Page:Popular Science Monthly Volume 58.djvu/325

Rh these planispheres. They were constructed by Schiaparelli in the following way: The entire sky was divided into 36 zones by parallels of declination 5° apart. Each zone was divided into spherical trapezia by hour-circles taken at intervals of 5° from the equator up to 50° of north or south declination; of 10° from 50 to 60; of 15° from 60 to 80; of 45° from 80 to 85, while the circle within 5° of the pole was taken as a single region. In this way 1,800 areas, not excessively different from each other, were formed.

The star density, as it actually is, might be indicated by the number

of stars of these regions. As a matter of fact, however, the density obtained in this way would vary too rapidly from one area to the adjoining one, owing to the accidental irregularities of distribution of the stars. An adjustment was, therefore, made by finding in the case of each area the number of stars contained in 1/200 of the entire sphere, including the region itself and those immediately round it. The number thus obtained was considered as giving the density for the central region. The total number of stars being 4,303, the mean number in 1/200 of the whole sphere is 21.5, and the mean in each area is 10.4.