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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 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.
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.
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 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 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 slam into and excite
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 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 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 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 -