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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 -
Light-Year (ly) 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
b
rightness. 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 hundreds
of 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.
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