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GLOSSARY OF TERMS
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Absolute magnitude
The apparent magnitude that a star would have if it were observed from a standard distance of 10
parsecs, or 32.6 light-years. The absolute magnitude of the Sun is 4.8. at a distance of 10 parsecs, it
would just be visible on Earth on a clear moonless night away from surface light.
Airy disk
The apparent size of a star's disk produced even by a perfect optical system. Since the star can never
be focused perfectly, 84 per cent of the light will concentrate into a single disk, and 16 per cent into
a system of surrounding rings.
Alt-Azimuth Mounting
A telescope mounting using two independent rotation axes allowing movement of the instrument in
Altitude and Azimuth.
Altitude
In astronomy, the altitude of a celestial object is its Angular Distance above or below the celestial
horizon.
Aperture
The diameter of a telescope's primary lens or mirror; the larger the aperture, the greater the
telescope's light-gathering power.
Apparent Magnitude
A measure of the relative brightness of a star or other celestial object as perceived by an observer on
Earth.
Arc minute
A unit of angular size equal to 1/60 of a degree.
Arc second
A unit of angular size equal to 1/3,600 of a degree (or 1/60 of an arc minute).
Asterism
A small unofficial grouping of stars in the night sky.
Asteroid
A small, rocky body that orbits a star.
Astrology The
pseudoscientific belief that the positions of stars and planets exert an influence on human
affairs; astrology has nothing in common with astronomy.
Astronomical unit (AU)
The distance between the Earth and the Sun. It is equal to 149,597,900 km., usually rounded off to
150,000,000 km.
