Jupiter, Saturn to line up in ‘Christmas star’ conjunction of planets

If you’ve been waiting for the right moment to cast a spell, chase an omen or simply witness a marvel of the solar system, Monday is the day.

Jupiter and Saturn will line up in the night sky on Dec. 21 in an extremely rare conjunction of the planets that hasn’t happened in nearly 400 years.

The two planets will appear close enough to shine as one bright point in the sky, providing a “Christmas star“-like effect ahead of the holiday, NASA says.

The so-called “Great Conjunction” will also occur during the Northern Hemisphere’s winter solstice, which is the shortest day of the year in terms of daylight hours.

It’s the kind of event that has been associated with myths, magic and doomsday prophecies in ages past — though you’ll surely be disappointed if you give one of those ideas a try. You’ll have a better time if you venture outside after dark on Monday to catch a glimpse of it yourself.

Jupiter and Saturn line up from Earth’s perspective approximately once every 20 years. However, this event is special because the planets will appear unusually close to one another after dark. Other planets line up from Earth’s perspective now and then, but Jupiter and Saturn are the largest and therefore easiest to see.

Jupiter and Saturn will be just one-tenth of a degree apart from our perspective or about one-fifth the width of a full moon. However, the solar system’s two largest planets will still be about 730 million kilometres apart in space.

The last such event happened in 1623, but that was during the daytime. NASA says it’s been nearly 800 years since the two planets lined up at night, in March 1226.

The best way to watch is to find an unobstructed view of the southwestern sky, NASA says. The two planets will appear low on the horizon as bright points of light, so they should be visible even from a city. They’ll be brightest an hour after sunset, provided there are no clouds.

“Saturn will be slightly fainter and will appear slightly above and to the left of Jupiter until Dec. 21, when Jupiter will overtake it and they will reverse positions in the sky,” NASA says.

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They’ll be so close that you could cover them both with a pinkie finger at arm’s length, according to NASA.

NASA astronomer Henry Throop compared the event to watching a race from the centre of the loop.

“From our vantage point, we’ll be able to be to see Jupiter on the inside lane, approaching Saturn all month and finally overtaking it on Dec. 21,” he said.

It could be a once-in-a-lifetime event, as the next conjunction is not expected until 2080.

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“What is most rare is a close conjunction that occurs in our nighttime sky,” said Vanderbilt University’s David Weintraub, an astronomy professor. “I think it’s fair to say that such an event typically may occur just once in any one person’s lifetime, and I think ‘once in my lifetime’ is a pretty good test of whether something merits being labelled as rare or special.”

It’s also a prime opportunity for professional and amateur astronomers, as it’ll be the first time humans have watched it through a telescope. The planets will be visible to the naked eye, but a telescope should reveal some of their moons as well.

“As far as we know, nobody will have seen Jupiter and Saturn so close together in a telescope eyepiece ever before this year,” Matthew Bate, a professor at the U.K.’s Exeter University, told The Guardian.

Even as Dec. 21 approaches, Jupiter and Saturn already appear close together, and amateur astronomers have been able to observe Saturn with its famous rings and Jupiter with some of its larger moons.

Bate pointed out that the last conjunction happened 13 years after Galileo Galilei first looked at Jupiter and Saturn through a telescope. He says telescopes simply were not widespread at the time, and even if they were, the last Great Conjunction happened close to the sun, where it would be nearly impossible to observe safely.

“There is no record of anyone observing the 1623 conjunction through a telescope,” he said.

The planets will appear close together in the days before and after the conjunction, meaning there’ll still be plenty to see if you forget on Monday.

However, you’ll only have one night to follow yonder “star.”

— with files from The Associated Press