As with any field of scientific study, astronomy has a long list of important terminology. So we've put together this glossary where you can check you fully understand the meaning of the common terms, as well as some of the more unusual words.
A -- B -- C -- D -- E -- F -- G -- H -- I -- J -- K -- L -- M -- N -- O -- P -- Q -- R -- S -- T -- U -- V -- W -- X -- Y -- Z
Words beginning with 'A' :
Accretion disk : a flow of diffused material (gas, plasma, particles or dust) in motion around an astronomical object. The spiraling material is pulled inward by the gravity of the object it orbits, and the friction within the disk causes it to heat up and emit electromagnetic radiation in a variety of wavelengths.
Albedo : the extent to which an object reflects light. It is a unitless measure. It goes from 0 (a black object that absorbs all light and doesn’t reflect any) to 1 (a white object reflects all light and absorbs none). For example Earth’s albedo is 0.30 while Venus’s is 0.75.
Angular size and distance: an object’s apparent size or the distance between two objects as seen from an observer on Earth measured as an angle. For example, at arm’s length your fist measures about 10°,and your little finger measures 1°.
Apastron : the point of maximum separation in an elliptical orbit between an object and its star, applies more specifically to binary systems (two stars orbiting each other).
Aphelion : in an elliptical orbit, the point at which an object is farthest from the Sun.
Aperture : is the diameter of a telescope’s main lens or mirror (the light collection region). The larger the aperture the more light is collected.
Apogee : refers to objects orbiting the Earth. When an object in an elliptical orbit around the Earth is at its maximum distance from the Earth.
Apparent magnitude : the measure of the brightness of an object as observed from Earth. The lower the number the brighter the object. For example the Sun has an apparent magnitude of -27, making it the brightest object in the sky.
Arcminute : is a unit of angular measurement, denoted with the symbol ', and is equal to 1/60 of a degree. It is divided into 60 arcseconds. For example, as seen from Earth, the Sun has an angular diameter of 30 arcminutes.
Arcsecond : is a unit of angular measurement, denoted with the symbol ", and is equal to 1/3600 of a degree. 60 arcseconds make 1 arcminute.
Asterism : a pattern of stars that maybe part of one or more than one constellation but is not in itself a complete constellation.
Asteroid : celestial bodies larger than comets and smaller than planets, sometimes also referred as minor planets or planetoids. They are remnants of protoplanetary disks which usually form large planets.
Astronomical Unit (AU) : the average distance between the Sun and Earth. It is an SI unit (International System of Units) determined in meters, and used as a yardstick to measure distances between the Sun and other Solar System objects as well as objects around other stars. 1 AU = 149,597,870,700 meters (1.50 x 1011 m), this is roughly equal to 150 million kilometers or 93 million miles.
Aurora : colorful light display occurring at high northern or southern latitudes. These are produced when solar wind disturbs the Earth magnetosphere. Energetic protons and electrons from solar winds travel along the Earth's magnetic field lines and collide with atmospheric atoms. The excited atoms emit photons of light when they drop back into a more stable state. The color emitted depends on the gas; for example oxygen atoms release photons that are more red or green.
Aurora austrealis (aka southern lights) : aurorae visible from the southern hemisphere. They can usually be seen from high southern latitudes.
Aurora borealis (aka northern lights) : aurorae visible from the northern hemisphere. They can usually be seen around 60-70 degrees latitude and further south during greater solar activity.
Axis : refers to the line around which an astronomical object rotates. It goes from the north pole to the south pole.
Axial tilt (aka axial inclination or obliquity) : is the angle between the rotational axis of an astronomical object and the line perpendicular to the orbital plane. Earth has an axial tilt of 23.5 degrees, which is the cause of the seasons. Mercury, however, has an axial tilt of just 2.11 degrees and thus has no seasons.
Azimuth : is the horizontal coordinate of an object on the celestial sphere. It is the angle from east to north parallel to the horizon.
To support the quizzes in each section, we've put together some extra resources to provide further information:
- Articles - Alongside the quizzes, you'll find short articles on related topics, to help widen your knowledge of that area.
- Glossary - As with any field of scientific study astronomy has a long list of important terminology. So we've put together a glossary where you can check you fully understand the meaning of the more unusual words!