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Focal length The distance between a lens (or mirror) a
brought to focus. The focal length divide
nd the point at which the image of an object at infinity is
d by the aperture of the mirror or lens is termed the focal
ratio.
J -
ets
r
(LY)
m/ sec.
the light-year equals a distance of 9.46 X 1 trillion km (5.87 X 1
estial body. The brightest stars are assigned
aintest star that can be
to a ratio of 2.5 in
Jovian Plan
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-Yea
A light-year is the distance light traverses in a vacuum in one year at the speed of 299,792 k
With 31,557,600 seconds in a year,
trillion mi).
M -
Magnitude Magnitude is a measure of the brightness of a cel
magnitude 1 and those increasingly fainter from 2 down to magnitude 5. The f
agnitude step corresponds
seen without a telescope is about magnitude 6. Each m
b
rightness. Thus a star of magnitude 1 is 2.5 times brighter than a star of magnitude 2, and 100 times
rightest star, Sirius, has an apparent magnitude of -1.6, the
o
the zenith. If you are facing South, the meridian starts from your Southern
Messier
N -
Nebula d of gas and dust. Also refers to any celestial object that has a cloudy appearance.
h Celestial Pole The point in the Northern hemisphere around which all the stars appear to rotate. This is caused by
Nova
O -
Open Cluster
of
stars.
Parallax fference in the apparent position of an object against a background when viewed by
lar direction of the
sured. The traditional method in
astronomy of determining the distance to a celestial object is to measure its parallax.
s one parfocal eyepiece all the other parfocal
eyepieces, in a particular line of eyepieces, will be in focus.
0,000,000 km. (Apart from the Sun, no star lies within
Point Source r 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.
Reflector A telescope in which the light is collected by means of a mirror.
brighter than a magnitude 5 star. The b
full moon is -12.7, and the Sun's brightness, expressed on a magnitude scale, is -26.78. The zer
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
horizon and passes directly overhead to the North celestial pole.
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.
Interstellar clou
Nort
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 refer
red to as the "Pole Star".
Although Latin for "new" it denotes a star that suddenly becomes explosively bright at the end of its
life cycle.
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
s
P -
Parallax is the di
an observer from two different locations. These positions and the actual position of the object form a
an be determined if
triangle from which the apex angle (the parallax) and the distance of the object c
n and the angu
the length of the baseline between the observing positions is know
object from each position at the ends of the baseline has been mea
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 focu
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 30,8000,00
one parsec of us.)
An object which cannot be resolved into an image because it to too fa
R -
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