Could the Star of Bethlehem have actually been a comet?

Could the Star of Bethlehem, which guided the 'three wise men' to the infant Jesus in the Christian Bible, have been a comet that came as close to the Earth as the moon?

That's the remarkable hypothesis from Mark Matney, a planetary scientist in NASA's Orbital Debris Program Office by day and a self-declared Christmas junkie. "I love Christmas," Matney told Space.com. "I love Christmas music, I love Christmas decorations — I love the whole thing!"

It was this love of Christmas, expressed in a festive show at the planetarium that Matney worked at when he was in college, that inspired his interest in the Star of Bethlehem. A passage in the Bible's Book of Matthew describes how the star went before the wise men — known as 'magi', who were probably astrologers looking for signs in the sky — and stood over Bethlehem. The planetarium show suggested that no known astronomical event behaves in this bizarre way, but rather than accept that, Matney saw it as a challenge. "I remember sitting there and thinking, I can think of one thing that can behave that way," he recalled.

For Matney, there are three ways to look at the story of the Star of Bethlehem. One, which is how those of a religious leaning might see it, is as a miraculous, divine event, the archangel Gabriel shining the way towards the baby Jesus.

Another, more cynical, view is to believe the whole story to be a myth, at best perhaps a misrepresentation or embellishment. If the Star of Bethlehem was either of these two things, then there's no point in looking for a scientific explanation.

On the other hand, the third way of looking at it is as a real astronomical event. Over the years, astronomers have suggested everything from a conjunction of Jupiter and Saturn to a supernova and, yes, a comet.

The problem with all previous astronomical explanations, says Matney, is that "objects in the sky, whether it be the sun, moon, planets, ordinary stars or normal comets, rise in the east and set in the west, they don't go before you and hover over a location."

However, Matney realized that if an object came close enough, at just the right time, moving in just the right direction at just the right speed through the sky, then it could appear to do these things.

"I came up with the idea of temporary geosynchronous motion," said Matney. "It has to be just right, but in principle it can happen."

The Chinese comet of 5 BCE

Matney filed his idea away in the back of his brain, until later when he learned that Chinese astrologers (astronomers and astrologers back then were synonymous) had seen a bright comet in 5 BCE, which is believed to be the year that Jesus was born.

The recorded observations of the comet are not sufficient to chart its exact orbit, but instead its measured positions in the sky could belong to a range of possible orbits. By running numerical simulations describing all these possible orbits, Matney found a subset of orbits that would have brought the comet close to Earth, and one possible orbit where it would have been close as Earth's moon.

Matney isn't saying that the comet definitely came that close — it's just one of a number of possible orbits the comet could have had. Had it done so, however, it could be a tantalizing explanation for the Star of Bethlehem, explaining a great many things.

The Christmas festivities tell us that Jesus was born on Dec. 25, but theologians and historians aren't actually sure of his birth date. However, the Chinese comet was discovered in mid-March, 5 BCE and, in the possible orbit flagged as being of interest to the Star of Bethlehem story, it would have reached its closest point to Earth on June 8 that year.

This doesn't necessarily mean that Jesus was born in June either; it's also not clear how long after Jesus' birth the magi were said to have visited him. We do know that when Herod later learned of Jesus' birth, he ordered all infant boys under the age of two to be killed, adding more uncertainty to when Jesus was born.

Unusual behavior at the comet's closest approach

Regardless, the comet would have continued along its orbit unperturbed by what was happening on Earth. In the possible orbit of interest discovered by Matney, the comet's closest approach to Earth would have been at a distance of 241,685 miles (388,954 kilometers). This would have been closer to Earth than any other comet in recorded history, so close that Earth itself would have been encased in the comet's coma, its expansive halo of dust around its icy nucleus.

As the comet neared its closest approach, its direction of origin would have meant that its motion in the sky began to accelerate eastwards, not westwards, fast enough to begin to counter Earth's rotation in the opposite direction. Between 10 a.m. and 11:30 a.m. on the morning of June 8, as seen from the Jerusalem/Bethlehem area, this motion would have given the comet the illusion of remaining still in the daytime sky, as bright as the full moon and appearing to be above Bethlehem from the point of view of the magi.

Afterwards, the comet would have resumed westward motion on an orbit that would have seen it skim the sun's corona. We call such comets 'sungrazers' and the close encounter with the sun would have probably resulted in the comet breaking up and being destroyed.

That the comet would have been visible during the daytime, in its guise as the Star of Bethlehem, even solves a minor mystery of the Christmas story, according to Matney.

"All the Christmas cards have the magi on camels at night, but during those times people typically did not travel at night," he said, citing hazards such as unlit paths and the danger from robbers. "So the fact that this comet would have been visible in broad daylight makes sense to me, as they were more likely to have travelled during the day."

Comets, omens and history

If Matney is right, why doesn't the Book of Matthew refer to a comet rather than a star? To the ancients, everything in the celestial sky barring the sun and the moon was a star. Planets were 'wandering stars', while comets were 'hairy stars' or 'broom stars' to the Chinese. And while comets are often seen as portents of doom, Matney explains that it's not as simple as that.

"Comet omens of that period are nuanced, but they were often omens of great change," said Matney. Often, it really depended on your point of view. Herod was very interested in the duration that the star was in the sky, and he would of course have seen the comet as a bad omen.

It seems that the Chinese were also influenced by the comet's presence in the sky. Although there's no record in their annals of the comet growing so bright as to rival the moon in the daytime sky, the comet seems to have affected the astrological reckoning surrounding the emperor of that time, Emperor Ai of the Han dynasty.

"The Chinese had periods in the Emperor's reign, and I'm not 100% sure of this, but according to my Chinese colleague, the Chinese temporarily changed the dating of these periods in part because of the comet," said Matney. "So it was enough that it got their attention."

Even so, it does seem like it would require a remarkable set of coincidences for the comet to have been the Star of Bethlehem, by being in the right direction, at the right time, moving at the right speed and at the right distance — a Goldilocks comet, if you will.

"It is a very unusual set of parameters," conceded Matney. "It had to come at the right time so that the right longitude could see it. Twelve hours sooner and it would have been on the other side of the world. But even though it is a highly unlikely set of circumstances, it's not out of the question. After all, every comet's orbit is a unique set of parameters."

Despite how rare it is for comets to come as close to the Earth as our moon — if Matney is right, this is the only comet in the past 2030 years — we have seen in the past decade that it is possible for a comet to sail close to a planet. In 2014, comet C/2013 A1 (Siding Spring) passed within 140,000 kilometers (87,000 miles) of Mars.

Searching for more evidence

Such a close approach could have physically left its mark on Earth. As the comet's dusty coma would have swept across our planet, there would have been one heck of a meteor shower with its radiant in the constellation of Capricornus, the Sea Goat, and some of that cometary dust would have drifted through the atmosphere and settled onto Earth, finding its way into sediment, just waiting to be found as a thin geological layer.

"There might be something in the ice cores, a sudden jump in cometary or meteoritic dust," said Matney. "I did look for something like that but didn't find anything obvious. Maybe someone who studies ice cores for a living can take a better look."

Another problem with Matney's hypothesis is that other than the short section in the Book of Matthew, which is believed to have been written after 70 AD, the only other source of information regarding the comet and its possible link with the Star of Bethlehem is the Chinese observations of the comet. If anyone else did see the star, they didn't leave any records — or at least, no records that have survived across the millennia since. Still, Matney is hopeful that something else might yet turn up.

"The weakest link in my story is that we don't have other records, which is why I'm still on the lookout for some untapped historical or archaeological source that might provide more clues," he said.

Matney is not claiming his hypothesis to be the final solution to the mystery of the Star of Bethlehem. "I have no proof that the comet came that close, I just show that it could have," he said. "Unless we can turn over more records from the first century AD that can help us pinpoint the comet's orbit, it will stay in the realm of speculation."

We might never know what the Star of Bethlehem was, or even if there was a star at all. Matney's motivation was just to show that no matter how rare it might be, there is an astronomical event that in principle could behave like the Star of Bethlehem is reported to have behaved.

It's ironic; were a comet to come that close today, there'd probably be panic about it possibly crashing into Earth, but a little over 2,000 years ago, it might have been seen as the rise of a new king, the birth of a savior and the dawn of a new religion.

Matney's research into the Star of Bethlehem and the comet hypothesis was published on Dec. 3 in the Journal of the British Astronomical Association.