Dionysius Exiguus

500?-560?

Scythian Theologian, Mathematician, and Astronomer

Dionysius Exiguus, a Roman theologian, mathematician, and astronomer, is known for inventing a Christian calendar that was later incorporated into the currently used Gregorian calendar. In addition to providing calculations for determining the date of Easter, Dionysius's calendar is notable for establishing its starting point from the birth of Jesus Christ, thus introducing the designations b.c. and a.d.

Little is known of Dionysius's early life. It is documented, however, that he arrived in Rome about the time of the death, in 496, of Pope Gelasius I, who had summoned him to organize the official archives of the church hierarchy. The Roman Empire was in ruins, and the city of Rome itself was dilapidated and nearly deserted.

A trained mathematician and astronomer, as well as a master theologian, Dionysius spent his days working at a complex now known as the Vatican. He wrote church canons and spent a great deal of time thinking about how to make order of time itself. He worked as a church scholar for many years and in 525, at the request of Pope John I, he began work on the calculations that would become the basis of the Gregorian calendar centuries later. At the time, the Julian calendar, devised by Julius Caesar (100-44 b.c.), was used as the working calendar of the Church. One of the most problematic calculations for these Roman timekeepers was figuring out where the holy days fell, especially Easter.

Pope John I asked Dionysius to calculate the dates upon which future Easters would fall. The Church had adopted a formula some 200 years earlier stating that Easter should fall on the first Sunday after the first full moon following the spring equinox. Dionysius carefully studied the positions of the Moon and Sun and was able to produce a chart noting the dates of upcoming Easters, beginning in the year 532. This compilation was received with some controversy, but was accepted. At that time, most people used the year designated as either 1285, dated from the founding of Rome, or 248, based on a calendar that began with the first year of the reign of Emperor Diocletian.

Dionysius defended his decision, suggesting that his calculation was taken from the reputed birth date of Jesus Christ. He "preferred to count and denote the years from the incarnation of our Lord, in order to make the foundation of our hope better known." The new Easter charts reflected this preference, which began with anno Domini nostri Jesu Christi DXXXII, or a.d. 532. Dionysius's system used the designations a.d. (after Christ's birth) and b.c. (before Christ). However, Dionysius miscalculated Christ's birth as a.d. 1, as there was no zero value in the Roman numeral system, instead of 4 or 5 b.c. (before Christ) as is now generally accepted. The sixteenth-century pope Gregory III later adapted and incorporated the Julian calendar, with its problem of loosing entire days over time, into what became known as the Gregorian calendar. This calendar is the most popular and widely used in the world today.

In addition to his skill as a mathematician and astronomer, Dionysius was highly regarded as a theologian. He is credited with writing a collection of 401 ecclesiastical canons or scriptures that would become important historical documents about the early years of Christianity. He is also responsible for compiling an important collection of decrees written by Pope Siricius (344?-399) and Pope Anastasius II (?-498). He translated many works from Greek into Latin, thus preserving many important documents for later scholars.

LESLIE HUTCHINSON