This article provides the first discussion of the unpublished treatise De crepusculis (“On twilight”), written in the second half of the thirteenth century by the Dominican scholar Giles of Lessines. It is shown that De crepusculis was intended as a critical supplement to a treatise on the height of the atmosphere by Ibn Muʿādh (eleventh century). In this supplement, Giles recapitulates Ibn Muʿādh’s geometrical arguments while furnishing proofs and covering relevant questions not included in the earlier work. One area where Giles greatly expands on his predecessor is in addressing relevant physical questions such as the causes of stellar luminosity. Giles here takes a nuanced view, acknowledging that all celestial bodies emit some form of light. He also provides a unique explanation of the blue colour of the sky, which draws on the extramission postulate used by thirteenth-century writers on optics.

本文首次讨论了多米尼加学者 Giles of Lessines 于 13 世纪下半叶撰写的未发表论文De crepusculis（“暮光之城”）。结果表明，De crepusculis旨在作为伊本·穆阿德（Ibn Muʿādh）（十一世纪）关于大气高度的论文的重要补充。在本增刊中，Giles 概括了 Ibn Muʿādh 的几何论证，同时提供了证据并涵盖了早期工作中未包含的相关问题。吉尔斯对其前任进行了极大扩展的一个领域是解决相关的物理问题，例如恒星光度的原因。贾尔斯在这里采取了细致入微的观点，承认所有天体都会发出某种形式的光。他还对天空的蓝色提供了独特的解释，该解释借鉴了 13 世纪光学作家使用的额外假设。