Dicuilus (or the more vernacular version of the name Dícuil) was a monk and geographer, born during the second half of the 8th century. Noble and Evans identify him as a Gael and suggest that he had probably spent time in the Hebrides. Background The exact dates of Dicuil's birth and death are unknown. Of his life nothing is known except that he probably belonged to one of the numerous Irish monasteries of the Frankish Kingdom, and became acquainted by personal observation with islands near England and Scotland. From 814 and 816 Dicuil taught in one of the schools of Louis the Pious, where he wrote an astronomical work, and in 825 a geographical work. Dicuil's reading was wide; he quotes from, or refers to, thirty Greek and Latin writers, including the classical Homer, Hecataeus, Herodotus, Thucydides, Virgil, Pliny and King Juba, the late classical Solinus, the patristic St Isidore and Orosius, and his contemporary the Irish poet Sedulius. In particular, he professes to utilize the alleged surveys of the Roman world executed by order of Julius Caesar, Augustus and Theodosius II. Based on similarities of style, it has been suggested that Dicuil may be the same person as the anonymous Hiberno-Latin poet and grammarian known as Hibernicus exul. Astronomical computus The astronomical work is a sort of computus of five books, in prose and verse. Four books are preserved in a manuscript which belonged formerly to the monastery of Saint-Amand in northern France, and is now at Valenciennes. A second manuscript is from the abbey of Saint Martin at Tours, and contains two chapters added to the fourth book, and two more chapters constituting a fifth book Book 1 contains material on calendars, on 19 year lunar cycles, and on versification. It also contains an account of the two methods of calculating triangular numbers: by summation of the natural numbers, or by the multiplication together of two consecutive numbers divided by two Book 2 contains material on the distance between the earth and the heavens, and between the seven planets; methods for counting the lunar months; the monthly age of the moon; rules for calculating Easter and Lent; intercalary days (extra days) and subtracted days; solar and lunar years; more on versification. Book 3 contains material on cycles of the stars; 19 year lunar cycles; other large cycles of the sun and moon; the first day of the natural