On sun-spot periodicities. —Preliminary notice

Abstract

In cases where it is necessary to separate true periodic changes from other variations, which during short periods of time often simulate periodicities, the method of the periodogram is at present the only one which can give definite results. In view of the importance of the questions connected with the changes in the frequency of the sun-spots, I have, therefore, undertaken the considerable labour of forming a complete periodogram of sun-spot variability as far as the data at my disposal allow me to do so. The following is a brief abstract of the results; the investigation will be presented shortly:— The periodogram, as already explained, is the diagram representing the intensity of periodic variations as determined from the sum of the squares of the two Fourier coefficients belonging to each assumed period. This diagram represents for any regular or irregular change exactly what the energy diagram gives us for a luminous disturbance which is analysed by a spectroscope.