monk, grammarian, and geographer whose work is important to the history of science and is a testament to Irish learning in the 9th century.

Much of Dicuil's astronomical knowledge was gained in calculating dates for religious festivals. Completed in 825, his De mensura orbis terrae (“Concerning the Measurement of the World”) contains the earliest mention of Irish hermits having visited Iceland (795), where they marveled at the midnight sun. The work also contains the most definite Western reference to the old freshwater canal between the Nile River and the Red Sea, which was blocked up in 767. Dicuil learned of the canal from one “Brother Fidelis,” probably another Irish monk, who sailed along the “Nile” into the Red Sea—passing the “Barns of Joseph,” or pyramids of Giza, which are well described. Dicuil quotes from, or refers to, 30 Greek and Latin writers as well as to the poet Sedulius, his Irish contemporary. The best edition of De mensura was made by G. Parthey in 1870.