Dicuil’s so-called Liber de astronomia has confused many modern scholars. Based on an analysis of its structure, this paper argues that the very nature of the work has often been misunderstood. It was never meant to be a textbook, but scholarly ad-hoc writing, shaped by some special, changing contexts of its presentations at the Carolingian court over the years 814-18. Dicuil himself later described the resulting work as De cursu solis lunaeque, which is more fitting than the common modern title.