'Christmas Star' will be closest visible conjunction of Jupiter and Saturn in 800 years

The conjunction of the planetary giants will look like one large star on the winter solstice, Dec. 21, 2020, in the southwest sky.

The galactic core area of the Milky Way over Maskinonge Pond in Waterton Lakes National Park, Alberta on July 14, 2020. Jupiter is the bright object at left, with Saturn dimmer to the left (east) of Jupiter.Alan Dyer / Universal Images Group via Getty Images file

The Great Conjunction of 2020 will brighten the darkest day of the year as the two giant planets of our solar system draw closer together in the night sky than they have been in centuries.

By chance, the day that Jupiter and Saturn will appear closest for Earth-based stargazers is Dec. 21, the winter solstice, which is the longest night of the year in the northern hemisphere.

The double planet view is also known by some astronomers as the "Christmas Star" because of a belief that the biblical tale of the Star of Bethlehem could have been a planetary conjunction. Although around two thousand years ago, Venus and Jupiter were closest — not Jupiter and Saturn, as is the case for the "Christmas Star" of 2020.

The last time the two planets were so close was 1623, but stargazing conditions at the time meant the astronomical event likely was not seen by earthlings. The last time such a close pairing was observable to the naked eye was in 1226, according to EarthSky.

This event is called a "great conjunction." These occur every 20 years this century as the orbits of Earth, Jupiter, and Saturn periodically align making these two outer planets appear close together in our nighttime sky.NASA/JPL-Caltech

On the evenings of December 15 through 18, stargazers can easily find Jupiter and Saturn moving in conjunction by looking toward the waxing crescent moon in the western sky 45 minutes after sunset, according to NASA's Night Sky Network.

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"Keep in mind that while the two gas giants may appear close, in reality they are hundreds of millions of miles apart," NASA writes."This will still be quite a striking sight, but you will need to look fast as both planets will set shortly after sunset."

On the solstice night, Dec. 21, the moon will be higher in the sky, but Jupiter and Saturn will remain closer to the horizon in the western sky and might look like one large star. From an amateur telescope, however, a stargazer might be able to clearly see both planets and some of their moons within one frame of view.

"I have been watching them daily," Pat Hartigan, an astrophysicist at Rice University, wrote in an email.

"Find a spot with a good horizon where you'd be able to see the sunset, and in that same general area of the sky you'll see Jupiter once it starts to get dark, and then a bit later, Saturn on the left now, getting closer to Jupiter every day."

Hartigan's "main takeaway," as published on his research site, is that the actual conjunction on the winter solstice is "a rare event definitely worth seeing, but it will be a challenge to observe from some locations."

The challenge is that the farther north a viewer is, the harder it is to see the planets on the winter solstice. On December 21, from New York City for instance, Jupiter and Saturn will be extremely close to the horizon, possibly obstructed by geography or blotted out by a bright sunset.

From Hartigan's home in Houston, he writes, the view of the two planets will be easier because they will be further above the horizon.

But there is still much to see in the days weeks leading to the conjunction, Hartigan writes.

"Near December 21, Jupiter and Saturn will appear like a binary planet in the dusk sky," he said. "However, they will not suddenly look like a brilliant 'Christmas Star.'"