Dec. 21 marks the winter solstice, the shortest day of the year in the northern hemisphere and the start of astronomical winter. This year’s winter solstice will be a bit special, though, with the darkness punctuated by a rare planetary conjunction.

Jupiter and Saturn will rendezvous in the night sky on Monday and may appear joined as one “double planet.” It’s been nearly eight centuries since the pair of planets appeared this close together in the night sky. In 1623, a similar conjunction of the planets occurred, but on the same side of the sky as the sun; that meant the conjunction wasn’t visible from Earth.

That makes Monday night’s show extra special, since we’ll be the first Earthlings to witness it since before the time of Marco Polo.

According to Earth Sky, astronomers aren’t sure if the planetary pairing will appear as one star or just super close together. Of the astronomers it consulted, most leaned toward the planets as appearing distinct. “Part of the fun is to wait and see,” said its lead sky writer Bruce McClure.

The show will be fleeting — lasting only about two hours after sunset. Skywatchers will need to look to their west-southwest as it gets dark, where they’ll see the bright, coupled planets shining through the twilight. If they appear merged to the naked eye, a telescope or binoculars will aid in telling them apart.

NASA writes that they’ll be bright enough to “be seen easily from large cities” and offers tips for photographing the spectacle. For best viewing, Sky and Telescope advises finding a location with an unobstructed view of the southwest sky.

Saturn will appear just to the right of Jupiter. Jupiter will be considerably brighter and somewhat rusty in color, while Saturn, a good deal dimmer, will have a more yellow hue. The two planets have been inching closer to each other since September.

Despite their proximity in the night sky, Jupiter and Saturn will be hundreds of millions of miles away from one another. Yet, with one virtually in front of the other, they will appear only a tenth of a degree apart.

Jupiter, the fifth planet from the sun, is the largest in our solar system. It is the first of the outer planets, a gas giant about 320 times more massive than Earth. Its atmosphere is primarily composed of hydrogen and helium. Giant swirling whirlpools of gas form storms that can last for decades or even centuries.

Saturn, on the other hand, is much smaller. It’s the next planet out from the sun after Jupiter and is a gas giant, too. One year on Saturn is equivalent to 29 years on Earth. Like Jupiter, its atmosphere is mostly made up of hydrogen and helium. Saturn is visually striking and popular with backyard astronomers due to its icy and slightly carbonic rings.

According to NASA, so-called “great conjunctions” of Jupiter and Saturn occur every 20 years or so, but very rarely do the two planets skirt by one another this closely in the night sky. The one in 1623 featured a slightly closer pass than Monday night’s.

If Monday night proves to be cloudy, however, some of us may have a chance to catch the planetary intermingling again during the next conjunction this close, which is slated for March 15, 2080.

After their super close appearance Monday, the pair will begin to separate, remaining close together even around Christmas, but Saturn will appear farther below and to the right of Jupiter each night.

Some have compared Monday night’s spectacle to the “star of Bethlehem” or “Christmas star” which, in the biblical tradition, guided three wise men to Bethlehem.

“The star which they had seen in the East went before them,” reads the translated Bible verse.

In the early 1600s, German astronomer Johannes Kepler calculated that a similar conjunction of Jupiter and Saturn happened shortly before the recognized birth of Jesus. While some state that could have appeared as the fabled star of Bethlehem, modern calculations have proved that theory unlikely.

Meanwhile, Monday also marks the winter solstice, or the shortest day of the year in the northern hemisphere. That’s when the sun’s most direct rays shine far south of the equator. Into the springtime, Earth’s orbit will begin to tilt the northern hemisphere back toward the light.

Jason Samenow contributed to this report.