I will use this fellowship to complete my book project on Apollonius’s Argonautica. The date of this unique and influential epic has been a source of controversy for over a century. My research, however, introduces a new and more objective approach to the question. By using the references to astronomical phenomena in the epic with calculations verified by colleagues in astronomy and by astronomical software, I have been able to date the poem to a specific/significant year during the Ptolemaic period. The astronomical references in books 1 and 2 allude, I submit, to the year 238 BCE, the year in which Ptolemy III celebrated his Jubilee and introduced a calendar reform, the first-known calendar based on the solar year. My findings challenge the prevailing view that Apollonius’s Argonautica was written sometime between 270 BCE and 240 BCE, during the reign Ptolemy II (284–246 BCE).