Astronomy

Aquarius (astrology): Sign, traits, history, and meaning Aquarius (astrology) Aquarius is the eleventh sign of the zodiac, an air sign symbolized by the Water Bearer. Associated with innovation, independence, and humanitarianism; traditionally ruled by Satur…

Aquarius (constellation): the Water-Bearer of the sky Aquarius (constellation) Aquarius is a zodiac constellation traditionally depicted as a water‑carrier. It contains several named stars and deep‑sky objects, has roots in ancient myth, and is one of Ptolemy…

Aquila (constellation) Aquila is a summer constellation located a few degrees north of the celestial equator. Represented as an eagle, it contains the bright star Altair and lies along the Milky Way with…

Ara (constellation) — the southern altar in the sky Ara (constellation) Ara is a small southern constellation whose name means 'altar'. Known since antiquity, it contains bright stars, open clusters and notable nebulae such as the Stingray Nebula and N…

Arche (Jupiter XLIII): a small irregular moon of Jupiter Arche (moon) Arche is a small, retrograde irregular satellite of Jupiter discovered in 2002. Part of the Carme group, it orbits far from the planet and offers clues about capture and fragmentat…

Arcturus (Alpha Boötis) — Bright Orange Giant Star Arcturus Arcturus is the brightest star in the northern sky, a nearby K-type orange giant about 36.7 light-years away. It is a prominent navigational and cultural marker and an example of s…

Argo Navis — the ancient ship constellation (former) Argo Navis Argo Navis was a large southern constellation representing Jason’s ship. No longer recognized, it was divided into Carina, Vela and Puppis; its legacy survives in star names, mytho…

Argument of periapsis The argument of periapsis is an orbital element measuring the angle from the ascending node to the orbit's periapsis; it locates where closest approach occurs and helps describe or…

Ariel (moon) — an icy, geologically varied satellite of Uranus Ariel (moon) Ariel is one of Uranus’s major moons. Discovered in 1851, it has a mixed ice–rock composition, a varied surface with canyons and craters, and evidence of past geological activity o…

Aries: constellation and astrological sign Aries Aries is the first sign of the zodiac and a northern constellation. This article explains its astronomical identity, astrological dates (tropical and sidereal), symbolism, origins…

Aries (constellation) Aries is a small zodiac constellation in the northern sky, traditionally depicted as a ram. It contains a few bright stars (Hamal, Sheratan), features in Babylonian and Greek myth,…

Artemis 1: Uncrewed Test Flight of SLS and Orion Artemis 1 Artemis 1 was the first integrated uncrewed test of NASA's Space Launch System and Orion spacecraft, demonstrating deep-space operations, heat-shield performance, and systems neede…

Artemis program — NASA-led return to the Moon Artemis program Artemis is NASA's program to land humans on the Moon and establish sustainable presence, using new spacecraft, commercial partners, and international cooperation to enable science…

Arthur Eddington: astronomer, astrophysicist and interpreter of relativity Arthur Eddington Biography and overview of Sir Arthur Eddington: his work on stellar structure, the Eddington limit, the 1919 eclipse test of general relativity, science popularisation and legacy.

Aryabhata (satellite) Aryabhata was India's first satellite, launched in 1975 and named after the ancient astronomer; it carried scientific experiments and marked the start of India's space programme.

AS-202: Second Uncrewed Test Flight of a Block I Apollo Command Module AS-202 AS-202 was a 1966 uncrewed suborbital test of a Block I Apollo command and service module on a Saturn IB. It validated guidance, fuel cells and reentry systems and cleared the way…

Ascending node (orbital mechanics) Ascending node The ascending node is the point where an orbiting body crosses a chosen reference plane from south to north; its longitude (Ω) fixes the orbit’s orientation and is one of the class…

Asteroid: Small rocky bodies of the Solar System Asteroid An asteroid is a small rocky object orbiting the Sun. This article explains what asteroids are, their composition and orbits, discovery history, roles in science and hazards, and h…

Asteroid belt (Main Belt) between Mars and Jupiter Asteroid belt The asteroid belt is a broad ring of rocky bodies and dust orbiting the Sun between Mars and Jupiter. It contains objects from dust grains to the dwarf planet Ceres and shows gaps…

Asteroid impact prediction: methods, limits, and planetary defense Asteroid impact prediction Overview of how scientists predict asteroid impacts, detection methods, warning times, historical examples, and the role of planetary defense and mitigation efforts.

Asteroid moon A small natural satellite that orbits an asteroid. Usually much smaller than its primary, asteroid moons are uncommon and offer clues about asteroid formation and evolution.

Asteroid spectral types: classification, causes, and importance Asteroid spectral types Overview of how asteroids are classified by reflectance spectra and albedo, main taxonomies and common classes, what spectra reveal about composition, distribution, and scientific…

Astraea — Greek goddess of justice and the star‑maiden Astraea Astraea is a figure from Greek myth associated with justice, innocence and the constellation Virgo. She symbolizes purity, fairness and the loss of a golden age, and appears in art…

Astro (television) — Malaysian satellite TV and radio service Astro (television) Astro is a Malaysian subscription satellite television and radio provider offering digital broadcast, on-demand and streaming services across Malaysia and parts of Southeast Asia.

Astro Boy (Tetsuwan Atomu) — Japanese manga and anime series Astro Boy Astro Boy (Tetsuwan Atomu) is a Japanese manga and anime created by Osamu Tezuka. It follows the robot child Astro and helped establish the visual language and global popularity of…

Astrobiology: the scientific study of life beyond Earth Astrobiology Astrobiology investigates the origins, distribution and future of life in the universe, studying conditions, environments and methods used to detect life on other worlds.

Astrolabe: ancient astronomical instrument and navigational calculator Astrolabe An astrolabe is a historical analog instrument that models the sky to solve problems in astronomy, timekeeping, and navigation. It was used from the Hellenistic world through the m…

Astrology: concepts, history, and contemporary practice Astrology Overview of astrology as a symbolic system linking celestial positions to human affairs; covers basic components, historical development, common practices, and distinctions from as…

Astrometry: Measuring Positions, Motions and Distances of Celestial Bodies Astrometry Astrometry is the branch of astronomy that precisely measures the positions, motions and distances of celestial objects using techniques such as proper motion and parallax, underpi…

Astronaut: People Trained to Travel and Work in Space Astronaut An astronaut (also cosmonaut, taikonaut, spationaute) is a trained person who travels into outer space to conduct missions, perform research, operate spacecraft and live aboard orb…

Astronomer — scientist who studies celestial objects and the universe Astronomer An astronomer observes, models, and interprets celestial phenomena. This article explains methods, tools, history, roles, and how astronomers contribute to science and technology.

Astronomer Royal Senior scientific office in the British Royal Household established in 1675; historically the Director of the Royal Observatory, now largely honorary and advisory to the Sovereign.

Astronomical Almanac — Annual Ephemerides, Time Standards, and Celestial Navigation Data Astronomical Almanac Annual reference from the US Naval Observatory and HM Nautical Almanac Office supplying precise ephemerides, time-scale conversions, eclipse predictions and navigational coordinate…

Astronomical clock: design, history, and cultural significance Astronomical clock Mechanical timekeepers that display celestial information—positions of Sun, Moon, zodiac, and sometimes planets—used in public towers, buildings, and watches for centuries.

Astronomical object: definition, types, formation, and significance Astronomical object A natural body or structure found in space—ranging from dust grains to stars, planets, moons, comets, nebulae and galaxies—its properties, origins, categories and importance to sci…

Astronomical spectroscopy: principles, history, and applications Astronomical spectroscopy Study of spectra from astronomical objects to determine composition, motion, temperature and other physical properties across the electromagnetic spectrum, from radio to gamma rays…

Astronomical unit (AU): definition, history, and uses Astronomical unit The astronomical unit (AU) is the standard length for Solar System distances — defined as exactly 149,597,870,700 meters — and used for planets, spacecraft and basic astronomy calc…

Astronomy: the scientific study of celestial objects, history and methods Astronomy Comprehensive overview of astronomy: what it studies, major objects, observational methods, instruments, history, key subfields and present research directions including multi‑mess…

Astrophysics: study of the physics of the cosmos Astrophysics Astrophysics applies physical laws to understand stars, planets, galaxies and the Universe, combining observation and theory to explain cosmic structure, evolution and phenomena.

Atacama Large Millimeter/submillimeter Array (ALMA) Atacama Large Millimeter Array ALMA is a high‑altitude radio interferometer in northern Chile that images the cold universe at millimeter and submillimeter wavelengths, operated by an international partnership.

Atlas (moon) — Saturn's small ring‑edge satellite Atlas (moon) Atlas is a small irregular moon of Saturn that orbits just outside the A ring. Discovered in Voyager images, it has a distinctive equatorial ridge and helps illustrate ring–moon in…

Auriga (constellation) Auriga is a northern-hemisphere constellation meaning “charioteer.” It contains the bright star Capella, several notable open clusters (M36–M38), and was catalogued by Ptolemy amon…

Australian Aboriginal astronomy: sky knowledge, traditions, and uses Australian Aboriginal astronomy An overview of the astronomical knowledge of Australian Aboriginal peoples: its features, cultural role, history, practical uses and key distinctions within diverse traditions.

Autonoe (Jupiter XXVIII) Autonoe (moon) Autonoe (Jupiter XXVIII) is a small retrograde irregular moon of Jupiter discovered in 2001. It orbits at about 24.26 million km, is roughly 4 km across, and belongs to the Pasipha…

Axial tilt (obliquity) Axial tilt Axial tilt, or obliquity, is the angle between a body's rotation axis and the perpendicular to its orbital plane. It determines seasons, daylight patterns, and influences long-term…

Axis of rotation The axis of rotation is an imaginary line through a body's center about which it spins. It defines day length, seasonal tilt, and many dynamical behaviours of planets, stars and ro…

Baby Boom Galaxy: an extreme early-universe starburst Baby boom galaxy The Baby Boom galaxy is an exceptionally luminous starburst galaxy seen 12.2 billion light years away. It formed stars at thousands per year, challenging simple models of early gal…

Barnard's Galaxy (NGC 6822): a nearby barred irregular galaxy Barnard's galaxy NGC 6822 (Barnard's Galaxy) is a dwarf barred irregular galaxy in Sagittarius, one of the nearest Local Group members and historically important for distance measurements using Cep…