Glossary
Astronomy glossary
514 definitions · page 1 of 9.
- aberration
- in optics, an imperfect focus caused when a mirror or lens fails to bring light to a sharp focus
- absolute magnitude
- the apparent brightness an object would have if it were 10 parsecs (32.6 light-years) from Earth
- absolute zero
- the coldest theoretical temperature, equal to 0 kelvin (-459.67° F or -273.15° C)
- absorption lines
- dark lines in a spectrum caused by the absorption of light by atoms or molecules in a star or planet’s atmosphere
- accretion disk
- a disk surrounding a black hole or star in which matter gravitationally falls onto the central object
- achromatic lens
- a two-element lens, or doublet, that significantly reduces chromatic aberration
- active galactic nuclei
- the exceptionally bright cores of some galaxies, thought to be fueled by matter falling into supermassive black holes
- active galaxy
- a galaxy emitting unusually large amounts of energy from a compact central source
- active optics
- The techniques by which corrections are made to the shape of a large mirror or radio dish to adjust for minute distortions in its shape. These variations arise as a telescope is subjected to forces such as gravity and temperature changes.
- adaptive optics
- a system of telescopes, computers, and deformable mirrors used to compensate for atmospheric blurring
- Airy disk
- the bright disk-like image of a point source of light, such as a star, as seen in an optical system with a circular aperture
- albedo
- the percentage of light that an object reflects
- altazimuth mount
- a mount that enables a telescope to move freely both vertically (in altitude) and horizontally (in azimuth)
- altitude
- 1. the height above sea level
- anaglyph
- a stereoscopic, composite image in which the right component (usually red in color) is superimposed on the left component (usually blue) to produce a three dimensional effect when viewed through correspondingly colored filters
- Andromeda Galaxy
- the largest member of the Local Group of galaxies; roughly twice the size of the Milky Way; also known as M31
- angular size
- the apparent width of an object as seen by an observer, usually expressed in degrees, arcminutes, or arcseconds
- anisotropies
- differences in physical properties depending on direction
- anisotropy
- the variation of a physical property depending on direction
- annular eclipse
- a solar eclipse in which the moon does not fully cover the sun’s disk, allowing observers to see a thin ring of sunlight.
- antimatter
- matter consisting of particles that have the same mass and properties as their matter counterparts but opposite electrical charges
- Barlow lens
- a lens attached behind the eyepiece of a telescope that increases magnification
- barred spiral galaxy
- a spiral galaxy with a central bar consisting of stars and gas
- Degree Angular Scale Interferometer (DASI)
- a 13-element microwave interferometer, located at the NSF Amundsen-Scott South Pole station, used to measure temperature and polarization in the Cosmic Microwave Background
- Hertzsprung-Russell diagram
- a diagram that plots luminosity against temperature for a group of stars
- anti-tail
- The name given to a comet's tail when it points toward the sun. This rare event typically occurs when Earth crosses the plane of the comet's orbit and the comet is relatively close to the sun.
- aperture
- the diameter of a telescope’s primary lens or mirror; the larger the aperture, the greater the telescope’s light-gathering power
- aphelion
- the point farthest from the sun in an object’s orbit
- apochromatic lens
- a lens with three or more elements that reduces chromatic aberration even more than an achromatic lens
- apogee
- the point in a satellite’s orbit when it is farthest from Earth
- Apollo
- 1. the U.S. space program that sent astronauts to the moon in the 1960s and '70s2. an asteroid with a perihelion less than 1.017 AU (and thus comes within the orbit of Earth)
- apparent field of view
- the angular diameter of the circle of light that the eye sees through an eyepiece
- apparent magnitude
- the measure of the brightness of an object as seen from Earth
- apparition
- the period of time during which a particular celestial object can be seen
- archeoastronomy
- the study of physical artifacts with astronomical connections
- arcminute
- a unit of angular size equal to 1/60 of a degree; abbreviated by 'Arcminutes are used to measure of the separation between two sky objects or the angular size of an object.
- arcsecond
- a unit of angular size equal to 1/3,600 of a degree (or 1/60 of an arcminute); abbreviated by "Arcseconds are used to measure of the separation between two sky objects or the angular size of an object.
- asterism
- a small grouping of stars in the night sky
- asteroid
- a small, rocky body that orbits a star
- asteroid belt
- the zone in which most asteroids orbit the sun, located between the orbits of Mars and Jupiter
- astrometry
- the study of the positions and motions of celestial objects
- astronomical unit
- the average distance from Earth to the sun, equal to about 93,000,000 miles (150,000,000 km)
- astronomical units
- a measurement used by astronomers within the solar system; one astronomical unit (AU) is the average distance between Earth and the sun (about 93,000,000 miles or 150,000,000 kilometers)
- astronomy
- the branch of science concerned with objects beyond Earth
- astrophotography
- the photography of astronomical objects
- astrophysics
- the branch of astronomy that deals with the physical characteristics of celestial objects
- atmosphere
- a gaseous envelope surrounding a moon, planet, or star
- atom
- the fundamental unit of matter; can consist of protons, neutrons, and electrons
- atomic nucleus
- the central region of an atom; can consist of protons and neutrons
- attitude
- the orientation of a spacecraft relative to the direction of its motion
- AU
- a measurement used by astronomers within the solar system; one astronomical unit (AU) is the average distance between Earth and the sun (about 93,000,000 miles or 150,000,000 kilometers)
- aurora
- the emission of light when charged particles from the solar wind slam into and excite atoms and molecules in a planet’s upper atmosphere
- aurora australis
- the southern lights; see definition for aurora
- aurora borealis
- the northern lights; see definition for aurora
- aurorae
- he plural of aurora (the emission of light when charged particles from the solar wind slam into and excite atoms and molecules in a planet’s upper atmosphere)
- autoguider
- a CCD camera used to automatically guide a telescope during long-exposure photography
- autumnal equinox
- the time of year around September 23 when the sun crosses the celestial equator heading south
- averted vision
- a technique that uses the more light-sensitive rods in the eye to better see a faint object by looking at it indirectly
- axis
- a straight line about which an object rotates
- azimuth
- the angle along the horizon measured eastward from due north to the point on the horizon directly below an object