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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 -
Light-Year
A light-year is the distance light traverses in a vacuum in one year at the speed of 299,792
km/ sec. With 31,557,600 seconds in a year, the light-year equals a distance of 9.46 X 1
trillion km (5.87 X 1 trillion mi).
M -
Magnitude
Magnitude is a measure of the brightness of a celestial body. The brightest stars are
assigned magnitude 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 magnitude 2, and 100 times brighter than a magnitude 5 star. The
brightest star, Sirius, has an apparent magnitude of -1.6, the full moon is -12.7, and the
Sun's brightness, expressed on a magnitude scale, is -26.78. The zero point of the
apparent magnitude scale is arbitrary.
Meridian
A reference line in the sky that starts at the North celestial pole and ends at the South
celestial pole and passes through the zenith. If you are facing South, the meridian starts
from your Southern horizon and passes directly overhead to the North celestial pole.
Messier
A French astronomer in the late 1700’s who was primarily looking for comets. Comets are
hazy diffuse objects and so Messier cataloged objects that were not comets to help his
search. This catalog became the Messier Catalog, M1 through M110.
N -
Nebula
Interstellar cloud of gas and dust. Also refers to any celestial object that has a cloudy
appearance.
North Celestial Pole
The point in the Northern hemisphere around which all the stars appear to rotate. This is
caused by the fact that the Earth is rotating on an axis that passes through the North and
South celestial poles. The star Polaris lies less than a degree from this point and is
therefore referred to as the "Pole Star".
Nova
Although Latin for "new" it denotes a star that suddenly becomes explosively bright at the
end of its life cycle.
O -
Open Cluster
One of the groupings of stars that are concentrated along the plane of the Milky Way. Most
have an asymmetrical appearance and are loosely assembled. They contain from a dozen
to many hundred stars.
P -
Parallax
Parallax is the difference in the apparent position of an object against a background when
viewed by an observer from two different locations. These positions and the actual position
of the object form a triangle from which the apex angle (the parallax) and the distance of
the object can be determined if the length of the baseline between the observing positions
is known and the angular direction of the object from each position at the ends of the
baseline has been measured. The traditional method in astronomy of determining the
distance to a celestial object is to measure its parallax.
Parfocal
Refers to a group of eyepieces that all require the same distance from the focal plane of
the telescope to be in focus. This means when you focus one parfocal eyepiece all the
other parfocal eyepieces, in a particular line of eyepieces, will be in focus.
Parsec
The distance at which a star would show parallax of one second of arc. It is equal to 3.26
light-years, 206,265 astronomical units, or 30,8000,000,000,000 km. (Apart from the Sun,
no star lies within one parsec of us.)
Point Source
An object which cannot be resolved into an image because it to too far away or too small is
considered a point source. A planet is far away but it can be resolved as a disk. Most
stars cannot be resolved as disks, they are too far away.
R -
Reflector
A telescope in which the light is collected by means of a mirror.
Resolution
The minimum detectable angle an optical system can detect. Because of diffraction, there
is a limit to the minimum angle, resolution. The larger the aperture, the better the
