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GLOSSARY OF TERMS
A-
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.
Aurora The emission of light when charged particles from the solar wind slams into and excites atoms and
molecules in a planet's upper atmosphere.
Azimuth The angular distance of an object eastwards along the horizon, measured from due north, between
the astronomical meridian (the vertical line passing through the center of the sky and the north and
south points on the horizon) and the vertical line containing the celestial body whose position is to
be measured. .
B -
Binary Stars Binary (Double) stars are pairs of stars that, because of their mutual gravitational attraction, orbit
around a common center of mass. If a group of three or more stars revolve around one another, it is
called a multiple system. It is believed that approximately 50 percent of all stars belong to binary or
multiple systems. Systems with individual components that can be seen separately by a telescope are
called visual binaries or visual multiples. The nearest "star" to our solar system, Alpha Centauri, is
actually our nearest example of a multiple star system, it consists of three stars, two very similar to
our Sun and one dim, small, red star orbiting around one another.
C -
Celestial Equator The projection of the Earth's equator on to the celestial sphere. It divides the sky into two equal
hemispheres.
Celestial pole The imaginary projection of Earth's rotational axis north or south pole onto the celestial sphere.
Celestial Sphere An imaginary sphere surrounding the Earth, concentric with the Earth's center.
Collimation The act of putting a telescope's optics into perfect alignment.
D -
Declination (DEC) The angular distance of a celestial body north or south of the celestial equator. It may be said to
correspond to latitude on the surface of the Earth.
E -
Ecliptic The projection of the Earth's orbit on to the celestial sphere. It may also be defined as "the apparent
yearly path of the Sun against the stars".
Equatorial mount A telescope mounting in which the instrument is set upon an axis which is parallel to the axis of the
Earth; the angle of the axis must be equal to the observer's latitude.
F -
Focal length The distance between a lens (or mirror) and the point at which the image of an object at infinity is
brought to focus. The focal lenSLTh divided by the aperture of the mirror or lens is termed the focal
ratio.