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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 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 axis 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
Apparent Magnitude
A measure of the relative brightness of a star or other celestial object as perceived by an observer on
Earth.
Arcminute
A unit of angular size equal to 1/60 of a degree.
Arcsecond
A unit of angular size equal to 1/3,600 of a degree (or 1/60 of an arcminute).
Asterism
A small
Asteroid
A small, rocky body that orbits a star.
Astrology
T
affairs; astrology has nothing in common with astronomy
(AU)
The distance between the Earth and the Sun. It is equal to 149,597,900 km., usually rounded off to
Astronomical unit
150,000,000 km.
Aurora
T
molecul
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
south points on the horizon) and the vertical line containing the celestial body whose position is to
be measured. .
B -
Binary Stars
Binary
around a common Center
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
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
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
T
Celestial Sphere
An imaginary sphere surrounding the Earth, concentric with the Earth's center.
Collimation
The act of putting a telescope'
D -
The angular distance of a celestial body north or south of the celestial equator. It may be said to
(DEC)
Declination
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
Earth; the angle of the axis must be equal to the observer's latitude.
F -
Focal length
brought to focus. The focal length divided by the aperture of the mirror or lens is termed the focal
ratio.
's light-gathering power.
unofficial
grouping of stars in the night sky.
he pseudoscientific belief that the positions of stars and planets exert an influence on human
he emission of light when charged particles from the solar wind slam
es in a planet'
s upper atmosphere.
(Double)
stars are pairs of stars that, because of their mutual gravitational attracti
of Mass. If a group of three or more stars revolve around one another, it
e imaginary projection of Earth
h
A telescope mounting in which the instrument is set upon an axis which is parallel to the axis of the
The distance between a lens (or mirror) and the point at which the image of an object at infinity is
.
's rotational axis north or south pole onto the celestial sphere.
s optics into perfect alignment.
58
s
s
into and excite
atoms and
center
of the sky and the north and
on, orbit
a
telescope
,
two very similar
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