Celestron 4 - page 38

A-

Absolutemagnitude Theapparent magnitude that a star would have if it were observedfrom a standard distance of 10

parsecs,or 32.6 light-years. The absolute magnitude of the Sun is4.8. at a distance of 10 parsecs, it

wouldjust be visible on Earth on a clear moonless night away from surface light.

Airydisk Theapparen t size of a star's disk produced even by a perfect optical system. Since the star can never

befocused perfectly,8 4 per cent of the light will concentrate into a single disk, and 16 per cent into

asystem of surrounding rings.

Alt-AzimuthMounting Atelescope mounting using two independent rotationaxis allowing movement of the instrument in

Altitudeand Azimuth.

Altitude In astronomy, the altitude of a celestial object is its AngularDistance above or below the celestial

horizon.

Aperture thediameter of a telescope's primary lens or mirror; the largerthe aperture, the greater the

telescope'slight-gathering power.

ApparentMagnitude Ameasureof the relative brightness of a star or other celestial object as perceivedby an observer on

Earth.

Arcminute Aunit of angular size equal to 1/60 of a degree.

Arcsecond Aunit of angular size equal to 1/3,600 of a degree (or 1/60 of an arcminute).

Asterism Asmall unofficialgrouping of stars in the night sky.

Asteroid Asmall, rocky body that orbits a star.

Astrology Thepseudoscientific belief that the positions of stars and planetsexert an influence on human

affairs;astrology has nothing in common with astronomy

Astronomicalunit (AU) Thedistance between the Earth and the Sun. It is equal to 149,597,900 km.,usually rounded off to

150,000,000km.

Aurora Theemission of light when charged particles from the solar windslams into and excites atoms and

moleculesin a planet's upper atmosphere.

Azimuth Theangular distance of an object eastwardsalong the horizon, measured from due north, between

theastronomical meridian (the vertical line passing through thecenter of the sky and the north and

southpoints on the horizon) and the vertical line containing the celestialbody whose position is to

bemeasured. .

B-

BinaryStars Binary(Double) stars are pairs of stars that, because of their mutual gravitational attraction, orbit

arounda common Center of Mass. If a group of three or more stars revolvearound one another, it is

calleda multiple system. It is believed that approximately 50 percent of allstars belong to binary or

multiplesystems. Systems with individual components that canbe seen separately by a telescope are

calledvisual binaries or visual multiples. The nearest "star" to our solar system, AlphaCentauri, is

actuallyour nearest example of a multiple star system, it consists of three stars, two very similar to

ourSun and one dim, small, red star orbiting aroundone another.

C-

CelestialEquator Theprojection of the Earth's equator on to the celestialsphere. It divides the sky into two equal

hemispheres.

Celestialpole Theimaginary projection of Earth's rotational axis north or south pole onto the celestialsphere.

CelestialSphere Animaginary sphere surrounding the Earth, concentric with the Earth's center.

Collimation Theact of putting a telescope's optics into perfect alignment.

D-

Declination (DEC) Theangular distanceo fa cel estial body north or south of the celestial equator. It may be said to

correspondto latitude on the surface of the Earth.

E-

Ecliptic Theprojection of the Earth's orbit on to the celestial sphere. It may also be defined as "the apparent

yearlypath o fth eS un against the stars".

Equatorialmount A telescope mounting in which the instrumentis 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.