A Day of Celebration for James Croll written by Professor Kevin J Edwards FRSGS (Departments of Geography & Environment and Archaeology, School of Geosciences, University of Aberdeen; McDonald Institute for Archaeological Research and Scott Polar Research Institute, University of Cambridge) After almost two years in the planning, but following an idea mooted five years ago, the RSGS with co-sponsorship from the Quaternary Research Association (QRA) was able to host a bicentenary commemoration of the birth of James Croll (1821-1890), the Perthshire-born, self-taught prodigy who laid the foundations for climate change science. James Croll – from janitor to genius was presented as a whole-day online meeting on 16th April 2021. Croll was a stonemason’s son from the Little Whitefield-Wolfhill area close to Perth. He became internationally celebrated as a proponent and developer of the astronomical theory of climate change with its implications for glaciation, oceanography and much else. The meeting, with 14 presentations and lively question and answer sessions, explored Croll as a person and as a scientist, with specialists drawn from the worlds of science, history and popularisation. The programme had two main, but interlocking strands: Croll the man, and Croll the scientist. I introduced the first, providing a context for the day and demonstrating that although Croll is not a major figure within popular consciousness, there is in fact a wealth of biographical evidence available for his life when it is sought. In a further talk, I argued that the year 1876, when many academic honours were bestowed on Croll, represented the summit of his scientific life. An informative video on the geographical, historical and scientific influences and achievements of Croll was presented by Mike Robinson (RSGS). Laura Brassington (Cambridge) explored Croll’s navigation of scientific societies and reflected upon the degree to which his social background was an impediment to institutional assimilation. Dr Diarmid A Finnegan (Belfast) examined the interplay of science, metaphysics and Calvinism as drivers for Croll’s beliefs. In a paper which bridged the humanistic and scientific dimensions of the meeting, Professor James Fleming (Maine) suggested that Croll was involved in a personal quest for a general theory of the unity of nature in which religion, philosophy and science meshed, and furthermore, what Darwin did for life forms, Croll accomplished for climate change. Professor Malcolm Longair (Cambridge) charted the transition from the celestial mechanics of classical times, through Croll’s development and augmentation of the astronomical background, to climate change as adopted by Milutin Milankovitch in the 1920s-1930s. The perception that Croll was too far ahead of his time for the real depths of his insights to be fully appreciated was taken further by several of the following contributions. Professor Alastair Dawson (Dundee) analysed the oceanographic contribution of Croll, demonstrating that he had calculated from first principles the quantities of heat delivered by ocean currents to high latitude areas, producing numerous far-sighted ideas. Acting as James Croll’s alter ego, Professor David Sugden (Edinburgh) showed how the pioneer’s modelled estimates of the Antarctic ice sheet were not improved upon for another 75 years. Professor James Rose (London) considered the differing perspectives of Charles Lyell, Archibald and James Geikie, and Croll regarding terrestrial glacial sediments and landforms, arguing persuasively that Croll may have been a reluctant geologist, but that he was a very good one. In a presentation by Professor Chronis Tzedakis (London; with Eric Wolff, Cambridge), it was revealed how the marine sedimentary record, as intimated by Croll, was able to deliver a test of astronomical theories as outlined by Croll and later by Milankovitch. Feedback mechanisms in the Earth system were the focus for Professor Roy Thompson (Edinburgh), who explained how Croll’s fundamental recognition of this concept foreshadowed global warming. Jo Woolf (RSGS) and Mike Robinson (RSGS) explored ways to popularise Croll. Their contributions included a broad spectrum of actual and potential events from seminar- and theatre-based presentations, to exhibitions, publications and educational tourist trails. It was an opportunity for expression and creativity in a manner which reminds us that Croll was once a young lad, ‘full of fun and frolic’. Papers based on the talks are published as a special issue of Earth and Environmental Science Transactions of the Royal Society of Edinburgh. The RSGS and QRA are to be congratulated for their support of this event in memory of an exceptional man with enduring relevance.