18.
In this chapter I shall give a short account of the publications in which the most complete information on the attributes of the stars may be obtained, with short notices of the contents and genesis of these publications. It is, however, not my intention to give a history of these researches. We shall consider more particularly the questions relating to the position of the stars, their motion, magnitude, and spectra.
19.
Place of the stars. Durchmusterungs. The most complete data on the position of the stars are obtained from the star catalogues known as “Durchmusterungs”. There are two such catalogues, which together cover the whole sky, one—visual—performed in Bonn and called the Bonner Durchmusterung (B. D.), the other—photographic—performed in Cape The Cape Photographic Durchmusterung (C. P. D.). As the first of these catalogues has long been—and is to some extent even now—our principal source for the study of the sky and is moreover the first enterprise of this kind, I shall give a somewhat detailed account of its origin and contents, as related by Argelander in the introduction to the B. D.
B. D. was planned and performed by the Swedish-Finn Argelander (born in Memel 1799). A scholar of Bessel he was first called as director in Åbo, then in Hälsingfors, and from there went in 1836 to Bonn, where in the years 1852 to 1856 he performed this great Durchmusterung. As instrument he used a Frauenhofer comet-seeker with an aperture of 76 mm, a focal length of 650 mm, and 10 times magnifying power. The field of sight had an extension of 6°.
In the focus of the objective was a semicircular piece of thin glass, with the edge (= the diameter of the semicircle) parallel to the circle of declination. This edge was sharply ground, so that it formed a narrow dark line perceptible at star illumination. Perpendicular to this diameter (the “hour-line”) were 10 lines, at each side of a middle line, drawn at a distance of 7′. These lines were drawn with black oil colour on the glass.
The observations are performed by the observer A and his assistant B. A is in a dark room, lies on a chair having the eye at the ocular and can easily look over 2° in declination. The assistant sits in the room below, separated by a board floor, at the Thiede clock.
From the beginning of the observations the declination circle is fixed at a certain declination (whole degrees). All stars passing the field at a distance smaller than one degree from the middle line are observed. Hence the name “Durchmusterung”. When a star passes the “hour line” the magnitude is called out by A, and noted by B together with the time of the clock. Simultaneously the declination is noted by A in the darkness. On some occasions 30 stars may be observed in a minute.
The first observation was made on Febr. 25, 1852, the last on March 27, 1859. In all there were 625 observation nights with 1841 “zones”. The total number of stars was 324198.
The catalogue was published by Argelander in three parts in the years 1859, 1861 and 1862[7] and embraces all stars between the pole and 2° south of the equator brighter than 9m.5, according to the scale of Argelander (his aim was to register all stars up to the 9th magnitude). To this scale we return later. The catalogue is arranged in accordance with the declination-degrees, and for each degree according to the right ascension. Quotations from B. D. have the form B. D. 23°.174, which signifies: Zone +23°, star No. 174.