Complex sentence: The work was written in Latin in or soon after AD 840 by an anonymous author who is conventionally called Astronomus, the Astronomer or the Limousin Astronomer, due to his many detailed comments on astronomical matters in the work.
Split into simpler sentences: The work was written in Latin in or soon after AD 840 by an anonymous author. He is conventionally called Astronomus, the Astronomer or the Limousin Astronomer, due to his many detailed comments on astronomical matters in the work.