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Made with the page turned horizontally, like adjacent drawings in blue wash in this series of studies of Caernarvon (see under folio 40 verso; D01856; Turner Bequest XLIII 39a), this seems to show the castle by moonlight, and relates to the small composition studies in the contemporary Dolbadarn sketchbook including Tate D02069 and D02071 (Turner Bequest XLVI 49a, 50a). It is possible that Turner intended his white gouache disc to represent the sun, as in the finished watercolour of Caernarvon Castle exhibited at the Royal Academy in 1799 (private collection),1 but here its identification as the moon seems virtually certain.
Andrew Wilton, J.M.W. Turner: His Life and Work, Fribourg 1979, p.328 no.254, pl.47 (colour).