The incidents whose absolute chronology may be determined with more or less probability are the year of Christ’s nativity, of the beginning of His public life, and of His death.
St. Matthew (2:1) tells us that Jesus was born “in the days of King Herod”. Josephus (Ant., XVII, viii, 1) informs us that Herod died after ruling thirty four years de facto, thirty seven years de jure. Now Herod was made rightful king of Judea A.U.C. 714, while he began his actual rule after taking Jerusalem A.U.C. 717.
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A.U.C. is an abbreviation for ab urbe condita, “from the founding of the city” of Rome. The year 1 A.U.C. is the same as the year 753 B.C., The year 1 B.C. is the same as the year 753 A.U.C., and the year 1 A.D. is the same as the year 754 A.U.C.. Unfortunately, a year divisible by 4 in the Roman calendar is not divisible by 4 in the Christian calendar. As an example, the year 4 A.D. is divisible by 4. The equivalent year by the Roman calendar is 757 A.U.C., which is not divisible by 4. This is very inconvenient when figuring leap years.
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As the Jews reckoned their years from Nisan to Nisan, and counted fractional parts as an entire year, the above data will place the death of Herod in A.U.C. 749, 750, 751. Again, Josephus tells us from that an eclipse of the moon occurred not long before Herod’s death; such an eclipse occurred from 12 to 13 March, A.U.C. 750, so that Herod must have died before the Passover of that year which fell on 12 April (Josephus, “Ant”., iv, 4; viii, 4). As Herod killed the children up to two years old, in order to destroy the new born King of the Jews, we are led to believe that Jesus may have been born A.U.C. 747, 748, 749. The enrollment under Cyrinus mentioned by St. Luke in connection with the nativity of Jesus Christ, and the remarkable astronomical conjunction of Mars, Jupiter, and Saturn in Pisces, in the spring of A.U.C. 748, will not lead us to any more definite result.