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Jupiter and Saturn's great conjunction: How to see the 'Christmas star' tonight

How to watch the closest observable rendezvous of the two giant planets since the Middle Ages.

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Saturn, captured here by the Hubble Space Telescope over the summer, will align with Jupiter on Monday.

NASA, ESA, A. Simon (Goddard Space Flight Center), M.H. Wong (University of California, Berkeley), and the OPAL Team

Skywatchers, get ready for a rare and spectacular sight. In an event dubbed a great conjunction, Jupiter and Saturn, the two biggest planets in our solar system, will appear very close to one another tonight. Closer, in fact, than they've been since the Middle Ages. 

The event is so legendary some have associated it with the famed Star of Bethlehem that guided the three wise men in the Bible's Nativity story. (For more on that angle, read below.) And even Monday's Google Doodle is marking the occasion, with an animation showing Saturn giving Jupiter a high-five as it slides on by in its solar system orbit.

In astronomy, a conjunction occurs when any two astronomical objects (asteroids, moons, planets, stars) appear close together in the sky when observed from Earth. A great conjunction specifically involves Jupiter and Saturn. This occurs only every 19.6 years, so the event is already rare, but Monday's event will be the closest observable conjunction of the two since the year 1226. (They also came about this close in 1623, but likely couldn't be seen from Earth.) And don't miss it -- you may not get another chance.

"This is the 'greatest' great conjunction between Jupiter and Saturn for the next 60 years, with the two planets not appearing this close in the sky until 2080," said Preston Dyches , a writer and producer from NASA's Jet Propulsion Laboratory, in a NASA video.

The event should be easy to see, says astronomy educator and former planetarium director Jeffrey Hunt, who has written about the event on his website, When the Curves Line Up.

"Step outside after sunset to find (the planets) in the southern sky," he advises. "A binocular is helpful; the pair is visible to the unaided eye as Jupiter overtakes and passes Saturn. On conjunction evening, the planets fit into the eyepiece of a spotting telescope or small telescope at low power." 

Saturn's rings and Jupiter's four brightest and largest moons will be visible with the aid of binoculars or a telescope as well.

Those who want to photograph the moment can do so easily. Hunt says a tripod-mounted camera with exposures ranging up to 10 seconds can capture the planets and the background stars. The event should be visible from anywhere on Earth that offers clear skies.

The conjunction is sometimes referred to as the Christmas star. Some claim a similar planetary meetup created the legendary Star of Bethlehem that led the biblical Magi, also known as the three wise men, to the Christ Child. Even German astronomer Johannes Kepler put forth the idea back in the 17th century.

But when you dig into the facts, that doesn't quite match up.

"Everyone is looking for a fantastic angle," Hunt says. "The problem with the Star of Bethlehem connection is the actual year, and season (or) month, of the birth. And there are other planetary alignments that could explain the Star of Bethlehem. This topic was thoroughly beat to death by the planetarium community in the 1980s."

Don't look for a sci-fi movie type merger of Jupiter and Saturn, Hunt says. This isn't an eclipse.

"The planets will not merge into a single point of light as is being reported in some media," he says. In other words, Jupiter won't pass directly in front of Saturn, cutting it off from view. 

There's really no need to embellish the sight, as it's amazing enough on its own.

"This should be a spectacular view with either the naked eye, or with a backyard telescope," NASA noted in an online article.

Hunt notes that, while this particular event is notably close, a great conjunction is a generational event, not a once-in-a-lifetime one.

"A great conjunction occurs three or four times during a human lifetime and it marks the passing of generations," he says. "I am encouraging families to get their children outside to look, tell the children that the planets will be near each other again in 20 years, and ask how old they will be then."

Although the planets come together over time, Monday will mark the actual conjunction -- the night when the two planets are closest, and Jupiter is very slowly passing Saturn. Monday, of course, also marks the winter solstice in the Northern Hemisphere and the summer solstice in the Southern Hemisphere.

If you're tied up Monday, you can keep heading outside through Christmas Eve to marvel at the sight. The planets will remain cozily close through Dec. 24.

At last, 2020 is giving us something positive to look forward to.