ASTRONOMY 233 Fall 2005
Ryan's Glossary #1

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.
Light-year: The distance light travels in a year, roughly 9.46 X 1015 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 light-years. 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 medium-sized 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.

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Tue Aug 30 11:19:12 EDT 2005 by martha for ryan