This paper examines a set oftreatises and observational reports on comets written by Portuguese astrologers/astronomers in the late sixteenth and seventeenth centuries. Focusing on the links between natural philosophy, astrology and politics, it analyses the change in comet-based astrological predictions in Portugal. In the late sixteenth century, the appearance ofcomets gave rise to numerous astrological texts predicting a transcendental role for the Portuguese monarchy under the Bragan~ dynasty. Nevertheless, as the seventeenth century progressed, astrological causality gradually disappeared from comet treatises. Increasingly. the appearance of comets in skies afforded an opportunity to discuss only the physical nature and effects ofcomets. not judicial astrological forecasts. In this paper, we argue that the process of rationalising and centralising the political system together with major changes concerning conceptions of nature and new 'scientific' methods partially explain this change in astrological beliefs.

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