Obvious and hidden periodicities. A variable quantity may show periodic changes which become obvious as soon as a sufficient record has been obtained; such are the semi‐diurnal changes of the tides, or the eleven years recurrence of sunspot maxima. We may call these obvious periodicities . Most often, however, small periodic variations are hidden behind irregular fluctuations, and their investigation then becomes a matter of considerable difficulty.
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Arthur Schuster (1898) studied this question.