The light of the night sky: its intensity variations when analysed by colour filters

Abstract

The light of the night sky has been discussed in previous papers more particularly with reference to the occurrence of the green auroral line. From a spectroscopic point of view this line is the most definite and interesting feature, and it is important to determine how its intensity varies with time, and whether any connection with the solar cycle of eleven years can be traced. It is also important to determine whether other regions in the spectrum the night sky undergo variations, and, if so, whether they are directly connected with the auroral line or not. In this paper methods are given which have been found practicable for attacking these questions, and systematic observations extending over more than a year are recorded. These establish the existence of a number of interesting variations, and seem to open up a wide field; but it will be necessary to pursue them for a good many years before the questions connected with the solar cycle can be answered. An individual worker cannot, of course, rely on being able to complete such a programme, and it seems desirable to record what has been done so far.