Jupiter and Saturn will meet up in tonight's Great Conjunction

Tonight, the two largest planets in our solar system will align in the sky in a special event that many people are referring to as the “Christmas star.” According to NASA, it has been nearly 400 years since Jupiter and Saturn passed this close to each other in the sky, and nearly 800 years since their alignment occurred at night.

The Great Conjunction happens to occur on the winter solstice, the longest night of the year in Earth’s Northern Hemisphere, and will be visible all over the world. 

For the last few months, Jupiter and Saturn have been moving closer together in the night sky, even though they are actually hundreds of millions of miles apart. Tonight, the planets will emerge after sunset so close together that they will appear to some as one remarkably bright object.

“This conjunction is exceptionally rare because of how close the planets will appear to one another,” said Professor Patrick Hartigan of Rice University. “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.”

“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. For most telescope viewers, each planet and several of their largest moons will be visible in the same field of view that evening.”

The best time to witness the conjunction is right after sunset. Professor Hartigan said that the further north you are located, the less time you will have to catch a glimpse of the conjunction before the planets sink below the horizon, where they will be difficult to view. 

With a telescope, you will be able to distinguish between the two planets and their brightest moons, and even get a glimpse of Saturn’s rings. 

“You can imagine the solar system to be a racetrack, with each of the planets as a runner in their own lane and the Earth toward the center of the stadium,” said Henry Throop, an astronomer in the Planetary Science Division at NASA Headquarters in Washington. “From our vantage point, we’ll be able to see Jupiter on the inside lane, approaching Saturn all month and finally overtaking it on December 21.”

“Conjunctions like this could happen on any day of the year, depending on where the planets are in their orbits,” said Throop. “The date of the conjunction is determined by the positions of Jupiter, Saturn, and the Earth in their paths around the Sun, while the date of the solstice is determined by the tilt of Earth’s axis. The solstice is the longest night of the year, so this rare coincidence will give people a great chance to go outside and see the solar system.”

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By Chrissy Sexton, Earth.com Staff Writer