Lighting up the Night: The Science Behind the Christmas Star

By Amie Knowles

December 5, 2020

This is a once in a lifetime chance for Virginia residents, as the event happens once every 800 years.

BLACKSBURG – Looking up in the night sky this month, you might need a pair of shades. Okay, so it might not be quite that drastic, but between Dec. 16 and 25, two planets will align so closely that they will light up the night sky in tandem. That alignment creates a sight people call the Christmas star.

Nahum Arav, a department of physics professor at Virginia Tech, revealed that while Saturn and Jupiter align once every 20 or so years, their upcoming close proximity alignment occurs once every eight centuries.

While major news media outlets just started publicly building up to the cosmic event this week, it’s something Arav’s students have been gearing up toward.

“This was well forecasted for a long time, and I indeed told my students that this will happen,” Arav said.

Henry Throop, an astronomer in NASA’s planetary science division in Washington, D.C., also anticipated the alignment.

“The orbits of the planets are known well and this close conjunction has been known for many years. It’s not a new discovery,” Throop said. “Astronomers have been able to predict conjunctions like this for hundreds of years.”

Prior celestial events

The fanfare – and fear – over the planetary alignment thus far hasn’t mimicked one of space’s most famous objects, Halley’s Comet, which appears once every 75 or so years.

First reported in 239 B.C., it wasn’t until the 1600s that Halley’s Comet gained notable recognition. That’s when English astronomer Edmond Halley, born in 1656, made a connection. He realized that a comet passed Earth in 1532, then again in 1607 and 1682. Although Halley didn’t live to see the comet pass by again in 1758, his discovery lives on through the comet’s name.

In 1835, people celebrated the comet’s appearance. Jewelers made special pieces, shaped after the celestial object. Antiques Roadshow appraiser Kevin Zavian came across one of the antique pins during the 2018 production tour. He appraised the one-carat-weight diamond pin at around $1,500 retail.  

By 1910, the comet’s appearance terrified many. Media outlets reported widespread panic that the comet’s gasses would wipeout life on earth – or at the very least, bring about pestilence.

Halley’s 1986 appearance saw new space-age excitement. Several spacecrafts approached the comet, sampling its composition. Earthlings also used high-powered telescopes to see the comet.

Returning in 2061, there’s buzz about Halley’s Comet every so often. However, the 800-year planetary alignment of the Christmas star just recently garnered attention.

“The thing is, these are two very bright planets and every time they’re coming close together, it stirs some interest with the public. This one will be the closest we’ve had in more than 20 years, even of lesser planets and especially the brighter planets. It is extremely close. This is why it will interest some people,” Arav said. “I think it is less interesting than a big comet just because it is less spectacular. A big comet gives you this long tail and all kinds of beautiful things like that.”

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A rare occurrence

The professor explained that the alignment happens because of the planets’ unique orbits around the sun. Jupiter orbits every 12 years, while Saturn makes a loop every 29 years.

“Basically, this is why about every 20 years Jupiter is overtaking Saturn once again,” Arav said. “Their exact ellipses are different from each other and their orientation is different from each other, so sometimes when they come close together, they are extremely close like this time. But most times, they are not that close. This time, they will be as close as one-fifth of the observed side of the moon. So it will be really close.”

Observed between Dec. 16 and 25, the peak alignment will occur on the Winter Solstice, Dec. 21. That night, the planets will look more like one giant star than two separate objects.

Given the seasonal appearance, folks dubbed the occurrence as the Christmas star. People quickly ran with the idea that the planets could have aligned the year of Jesus Christ’s birth, creating the Star of Bethlehem.

It’s possible, but there’s no definitive celestial explanation at present.

“Historians have studied many ideas for astronomical events which could be described by the Star of Bethlehem. It is not possible to know for sure because of uncertainty in the observations and in which year they were made,” Throop said. “However, a conjunction between Jupiter and Saturn is one possibility, as are alignments between the planets and the moon or bright stars.”

Look skyward for the Christmas star

The next occurrence of a close proximity alignment between Jupiter and Saturn won’t happen for another 800 years. Even a typical alignment is two decades in the future, following Dec. 25.

“Next year, you won’t see it together,” Arav said. “As we said, only once every 20 years does it even get close together.”

For those looking up, the bright objects are already visible to the naked eye – and they’ll put on a celestial show over the coming weeks.

“They are in a very good position to be observed. They should be observed about half an hour after sunset. Then you can see them very easily,” Arav said. “The other important thing to notice is they are already rather close to each other and every day they are becoming closer. So people can watch them, you know, the progression. Here in Virginia, we don’t have too many nights that are clear, so you look at clear nights after sunset and you can see for yourself how close they are becoming.”

Throop also spoke about the planets’ motion and encouraged viewers to take note.

“Yes, I hope that people will look up and see it. The closest alignment is on December 21. But for all of December, Jupiter and Saturn will be close in the sky and easy to see looking southwest just after sunset. The close alignment – within 0.2 degrees – will last for a few days,” Throop said. “For people who see it a few times during the period, it is a great chance to see the motions of the planets. You can imagine Jupiter and Saturn as runners on a track. Jupiter is moving faster, and looking from one night to the next, people will be able to see Jupiter approaching and passing Saturn in their paths around the sun.”

No equipment necessary

People don’t need to venture to their nearest observatory to see the Christmas star. In fact, they don’t even need a telescope.

“The good thing here is that you can definitely see [them] with the naked eye because both planets are very bright,” Arav said.

For a more distinctive view as the planets reach their closest proximity, Arav suggested using binoculars.

“To be sure that you see, you probably need any kind of binoculars. Even really small ones would probably do the trick,” Arav said. “Now if you have a small telescope, on the 21st of December, you can see something very unique. You can see both planets and some of their moons at the same time, in the same field of view.”

Viewers don’t need an empty cornfield or a dark mountain ridge to see the alignment. The event will be so bright, even areas experiencing extreme light pollution still get access to the show.

“The nice thing is, these things are very bright, so everyone in Virginia can see that. Even people in [Northern Virginia] can see it because Jupiter is the brightest planet, brighter than any star in the sky, so that can be seen in any place half an hour after sunset. Saturn is rather bright, so it can also be seen when there is significant light pollution,” Arav said. “That’s the good thing, that you don’t really need to go to dark places to see it. You can just do it from your backyard, as long as your southwestern horizon is clear.”

Amie Knowles reports for The Dogwood. She can be reached at [email protected]

  • Amie Knowles

    Amie is Dogwood's community editor. She has been in journalism for several years, winning multiple awards from the Virginia Press Association for news and features content. A lifelong Virginia resident, her work has appeared in the Martinsville Bulletin, Danville Register & Bee and NWNC Magazine.

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