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F-
Focall ength Thedistan ceb etween a lens (or mirror) and the point at which the image of an object at infinity is
broughtto focus. The focal length divided by the aperture of the mirroror lens is termed the focal
ratio.
J-
JovianP lanets Anyof the fo ur gas giant planets that are at a greater distance form the sun than the terrestrial
planets.
K-
KuiperBelt A region beyond the orbit of Neptuneextending to about 1000 AU which is a source of manyshort
periodcomets.
L-
Light-Year (ly) Alight- year is the distance light traverses in a vacuum in one year at the speed of 299,792 km/ sec.
With31,557,600 seconds in a year, the light-year equals a distance of 9.46 X 1 trillion km (5.87 X 1
trillionmi).
M-
Magnitude Magnitudeis a measure of the brightness of a celestial body. The brighteststars are assigned
magnitude1 and those increasingly fainter from2 down to magnitude 5. The faintest star that canbe
seenwithout a telescope is about magnitude 6. Each magnitude step correspondsto a ratio of 2.5 in
brightness.Thus a star of magnitude 1 is 2.5 times brighter than a star of magnitude2, and 100 times
brighterthan a magnitude 5 star. Thebrightest star, Sirius, has an apparent magnitude of -1.6, the
fullmoon is -12.7, and the Sun's brightness, expressed on a magnitudescale, is -26.78. The zero
pointof the apparent magnitude scale is arbitrary.
Meridian Areference l ine in the sky that starts at the North celestial pole and ends at the South celestial pole
andpasses through the zenith. Ifyou are facing South, the meridian startsfrom your Southern
horizon and passes directly overhead to the North celestial pole.
Messier AFrench astronomer in the late 1700’s who was primarilylooking for comets. Cometsare hazy
diffuseob jects and so Messier cataloged objects that were not comets to help his search. This
catalogb ecame the Messier Catalog, M1 through M110.
N-
Nebula Interstellar cloud of gas and dust. Alsorefers to any celestial object thathas a cloudy appearance.
NorthCelestial Pole Thepoint in the Northern hemisphere around which all the stars appearto rotate. This is caused by
thefact that the Earth is rotating on an axis that passes through theNorth and South celestial poles.
Thestar Polaris lies less than a degree from this point and is therefore referred to as the "PoleStar".
Nova AlthoughLatin for "new" it denotes a star that suddenlybecomes explosively bright at the end of its
lifecycle.
O-
OpenCluster Oneof the groupings of stars that are concentrated along the planeof the Milky Way. Most have an
asymmetricalappearance and are loosely assembled.They contain from a dozen to many hundreds
ofstars.
P-
Parallax Parallaxis the differencein the apparent position of an object against a backgroundwhen viewed by
anobserver from two different locations. These positions andthe actual position of the object form a
triangle from which the apex angle (the parallax) and the distance of the object can be determined i f
thelength of the baseline between the observing positions is knownand the angular direction of the
objectfrom each p osition at the ends of the baseline has been measured. The traditional method in
astronomyof determining the distance to a celestial object is to measure its parallax.
Parfocal Refersto a group of eyepieces that all require the same distance fro mth e focal plane of the
telescopeto be in focus. Thismeans when you focus one parfocal eyepiece all the other parfocal
eyepieces,i n 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 equalto 3.26 light-years,
206,265astronomical units, or 30,8000,000,000,000 km.(Apart from the Sun, no star lies within
oneparsec of us.)
Point Source Anobj ect which cannot be resolved into an image because it to too far away or too small is
considered a point source. Aplanet is far away but it can be resolved as a disk. Most stars cannot
beresolved asdisks, they are too far away.
R-
Reflector Atelescope in which the light iscollected by means of a mirror.