Historical records in solar physics.

The Sun has been observed for millennia, but systematic and quantitative records of its activity started being collected with the advent of telescopic observations in the early 17th century. These early observations revealed key signatures of solar magnetic activity such as sunspots and faculae. Subsequent advances in instrumentation and observing techniques enabled the study of different layers of the solar atmosphere and led to the establishment of numerous long-term observational programs. These records underpin our current understanding of solar variability across timescales from days to centuries. In this review, I survey the principal long-term historical archives of solar observations and their analyses, where "historical" refers to datasets spanning roughly 50 years or more. Because many of these observations were obtained when both instrumentation and the physical understanding of the Sun were evolving, their interpretation presents significant challenges. Modern analysis methods and improved calibration techniques, however, now allow these legacy datasets to be re-examined and homogenised, enabling the extraction of information that was not fully exploited at the time of their acquisition. I also discuss the main solar activity and variability indices derived from historical observations, as well as reconstructions of other solar parameters based on these records.