APPENDIX B - GLOSSARY OF TERMS
A –
Absolute Magnitude The apparent magnitude that a star
magnitude 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 A telescope mounting using two
Mounting 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 A measure of the relative brightness of a
Magnitude 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 The distance between the Earth and the Sun.
unit (AU) 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 The projection of the Earth’s equator
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 An imaginary sphere surrounding the Earth,
Sphere concentric with the Earth’s center.
Collimation The act of putting a telescope’s optics into
perfect alignment.
D –
Declination The angular distance of a celestial body
(DEC) 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 strument
mount 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.
G –
GoTo Term used to refer to a computerized
telescope or to the act of slewing (moving) a
computerized telescope.
J –
Jovian Planets Any of the four gas giant planets that are
at a greater distance from the sun than the
terrestrial planets.
K –
Kuiper Belt A region beyond the orbit of Neptune
extending 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 186,000
mi/sec. (299,792 km/sec.) With 31,557,600
seconds in a year, the light–year equals a
distance of 5.88 trillion miles (9.46 trillion km).
M –
Magnitude Magnitude is a measure of the brightness
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