As the sun sets on Monday evening, New Yorkers should gaze upon the western skies for an astronomical marvel: the Great Conjunction of Jupiter and Saturn, which will appear to be as close together as when Galileo himself first saw them.

Indeed, it’s the closest these two absolute units will appear to have been to each other since 1623, according to astrophysicists.

“No person currently alive has ever seen Jupiter and Saturn, the two largest planets in our solar system ever, this close to one another in the sky," said Dr. Jackie Faherty, astrophysicist at the American Museum of Natural History. "This is rare, given that the last time this was even a possibility for people was 400 years ago, and that wasn't even visible really, because Jupiter and Saturn were too close to the sun for you to see at any given time. You have to go back eight hundred years before anyone alive could have ever seen this."

"The phenomenon is called a 'great conjunction,' when astronomical objects appear to meet up with each other from Earth’s vantage point, because the two planets will be strikingly close to each other in the sky," the museum said in a press release.

The best time to spot the Great Conjunction -- which will appear as a double planet -- will be right at sunset, because that's when the planets are highest in the sky, Faherty said. "They'll come out right at dusk."

The majority of Earthlings should be able to see the two planets, weather permitting. And you won’t even need special equipment to witness the phenomenon. "You can walk outside, you're going to see they're very bright objects,” Faherty said. “They're two of the brightest objects in the sky. Jupiter is the brightest object in the sky right now. Saturn is not far behind."

"And then you'll see the two bright objects that pop out,” she added. “Jupiter will pop out to you first. Saturn will come second because it's almost two magnitudes fainter and the stars will start to appear. But you'll then start to really notice, wow, those are two things that are bright, that are next to each other," she said. The planets will set around 7 p.m.

You should be able to spot it every evening for the next two weeks.

"I completely suggest to people to get outside every night between now and the 21st,” Faherty said. “Then, continue to watch it after Christmas, even until the New Year, because the planets are passing each other from our vantage point in the sky."