The present inquiry examines the change in the intellectual climate of early Palaiologan Byzantium characterised by the prominence of mathematical astronomy and the importance of the successful demonstration of related disciplines. It traces their impact on the late Byzantine discourse concerning knowledge of the natural world as reflected by contemporary historiography, as well as on the related book production and circulation. In particular, the chapter focuses on Nikephoros Gregoras’ (c. summer 1293/June 1294–1358/1361) Roman History, which lists numerous astronomical events, such as solar and lunar eclipses, the observation of comets and the configuration of the stars at a given moment in time. Book VIII, in particular, offers important examples of the incorporation of astronomy-related material into the historiographical narrative as it comprises Gregoras’ Hortatory Letter Concerning Astronomy and his discourse on the date of Easter. The present contribution features a case study of the Hortatory Letter and analyses its insertion in both Gregoras’ Roman History and the astronomical miscellany Vat. gr. 1087 where it serves as introduction to the entire volume. Finally, the present chapter proposes that Gregoras’ Hortatory Letter, in both its contexts of preservation (in Vat. gr. 1087 and in the Roman History), illustrates the interplay between, firstly, the increased occurrence of significant astronomical phenomena such as the lunar and especially the solar eclipses observable in Constantinople; secondly, the exceptional and rare mathematical expertise Gregoras acquired and demonstrated; thirdly, the change in his social and intellectual status from student of astronomy to one of the very few masters of the quadrivium in the capital; and finally, his newly acquired role as Theodore Metochites’ intellectual heir and the one responsible for the preservation of the latter’s scholarly legacy, especially in the context of the revival of Ptolemaic astronomy.