Page:Popular Science Monthly Volume 57.djvu/394

384 himself to the northern hemisphere, but extended his list over the whole celestial sphere, from the north to the south pole.

The catalogue of the celebrated Tycho Brahe, prepared toward the end of the sixteenth century, though of great historic value, is of no special interest to the general reader at the present time. A supplement to it, continuing its list of stars to the south pole, was published by Halley, who made the necessary observations during a journey to St. Helena in 1677.

The catalogue of Hevelius, published in 1690, offers no feature of special interest, except the addition of several new constellations which he placed between those already known. Having the aid of the telescope, he was able to include in his catalogue stars which had been invisible to his predecessors.

Modern catalogues of the stars may be divided into two classes: Those which include only stars of a special class, or stars of which the observer sought to determine the position or magnitude with all attainable precision; and catalogues intended to include all the stars in any given region of the heavens, down to some fixed order of magnitude. It may appear remarkable that no attempt of the latter sort was seriously made until more than two centuries after the telescope had been pointed at the heavens by Galileo. A reason for the absence of such an attempt will be seen in the vast number of stars shown by the telescope, the difficulty of stopping at any given point, and the seeming impossibility of assigning positions to hundreds of thousands of stars. The latter difficulty was overcome by the improved methods of observation devised in modern times.

About the middle of the present century the celebrated Argelander commenced the work of actually cataloguing all the stars of the northern celestial hemisphere to magnitude 9½. This work was termed a Durchmusterung of the northern heavens, a term which has been introduced into astronomy generally to designate a catalogue in which all the stars down to a certain magnitude are supposed to be mustered, as if a census of them were taken. The work fills three quarto volumes and contains more than 310,000 stars, of each of which the magnitude and the right ascension and declination are given. This work was extended by Schönfeld, Argelander's assistant and successor, to 22° of south declination.

In the latitudes in which most of the great observatories of the world are situated, that part of the celestial sphere within 40° or 50° of the south pole always remains below the horizon. Around this invisible region a belt of somewhat indefinite breadth, 10° or more, can be only imperfectly observed, owing to the nearness of the stars to the horizon, and the brevity of the period between their rising and setting. Up to the middle of the nineteenth century, the few observatories