"You'd have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky". A conjunction occurs when two planets line up close to one another in the night sky, and these are relatively common occurrences.
The once-in-a-lifetime alignment will be visible just after sunset on 21 December - the winter solstice. Image adapted by Open Source Planet Software Stellarium Graphics.
But an hour after sunset, people looking up at the sky in NY or London will see the planets even closer to the horizon at 7.5 degrees and 5.3 degrees, respectively.
The two planets can be seen in the night sky for almost 60 minutes when the sun goes down in the eve every day.
"On the evening of closest approach on December 21, they will look like a double planet, separated by only 1/5th the diameter of the full moon", said Hartigan.
The pair will be separated by one fifth the diameter of a full moon as viewed from the Earth, in an extraordinary astronomical proximity event which has not occurred for 800 years. "For most telescopes, each planet and their many large moons are visible at a single glance that evening".
If you are watching the night skies right now, you can see Saturn and Jupiter very close to each other in the evening. They will not be as close to Earth's sky until the year 2400.
"An observer is still north, with less time to see the time the planets merge before they sink to the horizon", he said.
Like we've mentioned, you'd be best served catching it this time, because it'll be the last time until 2080 that you can see this.
"By the time skies are fully dark in Houston, for example, the conjunction will be just 9 degrees above the horizon", Hartigan added.
You can see Saturn and Jupiter right now by looking to the south to southwest right after the sun goes down.