cosmic alignment

"Christmas Star" — It's not, actually, and astronomy explains why

Jupiter and Saturn are scheduled for a once-in-a-millennium rendezvous.

christmas star
Max Dannenbaum / Getty Images

For centuries, people have looked at the skies as a way to guide them.

Celestial events have always held a cultural and spiritual significance to those who looked up and saw shooting stars, or other cosmic happenings.

In anticipation of the historic Jupiter and Saturn conjunction on Monday, December 21, 2020 Inverse asked astronomers why this particular alignment has attracted so much global attention, and whether or not it may be another version of the Biblical "Star of Bethlehem."

On Monday, December 21, 2020, Jupiter and Saturn are due for a rare encounter when they come very close to each other. Look to the Western horizon and you'll be able to see it until around 7 p.m. Eastern time. You will not need a telescope and it will be the brightest object in the sky other than the moon.

Here's the background:

  • Jupiter and Saturn's respective orbits will align, bringing them closer than they have been in almost 400 years.
  • Juper and Saturn's proximity to each other will be more visibile that it has been in 800 years.
  • There's a hypothesis circling the internet that this conjunction may have been the Star of Bethlehem.
  • The once-in-a-lifetime event will be visible on Monday, December 21.

The last time these two gas giants appeared this close in our night sky was in 1623, and even then they were situated too close to the Sun for people to see them with the naked eye. The planets' last visible, close conjunction was in 1226.

In other words, the last time people actually saw the planets this close was 800 years ago. This is perhaps best described as a once-in-a-millennium event.

"They will get so close to one another that some folks will actually have difficulty seeing them as two separate objects."

To see the conjunction on Monday, all you need to do is look up at the right time and with the right conditions. Inverse has a guide for how best to see it here.

How can two planets become one?

This rare sight in the sky is actually the result of an optical illusion, caused by the planets' orbital planes.

It takes Jupiter 12 years to orbit the Sun. Saturn takes a slower, more distant route that takes the planet 29 years to complete.

During their conjunction, Jupiter essentially laps Saturn, speeding up past the gas giant in their respective trips around the Sun.

Jupiter and Saturn meet this way once every 20 years. But this year's rare alignment is the closest they will be from our view on Earth since 1623.

"There’s a lot more to life than knowing what the facts of life are."

According to Billy Teets, director and outreach astronomer at the Vanderbilt Dryer Observatory in Tennessee, Monday's alignment is no supernatural phenomenon, but the result of the orbital planes of Jupiter, Saturn and Earth.

All of the planets orbit in roughly the same plane around the Sun, following a narrow band in the sky, but they all have slight differences in their orbital planes.

"The orbital planes of each of the planets are all titled with respect to one another," Teets tells Inverse. "Most years, their orbital planes have kind of separated them but this year, they’re near a point where their orbital planes are crossing one another."

When it happens, the two planets will look as though they are incredibly close to one another, but the actual physical distance between the two planets will not have changed. Jupiter will appear as a bright star and be easily visible, while Saturn will be a little less bright, visible slightly above and to the left of Jupiter.

"They will get so close to one another that some folks will actually have difficulty seeing them as two separate objects," Teets adds.

Planetary conjunction: History shows why it's not "Star of Bethlehem"

It is believed that in the 6th century B.C.E, the three Wise Men followed a star that led them to Bethlehem, where they found the newly born Jesus Christ.

For centuries, astronomers have tried to uncover the truth of this celestial event and what it may have referenced. Some astronomers believe that the bright 'Star of Bethlehem' was actually a planetary conjunction similar to the one taking place on Monday, but perhaps between different planets, such as Venus, Mars, and Jupiter.

"It is more than likely that an event similar to this one could explain what was seen then," Diana Hannikainen, observing editor at Sky & Telescope, tells Inverse.

But Patrick Hartigan, a professor of Physics and Astronomy at Rice University in Texas, believes that if we traveled back in time to look at the planetary conjunction that took place that year, we would have been less than impressed.

"From the standpoint of a modern observer if we were to go back and look at that conjunction, it wouldn’t appear to be anything special," he tells Inverse. "There wasn’t a brilliant star, it wasn't particularly spectacular."

The conjunction that occurred at the time of the Magi did have some interesting aspects. It appears to have been the alignment of three planets at once, but they were between eight and nine times further away from Earth than the one taking place this year, according to Hartigan.

This year's conjunction will not look like a single, bright star, either. The two planets will appear as two objects, one more bright than the other.

Ultimately, the idea that this is the "Christmas star" is also misleading.

"If this had been occurring any other year or any other time of the year, it probably wouldn’t be called a 'Christmas star,'" Teets says. "These kinds of events, just because they’re so rare, that’s what catches people’s attention."

Jupiter and Saturn: So much more than a planetary alignment

Although their work is based on the science behind these types of events, astronomers tend to not shy away from the cultural significance that people attach to celestial objects.

"There’s something really compelling to see that’s happening in the sky," Hartigan says. "There’s a lot more to life than knowing what the facts of life are."

Hartigan believes celestial events such as this one connect people to the natural world, and give them a sense of time and space. The next time this rare conjunction will be observed will be in 60 years, in the year 2080. These events connect us with our peers across millennia, Teets, says.

"For a lot of people this conjunction will be a once in a lifetime event," Teets says.

Hartigan echoes the same sentiment.

"It's a connection to the past, and it's also a good connection to the future," he says.

For my part, Hartigan encouraged me to look up on Monday night to see this rare event, and perhaps 60 years from now, watch the future conjunction take place in the sky and recall this very conversation. I'll be watching.

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