The star of Bethlehem will grace the night sky on Dec. 21, the day that marks the beginning of the winter solstice, in a rare celestial event that hasn't been seen in 800 years.

The two gas giants, Jupiter and Saturn, will align with each other and will look so close together that they will appear as a double planet, according to Rice University.

This gives rise to the star of Bethlehem, which is also known as the Christmas star.

Alignments between the two planets usually happen every 20 years or so. However, their alignment four days before Christmas this year will make them look very close to one another, making the celestial event "exceptionally rare," Patrick Hartigan, a physics and astronomy professor at 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," he said.

Although the gas giants will not actually be close to each other, "they will look like a double planet, separated by only 1/5th the diameter of the full moon," Hartigan explained.

He also said that those who will view the alignment on telescopes will be able to see not just the two planets but also several of their biggest moons.

The event can be seen anywhere on the Earth even with the naked eye, but it can best be viewed in areas near the equator about one hour after sunset. Those who live further north will have a shorter time to see the Christmas star, as it will be situated low in the western sky.

"By the time skies are fully dark in Houston, for example, the conjunction will be just 9 degrees above the horizon," Hartigan said. "Viewing that would be manageable if the weather cooperates and you have an unobstructed view to the southwest."

Jupiter and Saturn will not appear this close again until March 15, 2080, after which the star of Bethlehem will not be seen until 2400.

With the coming appearance of this rare event just a few days before Christmas, people are wondering if the star of Bethlehem is the same "star" that the Magi saw during the birth of Christ. The answer is no.

Some historians say Jesus was born on 7 B.C. At that time, a triple astronomical conjunction occurred in which Jupiter and Saturn aligned and separated three times, but not as close as they would be in the coming days. This triple conjunction was probably what the Magi saw, according to an article from The Independent.

The wise men considered Jupiter as the "king of the planets" and Saturn as the "planet of destiny." The event happened with the constellation Pisces, which the Magi connected with the Jews. These signs in the sky led them to believe that a King had been born to the Jewish people.   

Others say that the birth of Christ happened on 3 B.C. According to historians, five astronomical conjunctions happened from August of 3 B.C. to June of 2 B.C. involving the constellation Leo, the planet Jupiter, the planet Venus, and a star called Regulus, a report from CBN News said.

To the Magi, the constellation of Leo the Lion was symbolic of Israel, Venus represented motherhood, Jupiter represented fatherhood, and the Regulus star was symbolic of royalty. Thus, the Magi deduced that a King had been born in Israel.