We discuss how, in the 1970's, Eddy took clues from the historical researches of Spörer and Maunder in the 19th century to draw attention to the virtual absence of sunspot activity between 1645 and 1715. This ``Maunder Minimum'' is not only of interest to solar physicists in the context of the theory of solar magnetic activity, and to stellar astrophysicists working on the properties of cool stars, but may also be a vital clue to the influence of the variability of the Sun's power output on terrestrial climate. Without the availability of the historical documentary records the long-term variability of the Sun implied by the Maunder Minimum would not have come to light, and the consequent advances in stellar physics and in palaeoclimatology would not have been possible.