• Jupiter and Saturn will appear so close together that they will look like a single star during the "Great Conjunction" Monday
  • The gas giants have not been this close to one another in the sky in almost 400 years ago
  • This year marks the first time in almost 800 years since the alignment of Jupiter and Saturn occurred at night

Space enthusiasts will be treated to a rare cosmic event this holiday. Just a few days before Christmas, Jupiter and Saturn will appear so close together that they will look almost like a single star in the evening sky in an event known as a “Great Conjunction.”

Jupiter’s and Saturn’s 12- and 29-year orbits bring them together in what has become known as the "Christmas Star," making them appear brighter than they have in hundreds of years. The conjunction culminates on Monday night, according to NASA.

Jupiter and Saturn, which are about 400 million miles apart, regularly appear to pass each other in the solar system, with the positions of the two planets being aligned in the sky about once every 20 years. The last Great Conjunction was in May 2000, but it was difficult to see due to its position in the sky.

But what makes the Great Conjunction this year extra special is the fact that Jupiter and Saturn have not been this close to one another in the sky since 1623 — almost 400 years ago. This year would also mark the first time in almost 800 years since the alignment of the two gas giants occurred at night.

“You’d have to go all the way back to just before dawn on 4 March 1226 to see a closer alignment between these objects visible in the night sky,” said Patrick Hartigan, an astronomer from Rice University in Texas.

Those who miss out on the Dec. 21 event will have to wait until 2080 to witness another Great Conjunction that brings Jupiter and Saturn this close together, NBC Los Angeles noted.

For viewers who want to see the Christmas Star, it will appear low in the western sky just after sunset, NASA said. According to Hartigan, those near the equator would have the best viewing conditions, but Jupiter and Saturn's Great Conjunction will be visible from anywhere on Earth if the weather permits.

Since the two planets will be so close together, one should be able to see them in the same view when using a telescope or a pair of binoculars. Telescopes that are powerful enough would even be able to see the major moons of both of the planets. The moons will look like tiny dots of light along a line that cuts through the planet.

As for why the conjunction is occurring on the same day as the winter solstice, astronomers said the timing is merely a coincidence.

“Conjunctions like this could happen on any day of the year, depending on where the planets are in their orbits,” said Henry Throop, an astronomer in the Planetary Science Division at NASA Headquarters in Washington.

He continued, “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.”

NASA Solar System Montage - planets, comet, asteroid
This is an artist conception of a solar-system montage of the eight planets, a comet and an asteroid. NASA/JPL