What we learn in this chapter
Stars are located on the sky with two angular coordinates. Distances to them may be ignored by visualizing all of them as being on a celestial sphere at “infinite” distance. The angular coordinates define the star's location on the sphere. Any number of coordinate systems can be defined on this sphere. Astronomers use the equatorial, galactic, ecliptic, and horizon systems. The coordinates of a star differ from coordinate system to coordinate system so transformations between them are needed. In the equatorial system, the coordinates of a given star vary steadily and slowly due to precession of the earth, so one must define the epoch, e.g., J2000.0, of any quoted coordinates. “Areas” on the sky are defined as solid angles. Cataloging of stars is accomplished through photographic surveys, printed sky charts, and printed lists (“catalogs”). Unnamed stars can be specified unambiguously by marking the star on a finding chart, a sky photograph of the local region. The name of a star or galaxy may depend on its location and brightness within a constellation, its equatorial coordinates (“telephone number”), or simply its sequential number in a published catalog of objects together with the catalog name, e.g., Messier 42 is the Orion nebula.
A casual look at the sky confronts one with uncountable pinpoints of light. It is imperative that we be able to refer to particular celestial objects without waiting for a cloudless and moonless night.