Astronomy Glossary



Gravitational accumulation of mass in a planet or protoplanet.


A lens that corrects to some extent for the serious false colour seen in the image produced by a single lens. This usually consists of two lenses of different types of glass combined in a shell.


A type of stony meteorite that lacks chondrules


The ratio of the thickness of the atmosphere at the observing altitude to the thickness at the zenith.


The reflectance or reflectivity of an object.

altazimuth mount

A telescope mount with separate axes for moving in altitude and alzimuth.


In astronomy, angular distance above the horizon in degrees.


Observed from the same location very day, the Sun follows a figure-of-eight path through the sky. This pattern is due to the tilt of the Earth’s axis as it orbits the Sun.


Primary igneous rocks of the lunar highlands, a very rare type of rock on Earth.


The clear diameter of a telescope’s main mirror or lens.


The point in the orbit of a body at which it is furthest from the Sun.


A lens designed to overcome almost completely the problem of false colour, with better correction than an achromat.


A small unit of angular measurement, spanning 1/60 of a degree; an arcsecond is 1/60 of an arcminute.

arc lamp

A hollow cathode lamp whose spectrum is used to calibrate the wavelength of astronomical spectra.

ascending node

The point where the orbit of a body in the Solar System intersects the ecliptic plane when the body crosses from south to north of this plane.


A pattern of stars seen in the sky that does not make up an official constellation. e.g. Summer Triangle.


Any small body (less than 1000km in diameter) orbiting the Sun that does not display the atmosphere or tail associated with a comet; formerly called minor planets.

astronomical unit (AU)

The semimajor axis of the Earth’s orbit, or equivalently the average distance of the Earth from the Sun, approx. 150 million km.


Light emitted by atoms and icons in a planetary ionosphere, mostly in the magnetic polar regions; also called polar lights.

axial inclination (obliquity)

The angle between the rotation axis of a body and the perpendicular to its orbital plane.


Angular distance around the horizon, measured from north through east, south and west.


Barlow lens

A lens that increases the working focal length of an optical system. Placed before the eyepiece, it increases the magnification of that eyepiece.


The assemblage of all living things and their remains. The Earth is the only planet known to have a biosphere.



A crater created by volcanism and not by impact.


A chemical compound containing the chemical unit CO3


A group of asteroids with orbits that lie among the giant planets. They are icy-rocky bodies from the Edgeworth-Kuiper belt.

chromatic aberration

An abberation inherent in all lenses that causes light of different wavelengths to be brought to a focus at different points.


Small, icy-rocky bodies that develop huge fuzzy heads and tails when they are in the inner Solar System.


The corona is the plasma ‘atmosphere’ of the sun located outside of the visible solar surface, the photosphere. It can only be viewed when the light from the photosphere is blocked, namely during a total solar eclipse.

cosmic rays

Atomic particles that pervade interstellar space, moving at speeds close to that of light.


eye relief

The distance from the eyepiece at which you can see the full field of view. People with glasses observe further away from the eyepiece and therefore require longer eye relief.



Ratio of the focal length of an optical instrument to the diameter of the bundle of parallel light rays that is brought to a focus.

field of view

The area of sky that is accessible at one time, expressed in terms of an angular diameter.

flat field

An image obtained by exposing a CCD to a unformly illuminated light source. Flat fields are used to calibrate for the variation in pixel-to-pixel sensitivity of a CCD.


giant planet

In the Solar System , one of the four planets, Jupiter, Saturn, Uranus, Neptune. These massive planets usually consist of hydrogen, helium and icy materials.


igneous rocks

Rocks produced from magma.


light year

The distance travelled by light in a vacuum in a year of 365.2425 days. It equals 9.460536 x 10-15 metres.



This is the area in-between the Earth’s crust and its core (roughly 3,000km thick) that consists of hot solid rock and magma (molten rock).


A small body that enters the Earth’s atmosphere at high speed. They are commonly seen as bright, transient streaks of light across the sky.



‘Little Planets’, small bodies 0.1-10km across, formed from the dust in the solar nebula. In solar nebular theories, many are incorporated in embryos en route to forming planets and the cores of giant planets.


solar wind

A thin, gusty stream of high-speed particles (protons and electrons) that escapes from the Sun.



This is the point directly above your head in the sky. It also describes the highest point a celestial object appears in the sky.