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Who determined the year that Jesus was born? Did the person leave behind an explanation? How accurate is the estimated year (753 Ab Urbe Condita) of Jesus' birth?

December 21, 2013

When was Jesus born?
It is somewhat embarrassing for historians of antiquity when someone asks this question. Crazy facts come into play, something as boring as a calendar and then as fascinating as planets joining in the heavens.

In 1603 German mathematician and astronomer Johannes Kepler watched and charted a conjunction (the orbits of the two planets coming together visually) of Jupiter and Saturn. Kepler calculated that this event happened in almost exactly 800 year intervals. The conjunction happened again the next year, thus more studying of the event. On October 10 Kepler witnessed a supernova. He published a book about this event, but while working on this book he encountered the work of Laurence Suslyga, a Polish astronomer who had argued that, based on some other celestial signs, Christ had been born in 4 B.C. This led to Kepler's theory that the conjunction of Jupiter and Saturn 1600 years prior must have led to a supernova event that became known as the Christmas Star, or Star of Bethlehem.

To this day many believe that the conjunction of Jupiter and Saturn may have been the Christmas Star. Many have offered arguments against this theory. But how did we come to the actual year number 6 BC?

As with almost all great ancient cultures, the Romans devised their own calendar to record their greatness, "Anno Urbis Conditae" or "from the founding of the city." Unfortunately, the Romans failed to use this dating method as much as a more regional method, referring to the "Nth year of the reign of [a local consul or king]."

The initial Roman calendar had 10 months with leap years and was not very practical. In 46 BC Julius Caesar commissioned Sosigenes, a Greek astronomer, to change the calendar to 12 months and a leap year system. This calendar system was an improvement, but unfortunately, every four years a day was added to the recorded "time", but not accounted for, it just kind of got lost.

In 525 Dionysius Exiguus, a Scythian monk, decided that he no longer wanted his community to follow the Roman calendar and wanted to chart everything by the birth of Jesus. He calculated back to the year 1 as the birth of Jesus, but he made a few important mistakes. He did not realize that the Roman calendar was gaining a day each year and he failed to consider that, given his method, Jesus would have been born at the year zero, which does not exist. Sadly, it took more than 1,000 years for his mistakes to be discovered by - none other than Laurence Suslyga, the Polish astronomer who had postulated the birth of Jesus at 4 BC. It turns out that the Dionysius calendar was off a total of at least five years, thus the birth of Jesus would have to be approximately 4 BC plus or minus a few years for other mistakes which have possibly not been realized yet.

Fortunately, the planetary alignments can help us postulate a date. Thus 6 BC with the conjunction of Jupiter and Saturn discovered by Johannes Kepler is accepted by some as a good dating mechanism. This is still only a theory with plenty of scholars arguing for various small differences, but most agreeing to a date of between 6 BC and 2 BC.

Kidger, Mark R., "The Star of Bethlehem: An Astronomer's View" (1999 Princeton Univ Press)
Mosley, John, "Common Errors in 'Star of Bethlehem' Planetarium Shows" (1981 Planetarian)

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