The passage of Halley's Comet through the skies above the Earth in the spring of 837 was a spectacular event, with written records surviving from T'ang China, Heian Japan, the Abbasid Caliphate and the Carolingian empire. This article integrates all of these sources, together with the findings of modern historical astronomy, to revise the famous account from the Vita Hludowici imperatoris of the emperor Louis the Pious and the ‘Astronomer’ observing the comet from the rooftops of Aachen. It assesses how reliable early medieval witnesses were when describing the natural world and demonstrates that the Astronomer was able to produce a coherent description of the night sky. However, despite the claims of the text, close analysis and comparison with Chinese observations reveals the two men could not have been watching Halley's Comet that evening. Nevertheless, the Astronomer's account is sufficiently accurate and effective that two alternative suggestions can be derived from it.