerial telescopeAn aerial telescope is a type of very long focal length refracting telescope, built in the
- AerobrakingAerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit (
- AeronomyScience of the upper region of the Earth's or other planetary atmospheres
- AF LeporisVariable star in the constellation Lepus
- AfternoonTime of the day between noon and evening
- AG CarinaeLuminous variable star in the constellation Carina
- Age of the universeCosmological time duration
- AH VirginisStar in the constellation Virgo
- Ahuna MonsLargest mountain on Ceres
- AI VelorumVariable star in the constellation of Lynx
- Air mass (astronomy)Amount of air seen through in astronomical observations
- Air pumpPump for pushing air
- Air shower (physics)Cascade of atmospheric subatomic particles
- Airborne observatoryTelescopes carried by aircraft
- AirglowFaint emission of light by a planetary atmosphere
- AirworthinessIn aviation, airworthiness is the measure of an aircraft's suitability for safe flight. In
- Airy-0Crater on Mars whose location defines the position of the prime meridian
- AK ScorpiiBinary star in the constellation Scorpius
- Al-BattaniMuslim astronomer and mathematician (died 929)
- Al-Birjandi16th-century Persian polymath
- Alan C. CummingsAmerican astrophysicist (born 1944)
- Alan GuthAmerican theoretical physicist and cosmologist
- Alan Hale (astronomer)American astronomer; co-discoverer of Comet Hale–Bopp (1958–2026)
- Alba MonsMartian volcano
- Albategnius (crater)Albategnius is an ancient lunar impact crater located in the central highlands. It is name
- AlbedoRatio of how much light is reflected back from a body
- Albert AubletAlbert Louis Aublet (French pronunciation: ; 18 January 1851 in Paris – 3 March 1938 in Ne
- Albert Einstein Science ParkThe Albert Einstein Science Park is located on the Telegrafenberg hill in Potsdam, Germany
- Albiorix (moon)Albiorix is a prograde irregular satellite of Saturn. It was discovered by Holman and coll
- AlbireoDouble star system in the constellation Cygnus
- Alcyone (star)Star system in the constellation Taurus
- Alcyoneus (galaxy)Large radio galaxy
- AldebaranBrightest star in Taurus
- Alex FilippenkoAmerican astrophysicist (born 1958)
- Alexis BouvardFrench astronomer
- Alfred FowlerEnglish astronomer
- Alfvén surfaceBoundary between solar corona and wind
- AlgolEclipsing variable star in the constellation Perseus
- Algol variableClass of eclipsing binary stars
- Alicante 8Alicante 8, also known as RSGC4, (Red Supergiant Cluster 4) is an asterism, formerly thoug
- Alice HardingAlice Kust Harding is an American astrophysicist at the NASA Goddard Space Flight Center,
- Alinda asteroidGroup of near-Earth asteroids
- AliothStar in the constellation Ursa Major
- Alison CoilAmerican astrophysicist
- Alister GrahamAustralian astronomer known for work on galaxy structure and supermassive black holes
- AlkaidStar in the constellation Ursa Major
- Allan HillsThe Allan Hills (76°42′S 159°42′E / 76.700°S 159.700°E) are a group of hills, mainly ice f
- Allan Hills 77005Martian meteorite found in Antarctica
- Allan Hills 84001Martian meteorite discovered in Antarctica in 1984
- Allegheny ObservatoryResearch institution in Pennsylvania, US
- Allen Telescope ArrayRadio telescope array in California
- Allende meteoriteCV3 carbonaceous chondrite meteorite
- AlmagestAstronomical treatise by Claudius Ptolemy
- AlnilamStar in the constellation Orion, in the center of the Orion's Belt
- AlnitakTriple star system in the constellation Orion
- Alpha AquariiStar in the constellation Aquarius
- Alpha AraeStar in the constellation Ara
- Alpha CamelopardalisStar in the constellation Camelopardalis
- Alpha CassiopeiaeStar in the constellation Cassiopeia
- Alpha CentauriStar system in the Centaurus constellation
- Alpha Centauri BbAlpha Centauri Bb (α Cen Bb) was a proposed exoplanet orbiting the K-type main-sequence st
- Alpha CepheiStar in the constellation of Cepheus
- Alpha CetiRed giant star in the constellation Cetus
- Alpha CirciniStar in the constellation Circinus
- Alpha ColumbaeStar in the constellation Columba
- Alpha Comae BerenicesStar in the constellation Coma Berenices
- Alpha Coronae BorealisBinary star in the constellation Corona Borealis
- Alpha Cygni variableVariable stars which exhibit non-radial pulsations
- Alpha DoradusStar in the constellation Dorado
- Alpha FornacisStar in the constellation Fornax
- Alpha GruisStar in the constellation Grus
- Alpha HerculisMultiple star system in the constellation Hercules
- Alpha HydriStar in the constrellation Hydrus
- Alpha IndiStar in the constellation Indus
- Alpha LeporisBrightest star in the constellation Lepus
- Alpha LibraeQuadruple star in the constellation Libra
- Alpha LupiStar in the constellation Lupus
- Alpha LyncisStar in the constellation Lynx
- Alpha MuscaeStar in the constellation Musca
- Alpha nuclideAn alpha nuclide is a nuclide that consists of an integer number of alpha particles. Alpha
- Alpha OphiuchiStar in the constellation Ophiuchus
- Alpha PavonisStar in the southern constellation Pavo
- Alpha PegasiStar in the constellation Pegasus
- Alpha PerseiBrightest star in the constellation Perseus
- Alpha Persei ClusterOpen cluster in the constellation Perseus
- Alpha PhoenicisOrange-hued star in the constellation Phoenix
- Alpha PictorisBrightest star in the southern constellation of Pictor
- Alpha processNuclear fusion reaction
- Alpha ReticuliBrightest star in the constellation Reticulum
- Alpha SagittariiStar in the constellation of Sagittarius
- Alpha ScutiAlpha Scuti, Latinized from α Scuti, is an orange-hued star in the southern constellation
- Alpha SerpentisDouble star in the constellation Serpens
- Alpha TelescopiiAlpha Telescopii, Latinized from α Telescopii, is the brightest star in the southern const
- Alpha TrianguliStar in the constellation Triangulum
- Alpha Trianguli AustralisStar in the constellation Triangulum Australe
- Alpha TucanaeBinary star system in the constellation Tucana
- Alpha VulpeculaeAlpha Vulpeculae (α Vulpeculae, abbreviated Alpha Vul, α Vul), officially named Anser , is
- Alpha2 Canum Venaticorum variableVariable star type
- AlphardStar in the constellation Hydra
- Alpher–Bethe–Gamow paperScientific theory of the origin of chemical elements
- AlpheratzBinary star in the constellation of Andromeda
- Alphonse BorrellyFrench astronomer
- AltairBrightest star in the constellation Aquila
- Alvan ClarkAmerican astronomer and telescope maker
- AM Canum VenaticorumStar in the constellation Canes Venatici
- AM Canum Venaticorum starType of cataclysmic variable star
- AM HerculisStar in the constellation Hercules
- Am starType of chemically peculiar star
- Amalthea (moon)Moon of Jupiter
- Amanda BoshAmerican astronomer
- Amateur astronomyHobby of watching the sky and stars
- Amateur telescope makingActivity of building telescopes as a hobby
- Amazonian (Mars)Time period on Mars
- American Association of Variable Star ObserversOrganization of amateur and professional astronomers
- American Astronomical SocietyAmerican professional organization of astronomers
- American Ephemeris and Nautical AlmanacBook of astronomical data for navigation
- AMiBARadio telescope on Mauna Loa, Hawaii
- Amina HelmiArgentine astronomer (born 1970)
- Ammonium hydrosulfideChemical compound
- Amor asteroidGroup of near-Earth asteroids
- Amorphous carbonCarbon that has no crystalline structure
- AN Ursae MajorisAN Ursae Majoris is a binary star system in the northern circumpolar constellation of Ursa
- AnalemmaDiagrammatic representation of Sun's position over a period of time
- AnaximanderAncient Greek philosopher (c. 610 – c. 546 BC)
- Ancient Greek astronomyAncient Greek astronomy is the astronomy written in the Greek language during classical an
- Anderson Mesa StationAnderson Mesa Station is an astronomical observatory established in 1959 as a dark-sky obs
- André-Louis DanjonFrench astronomer (1890–1967)
- Andrea BoattiniItalian astronomer (born 1969)
- Andrew Ainslie CommonEnglish astronomer
- Andromeda (constellation)Constellation in the northern celestial hemisphere
- Andromeda GalaxyBarred spiral galaxy in the Local Group
- Andromeda IDwarf galaxy in the constellation Andromeda
- Andromeda XXIAndromeda XXI (And 21, And XXI) is a moderately bright dwarf spheroidal galaxy about 859 ±
- Andromeda–Milky Way collisionPostulated future astronomical event
- Anemic galaxyAn anemic galaxy is a type of spiral galaxy characterized by a low contrast between its sp
- Angeline StickneyChloe Angeline Stickney Hall (November 1, 1830 – July 3, 1892) was an American mathematici
- Anglo-Australian Planet SearchThe Anglo Australian Planet Search or (AAPS) is a long-term astronomical survey started in
- AngriteAngrites are a rare group of achondrites consisting mostly of Al-Ti bearing diopside, hede
- Angular diameterHow large a sphere or circle appears
- Angular diameter distanceAstronomical concept
- Angular resolutionAbility of any image-forming device to distinguish small details of an object
- Annibale de GasparisAnnibale de Gasparis (Italian pronunciation: ; 9 November 1819 – 21 March 1892) was an Ita
- Annual Review of Astronomy and AstrophysicsAcademic journal
- ANSMETProgram funded by the National Science Foundation
- AntaresBinary star in the constellation Scorpius
- Antennae GalaxiesGalaxies in the constellation Corvus
- Anthropic principleHypothesis about sapient life and the universe
- Anti-de Sitter spaceMaximally symmetric Lorentzian manifold with a negative cosmological constant
- Antimatter cometHypothetical object
- Antisolar pointPoint on the celestial sphere opposite Sun
- AntliaConstellation in the Southern Celestial Hemisphere
- Antlia DwarfThe Antlia Dwarf is a dwarf spheroidal/irregular galaxy. It lies about 1.3 Mpc (4.3 millio
- Antlia-Sextans GroupThe Antlia-Sextans Group is a small Galaxy group in the constellations Hydra, Sextans, Ant
- Antonín MrkosCzech astronomer
- Ap and Bp starsChemically peculiar stars of spectral types A and B
- AP ColumbaeStar in the constellation Columba
- AP LibraeActive galactic nucleus in the constellation Libra
- Apache Point ObservatoryThe Apache Point Observatory (APO; obs. code: 705) is an astronomical observatory located
- Aperture masking interferometryAperture masking interferometry (or Sparse aperture masking) is a form of speckle interfer
- Aphrodite TerraHighland region on Venus, near the equator
- Apollo 1 HillsHills on Mars
- Apollo 10Second crewed mission to orbit the Moon
- Apollo 11 missing tapesLost NASA telemetry data tapes
- Apollo 12Second crewed Moon landing
- Apollo 13Aborted Moon landing mission in the Apollo program
- Apollo 14Third crewed Moon landing
- Apollo 15Fourth crewed Moon landing
- Apollo 16Fifth crewed Moon landing
- Apollo 17Sixth and most recent crewed Moon landing
- Apollo 4First test flight of the Apollo Saturn V rocket
- Apollo 5Uncrewed first test flight of the Apollo Lunar Module
- Apollo 61968 American Saturn V rocket test flight
- Apollo 7First crewed flight of the Apollo space program
- Apollo 8First crewed space mission to orbit the Moon
- Apollo 9Third crewed mission of the Apollo space program
- Apollo asteroidGroup of near-Earth asteroids
- Apollo command and service moduleComponent of the Apollo spacecraft
- Apollo Lunar Surface Experiments PackageScientific instruments left by the Apollo astronauts on the Moon
- Apparent magnitudeBrightness of a celestial object observed from the Earth
- Apparent placeThe apparent place of an object is its position in space as seen by an observer. Because o
- Apparent retrograde motionApparent backward motion of a planet
- Applegate mechanismStellar phenomenon
- ApsisEither of two extreme points in a celestial object's orbit
- ApusConstellation in the southern celestial hemisphere
- Aquarius (astrology)Eleventh astrological sign of the zodiac
- Aquarius (constellation)Zodiac constellation straddling the celestial equator
- Aquarius DwarfGalaxy in the constellation Aquarius
- Aquila (constellation)Constellation near the celestial equator
- AR LacertaeStar system in the constellation Lacerta
- Ara (constellation)Constellation in the southern celestial hemisphere
- Arachnoid (astrogeology)In astrogeology, an arachnoid is a large geological structure resembling a spider web. The
- Arago (lunar crater)Arago is a lunar impact crater located in the western part of the Mare Tranquillitatis. It
- Aram ChaosCrater on Mars
- Arandas (crater)Arandas is a crater in the Mare Acidalium quadrangle of Mars, located 42.77° North and 15.
- ArchaeoastronomyInterdisciplinary study of astronomies in cultures
- Arches ClusterDensest star cluster known
- ArcturusBrightest star in the constellation Boötes
- Area code 831Area code 831 is a telephone area code in the North American Numbering Plan for a small re
- Arecibo TelescopeFormer radio telescope in Puerto Rico, United States
- AreographyDelineation and characterization of Martian regions
- Ares IAres I was the crew launch vehicle that was being developed by NASA as part of the Constel
- Ares VThe Ares V (formerly known as the Cargo Launch Vehicle or CaLV) was the planned cargo laun
- Argelander (crater)Argelander is a lunar impact crater that is located in the south-central highlands of the
- Argo NavisObsolete Southern constellation
- Argument of periapsisSpecifies the orbit of an object in space
- Ariel (moon)Fourth-largest moon of Uranus
- Aries (astrology)First astrological sign of the zodiac
- Aries (constellation)Zodiac constellation in the northern hemisphere
- ArietidsAnnual meteor shower in May and June
- Aristarchus (crater)Crater on the near side of Earth's Moon
- Aristarkh BelopolskyRussian astronomer (1854–1934)
- Armagh ObservatoryAstronomical research institute in Armagh, Northern Ireland
- ARO 12m Radio Telescope12-meter dish on Kitt Peak in Arizona, US
- Arp 220Luminous infrared galaxy in the constellation Serpens
- Arp 273Pair of interacting galaxies in the constellation Andromeda
- Arsia MonsMartian volcano
- Artemis AccordsMultilateral agreement on human moon exploration
- Artemis I2022 uncrewed Moon-orbiting NASA mission
- Artemis II2026 crewed flyby of the Moon
- Artemis IIIUpcoming crewed mission of the Artemis program
- Artemis programNASA-led lunar exploration program
- Arthur AuwersGerman astronomer (1838–1915)
- Arthur BeerGerman astronomer
- Arthur CodeArthur Dodd Code (August 13, 1923 – March 11, 2009) was an astronomer who designed orbitin
- AryabhatiyaSanskrit astronomical treatise by Aryabhata
- Asaph HallAmerican astronomer (1829–1907)
- ASASSN-15lh2015 hypernova event in the constellation Indus
- Ascraeus MonsMartian volcano
- Associated Universities, Inc.Associated Universities, Inc. (AUI) is a research management corporation that builds and o
- Association of Space ExplorersThe Association of Space Explorers is a non-profit organization with a membership composed
- ASTER (spacecraft)Planned asteroid orbiter mission
- Asterism (astronomy)Pattern of stars recognized on Earth's night sky
- AsteroidMinor planet found within the inner Solar System
- Asteroid beltRegion between the orbits of Mars and Jupiter
- Asteroid familyAsteroid population sharing similar proper orbital elements
- Asteroid impact avoidanceMethods to prevent destructive asteroid hits
- Asteroid pairAn asteroid pair, or (if more than two bodies) an asteroid cluster, are asteroids which at
- Asteroid spectral typesClassification type of a class of astronomical objects
- AsteroseismologyStudy of oscillations in stars
- AstrobiologyScience concerned with life in the universe
- AstrobotanyStudy of plants grown in spacecraft
- AstrochemistryStudy of molecules in the Universe and their reactions
- AstroecologyStudy of the interactions of biota with space environments
- AstrographType of telescope
- AstroidIn mathematics, an astroid is a particular type of roulette curve: a hypocycloid with four
- AstroinformaticsInterdisciplinary field of study
- Astrological ageTime period in ancient historical and astrological theories of time
- Astrological signTwelve 30° sectors of the ecliptic, as defined by Western astrology
- Astrological symbolsSymbols denoting astrological concepts
- Astrology and astronomyAstrology and astronomy were archaically treated together (Latin: astrologia), but gradual
- Astrology and scienceAstrology consists of a number of belief systems that hold that there is a relationship be
- Astrology in the medieval Islamic worldIslamic astrology of the Golden Age
- AstrometeorologyBelief that astrology can be used for weather prediction
- AstrometryBranch of astronomy involving positioning and movements of celestial bodies
- Astronaut trainingPreparing astronauts for space missions
- AstronomerScientist in the field of astronomy
- Astronomer RoyalPosition in the Royal Households of the United Kingdom
- Astronomical chronologyMethod of determining the age of events or artifacts
- Astronomical constantAny of several physical constants used in astronomy
- Astronomical coordinate systemsSystem for specifying positions of celestial objects
- Astronomical naming conventionsSystematic designations to unambiguously identify astronomical objects
- Astronomical nutationVariance in a celestial body's axis of rotation over time
- Astronomical objectNatural physical entity in space
- Astronomical Observatory (University of Illinois Urbana-Champaign)Observatory in Urbana, Illinois, U.S.
- Astronomical optical interferometryAstronomical methodology
- Astronomical radio sourceObject in outer space which strongly emits radio waves
- Astronomical seeingAtmospheric distortions of light
- Astronomical Society of IndiaLearned society in India, of professional astronomers
- Astronomical Society of Southern AfricaAstronomical society based in South Africa
- Astronomical Society of the PacificAmerican scientific and educational organization
- Astronomical spectroscopyMeasurement of electromagnetic radiation for astronomy
- Astronomical surveyGeneral map or image of a region of the sky with no specific observational target
- Astronomical transitTerm in astronomy
- Astronomische GesellschaftGerman astronomical association
- Astronomische NachrichtenAcademic journal
- AstronomyScientific study of celestial objects
- Astronomy and religionAstronomy has been a favorite and significant component of mythology and religion througho
- Astronomy educationAstronomy education or astronomy education research (AER) refers both to the methods curre
- Astronomy in ChileIn 2011, Chile was home to 42% of the world's astronomical infrastructure, consisting prin
- Astronomy in the medieval Islamic worldMedieval Islamic astronomy comprises the astronomical developments made in the Islamic wor
- AstrophotographyImaging of astronomical objects
- Astrophysical jetStream of ionized matter flowing away from a rotating astronomical object
- Astrophysical maserNatural source of microwave radiation emission
- Astrophysical plasmaAstrophysical plasma is plasma outside of the Solar System. It is studied as part of astro
- Astrophysical X-ray sourceAstronomical object emitting X-rays
- AstrophysicsSubfield of astronomy
- Astrophysics (journal)Astrophysics is a peer-reviewed scientific journal of astrophysics published by Springer.
- Astrophysics Data SystemDigital library portal operated by the Smithsonian
- Asymptotic giant branchGrouping of evolved cool luminous stars
- AT MicroscopiiAT Microscopii is a binary star system located at a distance of 35 ly (11 pc) from the Sun
- Aten asteroidGroup of near-Earth asteroids
- Athabasca VallesOutflow channel on Mars
- Athena CoustenisGreek-French astrophysicist
- Atira asteroidGroup of near-Earth asteroids
- Atlas CoelestisStar atlas
- Atlas of Peculiar GalaxiesCatalogue of peculiar galaxies produced by Halton Arp
- AtmosphereLayer of gases surrounding an astronomical body held by gravity
- Atmosphere of JupiterGas layer surrounding Jupiter
- Atmosphere of MarsGas layer surrounding Mars
- Atmosphere of MercuryGas layer surrounding planet Mercury
- Atmosphere of PlutoLayer of gases surrounding the dwarf planet Pluto
- Atmosphere of TitanGas layer surrounding the moon Titan
- Atmosphere of TritonLayer of gasses surrounding the moon Triton
- Atmosphere of UranusGas layer surrounding Uranus
- Atmosphere of VenusGas layer surrounding Venus
- Atmospheric entryPassage of an object through the gases of an atmosphere from outer space
- Atomic and molecular astrophysicsField of study on atomic and molecular particles in space
- AU MicroscopiiStar in the constellation Microscopium
- AubriteAubrites are a group of meteorites named for Aubres, a small achondrite meteorite that fel
- Audouin DollfusFrench astronomer and aeronaut
- August 1972 solar stormsSolar storms during solar cycle 20
- August KopffGerman astronomer
- AurigaConstellation in the northern celestial hemisphere
- Aurora programmeESA human spaceflight programme from 2001
- Australia Telescope Compact ArrayThe Australia Telescope Compact Array (ATCA) is a radio telescope operated by CSIRO at the
- Australia Telescope National FacilityThe Commonwealth Scientific and Industrial Research Organisation (CSIRO)'s radio astronomy
- Australian Astronomical ObservatoryObservatory
- Australian Square Kilometre Array PathfinderThe ASKAP radio telescope is a radio telescope array located at Inyarrimanha Ilgari Bundar
- Autograph of Nicolaus Copernicus' De revolutionibus1543 Latin manuscript by Nicolaus Copernicus
- Autolycus of Pitane4th-century BC Ancient Greek astromer, mathematician and geographer
- Auwers synthesisThe Auwers synthesis is a series of organic reactions forming a flavonol from a coumarone.
- Auzout (crater)Auzout is a lunar impact crater that is located to the southeast of the Mare Crisium, near
- AW Ursae MajorisVariable star in the constellation Ursa Major
- Axial parallelismCharacteristic of a spinning body in space
- Axial precessionChange of rotational axis in an astronomical body
- Axial tiltAngle between the rotational axis and orbital axis of a body
- Axis of evil (cosmology)Anomaly in cosmic microwave background
