Accretion (aggregation) – the increase in mass of a body by addition of smaller bodies that collide and stick to it. (Not resolved by dieting!) Accretion is now assumed to have had an important role in the formation of planets from swarms of dust grains.

Achernar – (a Eri) – A conspicuous bluish-white star that is brightest in the constellation Eridanus. Spectral type B3 and at 21pc distant.

Achromatic lens – a two-element lens (doublet) that greatly reduces Chromatic Aberration in an optical system i.e. reducing the occurrence of different wavelengths e.g. red and blue light to be brought to a focus at different distances from the lens – causing false colour arising in the image.

Airy disk – the bright disklike image of a point of source of light, such as a star, as seen in an optical system with a circular aperture. George Airy calculated in 1834 the disk diameter that limits the angular resolution of the telescope.

Aldebaran (a Tau) – a conspicuous read giant and brightest star in the southern summer constellation of Taurus. Spectral type K5 III and at 19pc distance.

Alpha Centauri (Rigil Kentaurus – a Cen) – A binary star that is the brightest star in the constellation Centaurus . Second nearest star to our sun consisting of Proxima Centauri being nearest to the sun @ 1.33pc distance Spectral type G2 V.

Altazimuth – often used to refer to telescope mountings that allow the telescope to swing about a vertical (azimuth) axis. The other axis with is referred to as altitude.

Aluminizing – a process whereby a very thin but perfectly uniform coating of aluminium is deposited by evaporation on a suitable base (e.g. a glass disk) giving it mirrorlike qualities. Used for primary and secondary telescope mirrors.

Anglo-Australian Telescope (AAT) – the 3.9 m reflecting telescope which began operating in 1975 at Siding Spring Australia at an altitude of 1 150m.

Andromeda galaxy (M31, NGC 224) – the largest of the nearby galaxies, visible to the unaided eye as a faint oval patch of light in the constellation of?……… Yes you guessed right! Andromeda! Distance is estimated at 725kpc or 2,35m light years and total luminosity estimated at about double that of our Milkyway galaxy. Great object to observe through a telescope of 10" or more in dark areas!

Antares (a Sco) – huge remote but conspicuous red super giant star in the southern winter constellation of Scorpius., it has a 5^th magnitude B type companion, has a size of 256 solar diameters and is at 160pc distance.

Aperture – clear diameter of the object lens in a refracting telescope or of a primary mirror in a reflector.

Aphelion – the point in the orbit of a planet, comet, or satellite in solar orbit that is furthest from the sun. The earth is e.g. at aphelion on or about July 3^rd.

Aquila (Eagle) – an equatorial constellation near Cygnus (swan). Altair is its brightest star at 1^st magnitude. Aquila and Cygnus are observable in our southern hemisphere flying in formation above the northern horizon in and around September.

Asteroids (minor planets) – small rocky solar system bodies that orbit our sun of which about 95% orbit in the asteroid belt between the orbits of Mars and Jupiter at a distance of about 1.7 to 4 AU from the sun. Some have different orbits and more than 100 000 are bright enough to photographically observed. 5 500 have received official asteroid numbers. Vesta is bright enough to be seen with the naked eye. Total mass estimated to be 15% of the moon’s mass. About 120 are larger than 130km in diameter and 10 with diameters exceeding 250km.

Aurora – a colourful display of diffuse changing coloured light seen high in the earth’s atmosphere mainly in the polar regions. Caused by charged particles from the solar wind and solar flares that become trapped in the earth’s atmosphere.