An Overview of the Planetary Theory of Solar Activity Variability

A summary of current knowledge of solar activity's long-term behaviour on a multi-millennial timescale, as rebuilt using the indirect proxy method, is presented here. The notion of solar activity is addressed, as well as an overview of the particular indices that are used to quantify various elements of changing solar activity, with a focus on sunspot number. Over long periods, only indirect proxies, such as the cosmogenic isotopes ¹⁴C and ¹⁰Be in natural stratified archives, can be used to derive quantitative information on previous solar activity (e.g., tree rings or ice cores).