The presentation of the magical and mantic in Celtic literature has persistently been dogged by misunderstanding and over-romanticized readings. Among the misconceptions about the ancient and medieval Celtic peoples, the notion of a specifically 'Celtic' astrology remains widespread in the popular mind. This study aims to counter such myth-making, and to demonstrate how a number Irish and Welsh literary writers in the medieval and Early Modern period conceived of portents in the heavens - comets, blood-coloured moons, darkened suns - and what they knew of the complex art of astrology.