Dictionary of Astronomy
By Students' Academy
Copyright@2011Students' Academy
Smashwords Edition
Chapter 1
A
Aberration of Starlight
Aberration of Starlight refers to the obvious displacement of a star's position as a consequence of Earth's motion through space and the finite speed of light.
Ablation
Ablation refers to the process of vaporization of the surface layers of a body when it enters the atmosphere; it occurs due to the heating that results from the compression of air ahead of it.
Absolute Magnitude
Absolute Magnitude represents the evident magnitude which an object has if it was placed at a distance of 10 parsecs from the observer.
Achromatic
Achromatic literally means no colours. Achromatic also refers to the combination in which chromatic aberration is corrected by bringing two colours to the same focus.
Accretion
Accretion refers to the accumulation of dust and gas into much bigger celestial bodies such as stars, planets and moons, or as discs around existing bodies.
Airy disc
Airy Disc represents the bright central part of the image of a star.
Altazimuth Mount
Altazimuth Mount refers to a mounting in which the axes of rotation are vertical and horizontal, i.e. in altitude and azimuth.
Altitude
Elevation especially above sea level or above the earth's surface
Albedo
Albedo is the ratio of reflected to incident light
Analemma
Analemma refers to a lemniscate-shaped form that results from plotting the position of the Sun at the same time every day.
Anomaly
Anomaly refers to the angle at the Sun between a planet and it perihelion.
Ansae
In the past it referred to a description of the appearance of Saturn's rings before they were recognised as being a ring system. Now this term is used to describe the extension of Saturn's rings outside the disc of the planet, and extensions from the central star of some planetary nebulae
Apastron
Apastron refers to the position in an orbit about a star whence the object is at its greatest distance from the star.
Aperture
A variable-sized hole that controls the amount of light admitted to a camera.
Aphelion
Aphelion refers to a position in a heliocentric whence the object is at its greatest distance from the Sun.
Apoapse
Apoapse is also called Apocente. It refers to the position in an orbital path that is the greatest distance from the primary body.
Apoapsis
Apoapsis refers to a point in an orbit when a planet is farthest from any body other than the Sun or the Earth.