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 |
| 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. |
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 |
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 |
| 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 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 (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. |
31