Altitude: 
measurement of angular elevation of object in the local observer's coordinate
system. The altitude is the angular measurement from the horizon. An altitude
of 90 degrees is known as zenith.
See the Astro 201 explanation of
altitude and azimuth for a graphical illustration. 
Arc second: 
angular separation equal to 1/60th of an arc minute. An
arc minute is an
angular separation equal to 1/60th of one degree. For example, the full moon
has an angular measure of about half a degree, or 30 arc minutes. 
Azimuth: 
measure of an object's angular displacement from due north in observer's
coordinate. See the Astro 201 explanation of
altitude and azimuth for a graphical illustration. 
Chromatic aberration: 
an effect that occurs in some refracting telescopes. The glass lenses
make different wavelengths of light bend at slightly different angles, with
shorter wavelengths (blue) bent more. The effect can be noticed as a slight
blue ring or rainbow effect. Reflecting telescopes do not have this problem.
See the Astro 201 page on
telescopes to see examples of refracting and reflecting designs.

Declination: 
measurement of angular elevation of object in celestial coordinates,
with zero set at the celestial equator and +90 degrees at the north celestial
pole. (analogous to latitude) See the Astro 201 page on
celestial sphere and the general Astro 201 topical page on the
night sky. 
Focal length: 
in a refracting telescope, the distance from the center of the primary
lens at the end of the tube to where light is focused to a point. See the Astro 201 page on
telescopes. 
Lightyear: 
The distance light travels in a year, roughly 9.46 X 10^{15} m. 
Parsec: 
distance corresponding to a parallax angle of one arc second using
observations from opposite sides of Earth's orbit. One parsec is equal to
about 3.26 lightyears. See the Astro 201 explanation of
parallax for a graphical illustration. 
Parallax: 
technique used to measure distances to nearby objects. By observing
the position of a star from two different locations in the Earth's orbit around
the Sun (i.e. several months apart), it is possible to calculate the distance
to the object. See the Astro 201 explanation of
parallax. 
Redshift: 
the shifting of light towards redder wavelengths due to the source moving
away from the observer. Observed by Hubble and led to the conclusion that the
universe is expanding. See also the Astro 201 explanation of
doppler shift and its
relativistic formulation 
Right ascension: 
measurement of angular displacement in celestial coordinates. The zero of
right ascension is known as the first point of Aries (actually in Pisces now,
thanks to precession) on the vernal equinox. See the Astro 201 page on
celestial sphere and the general Astro 201 topical page on the
night sky. 
Seeing: 
measurement, typically in arc seconds, of the limit of image
detail during astronomical
observations. Seeing is limited by atmospheric turbulence and water vapor or
by the telescope's diffraction limit.
Good seeing corresponds to about 1 arc second for mediumsized optical
telescopes at typical sites. Some modern observatories improve their
resolution by using adaptive optics systems. See also the Astro 201 explanation of
seeing, diffraction limit, etc.

Zenith: 
the position directly above the observer. See altitude (above) and
the Astro 201 explanation of the night sky
local perspective for a graphical illustration. 