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Appendix B -
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 concen-
trate 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 bright-
ness 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 round-
ed off to 150,000,000 km.
Aurora: The emission of light when charged particles
from the solar wind slams into and excites atoms and mol-
ecules 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 mul-
tiple 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 rota-
tional 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 length divided by the aperture
of the mirror or lens is termed the focal ratio.
J -
Jovian Planets: Any of the four gas giant planets that are
at a greater distance form the sun than the terrestrial plan-
ets.
K -
Kuiper Belt: A region beyond the orbit of Neptune extend-
ing to about 1000 AU which is a source of many short
period comets.
L -
Light-Year (ly): A light-year is the distance light traverses
in a vacuum in one year at the speed of 299,792 km/ sec.
With 31,557,600 seconds in a year, the light-year equals a
distance of 9.46 X 1 trillion km (5.87 X 1 trillion mi).
M -
Magnitude: Magnitude is a measure of the brightness of a
celestial body. The brightest stars are assigned magnitude
1 and those increasingly fainter from 2 down to magnitude
5. The faintest star that can be seen without a telescope
is about magnitude 6. Each magnitude step corresponds
to a ratio of 2.5 in brightness. Thus a star of magnitude