Appendix A -
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
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 sur- rounding rings.
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 mir- ror; the larger the aperture, the greater the telescope’s
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 mole- cules 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 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 per- fect 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 instru- ment 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 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 -
M -
Magnitude: Magnitude is a measure of the brightness of a celestial body. The brightest stars are assigned mag- nitude 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 1 is 2.5 times brighter than a star of magni- tude 2, and 100 times brighter than a magnitude 5 star. The brightest star, Sirius, has an apparent magnitude of
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