Dec. 24, 2012 at 11:46 AM ET
The Star of Bethlehem is one of the best-known parts of the Christmas story, celebrated in the Gospel of Matthew as well in as a constellation of holiday songs. It was that star that led the Three Wise Men to the infant Jesus — at least if you believe the Bible. But is there anything in the astronomical record that supports the story of the "Star of Wonder"?
The answer is, maybe. The case of the Christmas Star illustrates how slippery things can get when you try to mix scripture and science.
First of all, there's no way to show a definitive connection between any astronomical phenomenon and the tale of the Nativity. On one hand, you could just say that the star was a miraculous apparition. In that case, no further evidence would be needed. On the other hand, you could say that the whole Nativity story, including the part about the Three Wise Men, is fictional. In that case, trying to find the Christmas Star would be as fruitless as trying to determine the real-life location of Dumbledore's tomb in the "Harry Potter" saga.
But if you go along with the astronomers who have looked into the likeliest scenarios to explain Matthew's references to the Christmas Star, the line of reasoning takes some surprising twists: The star could have been a series of planetary conjunctions, or a comet, or perhaps a nova. These events didn't occur during the year A.D. 1, which most people assume was the year Jesus was born. Instead, they occurred at least a couple of years earlier. They also didn't occur anywhere close to Dec. 25.
And here's what might be the most surprising twist: All this meshes with the views generally held by scriptural scholars.
Matthew's story tells of "wise men from the east" — who were actually priestly astrologers. What they saw in their astronomical calculations led them to alert Judea's king, Herod the Great, to the birth of the "king of the Jews." Herod told the astrologers to look for the infant in Bethlehem and let him know what they found. Matthew says they came upon the infant Jesus, but were warned in a dream not to go back to Herod.
Historical accounts suggest that Herod died around the 4 B.C. — although some scholars suggest the date could have been as early as 5 B.C. or as late as 1 B.C. Using this time frame, astronomers have checked the historical records and run computer simulations of the night sky — and they've come up with these leading candidates for the Christmas Star:
Planets: The simulations show that there was a rare series of planetary groupings, or conjunctions, in 3 B.C. and 2 B.C. The first conjunction was on the morning of June 12 in 3 B.C., with Venus close to Saturn in the eastern sky. The second conjunction was a spectacular pairing of Venus and Jupiter on Aug. 12 in the constellation Leo, which ancient astrologers associated with the destiny of the Jews.
Between September of 3 B.C. and June of 2 B.C., Jupiter would have passed by the star Regulus in Leo, reversed itself and passed it again, then turned back and passed the star a third time. This reversal was due to the planet's apparent retrograde motion — a phenomenon familiar to the astrologers but not necessarily noticed by the casual observer. In his book on the Christmas Star, astronomer John Mosley says this would have been a significant event, because ancient astrologers considered Jupiter the kingly planet and regarded Regulus as the "king star."
The crowning touch came on June 17 of 2 B.C., when Jupiter was so close to Venus that "they would have looked like a single star," Mosley said. His scenario implies that the climax of the Nativity story came in the spring of 2 B.C.
There's a problem with this scenario, however: It doesn't work if Herod died in 4 B.C. An astronomer at the University of Sheffield, David Hughes, has proposed a different series of planetary conjunctions in 7 B.C. This was a triple conjunction, in which Jupiter and Saturn would appear to approach each other three times between May and December. "Events indicate that Jesus Christ was probably born in the autumn of that year, around October, 7 B.C.," Hughes wrote in a paper published by the journal Nature.
Comet: Other astronomers have considered the idea that the "star" was actually a comet. The likeliest candidate would be a comet recorded by Chinese astronomers in the year 5 B.C., in the constellation Capricorn or Aquila. Comet Halley would have been visible in 11 B.C., and the record suggests that other comets might have been seen in the time frame between those two dates. "The snag is that they're not that rare," Hughes told the BBC. "They were also commonly associated with the 'four Ds' — doom, death, disease and disaster. So if it did contain a message, it would have been a bad omen."
Nova or supernova: The Chinese were particularly good at chronicling supernovae, and the fact that none was recorded during the time frame in question has led most astronomers to discount a supernova as the explanation for the Christmas Star. However, astronomer Mark Kidger argues in his book, "The Star of Bethlehem," that the comet seen by the Chinese in 5 B.C. was actually a nova — that is, a suddenly brightening star. The temporary brightening may not have caused a worldwide marvel, but if it came after a series of planetary conjunctions, it could have been enough of a signal to send the wise men on their way.
Kidger's scenario calls for the climax of the Christmas Star story to come in March of 5 B.C., after months of buildup. He even names his candidate for the Christmas Star: DO Aquilae, which is just faintly visible today.
What scholars say: None of these scenarios would be consistent with Western Christianity's traditional schedule for the Christmas season, which calls for the "12 Days of Christmas" to begin on Dec. 25 and wind up with the arrival of the Three Kings on Epiphany, Jan. 6. However, scriptural scholars have pointed out that none of the Gospels refers to the date of Jesus' birth. In fact, the Gospel of Luke’s account about shepherds being out in their fields might make more sense if the birth occurred during the spring lambing season.
So how did Dec. 25, A.D. 1, get set as Jesus' birthdate? The current counting system for years (A.D. and B.C.) was set up in the sixth century by a monk named Dionysius Exiguus, who combined his reading of scripture, Roman history and end-of-the-world numerology to pick Year 1. Scriptural scholars now agree, however, that the timing of the Nativity story would make more sense if the birth occurred earlier than that — because of the timing of Herod's death as well as a better understanding of the chronology for Roman emperors and governors.
As for the December date: Scholars say that the early Christian church wasn't all that interested in marking the day of Jesus' birth. For example, a 3rd-century theologian named Origen mocked the Romans for making such a big deal over divine birthdays.
Around the year 200, Clement of Alexandria noted that the favored dates for the birth were in the March-April-May time frame — which would be consistent with the astronomical scenarios for the Christmas star.
It wasn't until the mid-4th century that Dec. 25 started showing up in church literature. The conventional wisdom is that Christmas was set in December after Constantine the Great's conversion to Christianity in 312, to bring the Christian holiday into line with pagan celebrations of the solstice. But Andrew McGowan, warden and president of Trinity College at the University of Melbourne in Australia, argues in favor of an alternate explanation: that church leaders wanted to link their date for Jesus' conception with the presumed date of his death, on March 25. If you add nine months to March 25, you get Dec. 25 as the date of birth.
"Connecting Jesus' conception and death in this way will certainly seem odd to modern readers, but it reflects ancient and medieval understandings of the whole of salvation being bound up together," McGowan writes in this month's essay for Bible History Today.
The tale of Christmas and the Star of Wonder shows how astronomy and numerology can get tangled up with religion. But we're familiar with that, right? After all, we've just been through the apocalyptic angst surrounding the turnover of the Maya Long Count calendar. Fortunately, this week's turn of the calendar has a much more positive spin. So here's wishing you a wonderful holiday season of your choice — whether it celebrates Christmas or the solstice, the new year on the Gregorian calendar, or the new baktun for the Maya.
More about the science of the season:
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.