A catalogue or list of stars is a work giving for each star listed its magnitude and its position on the celestial sphere, with such other particulars as may be necessary to attain the object of the catalogue. If the latter includes only the more conspicuous stars, it is common to add the name of each star that has one; if none is recognized, the constellation to which the star belongs is frequently given. The position of a star on the celestial sphere is defined by its right ascension and declination. These correspond to the longitude and latitude of places on the earth, in the following way: Imagine a plane passing through the center of the earth and coinciding with its equator, to extend out so as to intersect the celestial sphere. The line of intersection will be a great circle of the celestial sphere, called the celestial equator. The axis of the earth, being also indefinitely extended in both the north and the south directions, will meet the celestial spheres in two opposite points, known as the north and south celestial poles. The equator will then be a great circle 90° from each pole. Then as meridians are drawn from pole to pole on the earth, cutting the equator at different points, so imaginary meridians are conceived as drawn from pole to pole on the celestial sphere. Corresponding to parallels of latitude on the earth we have parallels of declination on the celestial sphere. These are parallel to the equator, and become smaller and smaller as we approach either pole. The correspondence of the terrestrial and celestial circles is this: To _latitude_ on the earth’s surface corresponds _declination_ in the heavens. To _longitude_ on the earth corresponds _right ascension_ in the heavens. A little study of these facts will show that the zenith of any point on the earth’s surface is always in a declination equal to the latitude of the place. For example, for an observer in Philadelphia, in 40° latitude, the parallel of 40° north declination will always pass through his zenith, and a star of that declination will, in the course of its diurnal motion, also pass through his zenith. So also to an observer on the equator the celestial sphere always spans the visible celestial hemisphere through the east and west points.