Astronomy

Conjunction (astronomy and astrology) Conjunction: when two celestial bodies appear close together from an observer’s viewpoint. Covers astronomical definition, types, observation notes, astrological meaning, symbol, a…

Constellations: cultural meanings, history, and the 88 official star regions Constellation Explore constellations: how cultures worldwide interpreted star patterns, their roles in myth and navigation, and the modern mapping that divides the sky into 88 official star regi…

Cordelia (moon of Uranus) Cordelia (moon) Cordelia is the innermost known moon of Uranus, a small, irregular satellite discovered in Voyager 2 images in 1986, named for a character in Shakespeare's King Lear and associated…

Corona (solar and stellar outer atmosphere) Corona The corona is the tenuous, extremely hot outer atmosphere of the Sun and other stars. This article explains its structure, temperature paradox, observable features, methods of stud…

Corona Australis (Southern Crown) Corona Australis Small southern constellation shaped like a crown or wreath, noted for a nearby star‑forming molecular cloud and modest, widely spaced stars; visible primarily from southern latitud…

Corona Borealis — the Northern Crown constellation Corona Borealis Corona Borealis, the Northern Crown, is a small but distinctive northern constellation noted for its semicircular pattern, mythological origins, bright star Alphecca, and the irreg…

Coronagraph (instrument for observing faint structures near bright stars) Coronagraph A coronagraph is an optical device used with telescopes to block a bright star (most often the Sun) so faint nearby features — the solar corona, dust, or exoplanets — can be studie…

CoRoT-7b — a rocky exoplanet with an ultra-short orbital period COROT-7b CoRoT-7b is a small, likely rocky exoplanet discovered by the CoRoT mission in 2009. It orbits its star in about 20 hours and was one of the first transiting worlds with measured s…

Corvus: the Raven constellation Corvus (constellation) Corvus is a small southern constellation — Latin for ‘raven’ or ‘crow’ — known for a compact quadrilateral of bright stars, classical mythology, and its listing among Ptolemy's 48…

Cosmic distance ladder: how astronomers measure distances in the universe Cosmic distance ladder A structured overview of the cosmic distance ladder: its purpose, principal methods (parallax, standard candles and rulers, redshift), history, strengths and limitations, and why i…

Cosmic dust: origin, composition, observations and role in planetary systems Cosmic dust Tiny solid particles found throughout space. Covers composition, formation, locations (interstellar, interplanetary, circumplanetary), observational signatures, and importance for…

Cosmic microwave background radiation Relic microwave radiation from the early universe: a near–blackbody glow with small anisotropies that records conditions at photon decoupling and constrains cosmological models.

Cosmogony: Theories and Traditions on the Origin of the Universe Cosmogony Cosmogony examines accounts and theories about how the universe began, covering mythic traditions, philosophical speculation, and scientific models such as the Big Bang and alterna…

Cosmology — the scientific study of the universe Cosmology Cosmology is the scientific discipline that investigates the origin, large-scale structure, evolution and ultimate fate of the universe using observation and theoretical physics.

Crab Nebula: Remnant of SN 1054 and its Pulsar Crab Nebula Overview of the Crab Nebula (M1): its structure, history as the remnant of SN 1054, the central Crab pulsar, and why it is important to astronomy and astrophysics.

Crater (constellation) Crater is a small southern constellation whose name means 'cup' in Latin. It has ancient mythic associations with Apollo and the crow, was catalogued by Ptolemy, and remains one of…

Cressida (Uranian moon) Cressida (moon) Cressida is a small inner moon of Uranus discovered in Voyager 2 images in 1986. It is elongated, dark, and a member of the Portia group; most knowledge comes from brief spacecraft…

Crux (Southern Cross): constellation, history, navigation and cultural symbols Crux Crux, known as the Southern Cross, is the smallest modern constellation; a compact, bright cross of stars used for navigation and widely depicted on flags and emblems across the So…

Cupid (moon) — small inner satellite of Uranus Cupid (moon) Cupid is a tiny inner moon of Uranus, discovered in 2003 by Hubble. Roughly 18 km across and dark, it orbits close to Belinda, carries the designation S/2003 U 2 (Uranus XXVII), an…

Curiosity rover Curiosity is NASA's car-sized, nuclear-powered Mars rover that landed in Gale Crater in 2012 to study Mars' climate, geology, and past habitability as part of the Mars Science Labo…

CW Leonis (IRC+10216): a carbon-rich red giant star CW Leonis CW Leonis (IRC+10216) is a carbon-rich asymptotic giant branch star in Leo, heavily obscured by dust and extremely bright in the infrared; a key object for studying circumstellar c…

Cybele asteroids: the outer main-belt group beyond 3.27 AU Cybele asteroid A compact overview of the Cybele asteroids: their location in the outer asteroid belt, key orbital and physical traits, history of the group, scientific importance, and how they di…

Cygnus — the Swan: a northern constellation with the Northern Cross Cygnus (constellation) Cygnus is a prominent northern constellation shaped like a swan and home to Deneb, the Northern Cross asterism, rich Milky Way fields, notable nebulae, and the famous X-ray source…

Cygnus OB2: a massive OB association in the constellation Cygnus Cygnus OB2 Cygnus OB2 is a very large, young association of massive stars in the constellation Cygnus. It is heavily obscured by dust, contains many O-type and luminous stars, and lies within…

Cygnus X-1: a well-known stellar-mass black hole and X‑ray binary Cygnus X-1 Cygnus X‑1 is a bright X‑ray source in the constellation Cygnus, widely accepted as a stellar‑mass black hole in a binary system. It has been central to studies of accretion, compa…

Cyllene (Jupiter XLVIII) — small retrograde moon of Jupiter Cyllene (moon) Cyllene (Jupiter XLVIII, S/2003 J 13) is a ~2 km irregular, retrograde moon of Jupiter discovered in 2003. It orbits far from the planet and is a member of the Pasiphaë group; name…

Damocloid: dormant comet-like small bodies of the outer Solar System Damocloid Damocloids are small bodies with long, comet-like orbits but no visible coma. Thought to be inactive comet nuclei from distant reservoirs, they have high inclinations, often retrog…

Daphnis (moon of Saturn) Daphnis (moon) Daphnis is a tiny shepherd moon of Saturn that orbits in the Keeler Gap. Discovered by Cassini in 2005, it is roughly 6–8 km across and produces visible waves and wakes in the plan…

Dark Energy: The Mysterious Force Driving Cosmic Acceleration Dark energy Dark energy is the name given to the unknown cause of the universe's accelerating expansion. It appears uniform, resists clumping, and dominates the cosmos, shaping its fate and ch…

Dark nebula Dense interstellar clouds of gas and dust that block visible light, appearing as dark silhouettes against star fields; important sites for molecular chemistry and early star format…

Dawn (spacecraft) NASA's Dawn was a pioneering unmanned mission that used ion propulsion to orbit and study the protoplanets Vesta and Ceres in the main asteroid belt.

Day (Earth's rotation and civil time) Day A day is one full rotation of Earth relative to the Sun, commonly divided into 24 hours of daylight and night. This article explains solar and sidereal days, causes of variation, m…

Daylight Saving Time Seasonal practice of advancing clocks in warmer months to extend evening daylight. Adoption, timing, benefits and drawbacks vary by country and locality; many jurisdictions debate…

De revolutionibus orbium coelestium — Copernicus and the heliocentric work De revolutionibus orbium coelestium Nicolaus Copernicus's 1543 De revolutionibus proposed a Sun-centered system, challenged the Ptolemaic universe and helped launch the Copernican and Scientific Revolutions.

Declination (astronomy): the celestial latitude coordinate Declination Declination (dec or δ) is the angular distance of a celestial object north or south of the celestial equator, forming one coordinate of the equatorial system paired with right asce…

Deep Impact (NASA comet impactor mission) Deep Impact (space mission) Deep Impact was a NASA mission that struck comet Tempel 1 in 2005 to expose and study subsurface material, revealing ice, dust and organic-rich constituents and advancing comet for…

Deimos — moon of Mars Deimos (moon) Deimos is the smaller, outer moon of Mars. About 15 km across and heavily cratered, it completes an orbit in ~30.3 hours and is of interest as a possible captured asteroid and miss…

Delphinus (constellation) Delphinus, the Dolphin, is a small northern constellation notable for its distinctive diamond-and-tail asterism, several bright stars with unusual names, and a handful of telescopi…

Delta Cephei: a fundamental Cepheid variable and distance calibrator Delta Cephei Overview of Delta Cephei, a nearby classical Cepheid in Cepheus: its variability, physical properties, discovery, role as a distance calibrator, mass loss and surrounding nebula, a…

Delta II — American medium-lift launch vehicle and workhorse rocket Delta II Delta II, a reliable medium-lift expendable launch vehicle derived from the Thor family, served from 1989 to 2018 for civil, military and scientific payloads including GPS and plan…

Delta IV (Evolved Expendable Launch Vehicle) Delta IV American family of expendable launch vehicles used for government and commercial satellites; available in Medium, Medium+ and Heavy configurations; launched from Cape Canaveral and…

Delta-v (Δv): change in velocity and its role in spacecraft missions Delta-v Delta-v (Δv) quantifies the change in velocity a spacecraft can achieve. It is a core planning metric used for fuel sizing, transfer design and evaluating mission feasibility.

Denpa: Japan's 1972 Radiation Research Satellite (REXS) Denpa (satellite) Denpa (電波) was Japan's 1972 radiation-experiments satellite (REXS), built by the Institute of Space and Astronautical Science to measure energetic particles and characterize near-E…

Desdemona (Uranian moon) Desdemona (moon) Desdemona is a small inner moon of Uranus discovered by Voyager 2 in 1986. It belongs to the Portia group and is known mainly from brief spacecraft imaging and limited photometry.

Despina — inner moon of Neptune Despina (moon) Despina is a small, irregular inner satellite of Neptune discovered in Voyager 2 images in 1989. It orbits close to the planet, shows no signs of geological activity, and is named…

Dia (S/2000 J 11): an irregular prograde moon of Jupiter Dia (moon) Dia is a small, irregular prograde satellite of Jupiter (S/2000 J 11), roughly 4 km across, a member of the Himalia group; discovered in 2000, lost for a time and later recovered t…

Didier Queloz — Swiss astronomer and Nobel laureate Didier Queloz Didier Queloz is a Swiss astronomer who co-discovered the first exoplanet around a Sun-like star. He holds posts at the University of Geneva and the University of Cambridge and won…

Dione (Saturnian moon) Dione (moon) Dione is a mid-sized icy moon of Saturn, noted for its craters, bright ice cliffs and "wispy" terrain. Discovered in 1684, it was imaged in detail by the Cassini mission, which rev…