The once-in-a-lifetime sight is the product of an astronomical event known as a “conjunction,” in which two objects line up with each other in the sky. When it involves Jupiter and Saturn catching up to each other, it’s sometimes called a “great conjunction.”

“You can actually see it with your own eye. It doesn’t have to be measured with sophisticated instruments,” Michael Brown, an astronomer at Monash University in Australia, told The Washington Post. “The two objects are appearing very close in the sky, but ultimately they’re very far away from each other.”

While Jupiter and Saturn will be separated on Dec. 21 by 0.1 degrees, or less than a third of the moon’s width, the two planets will nonetheless remain separated by about 450 million miles in space, he said.

Emily Lakdawalla, a freelance space writer, said planetary orbits can be compared to a kind of running track, with the sun in the middle. If Jupiter is running in circles closer to the inside, Saturn is walking at a slower pace farther out.

“Jupiter is lapping Saturn,” she said.

Given the pace of their orbits — Jupiter takes about 12 Earth years to circle the sun compared to Saturn’s 30 — the two actually align in their paths roughly every two decades.

But there’s a catch: Because each track has a slightly different tilt, very close conjunctions like the one set for later this month are rare. The last time Saturn and Jupiter were close enough to create a “double planet” seen from Earth was in March 1226, Brown said.

The two planets came equally close in 1623, but that phenomenon was impossible to see from Earth because of glare from the sun, he added. So the conjunction later this month will be an extraordinarily rare event.

Since the summer, Jupiter and Saturn have been getting closer to one another, often visible at dusk, low in the western sky. Right around the solstice, they may appear as one overlapping body above the horizon.

Luckily, Earth will not need to await another eight centuries to view another “double planet.” Given the tilts of each orbit, the next conjunction will be visible in 2080, according to projections from Rice University astronomer Patrick Hartigan.

But for many, this year will mark their first and only opportunity to get a glimpse of the two planets seemingly merging in the sky. Alan Duffy, the lead scientist at the Royal Institution of Australia, recommends that stargazers hoping to get a clear view of both Jupiter and Saturn might want to look west before then.

“For those who are in the business of taking pretty pictures, you probably want to snap them a little earlier,” he said. Duffy offered instructions to look for two dots that, unlike stars, do not twinkle: a pure white dot for Jupiter and one with more of a golden hue for Saturn.

The meaning behind that image, however, may be more difficult to visualize.

“These sorts of alignments are all reminders that we are all sitting on very large spheres of gas and rocks that are rolling around the solar system in a very regular manner,” he said.