On Monday night, the sky will offer a sight that hasn't been seen since the Middle Ages and may have inspired one of the Bible's most famous stories.
The two largest planets in the solar system, Jupiter and Saturn, will be so close to one another in the sky that they will appear to be fused into a single point of light.
The last time this is believed to have been witnessed was in the year 1226, according to Michael Shanahan, the director of the Liberty Science Center Planetarium in New Jersey.
"The interesting thing about these long cycles in astronomy is that they come back at very different epochs of human history," Shanahan told TODAY. "The event that happened in the Middle Ages in 1226 occurred before dawn, so there was about an hour and a half before the sun rose to see it."
The last time astronomers believe it was possibly visible was in 1623, but it occurred right at sunset and Shanahan said there is no record of anyone having noticed it because the two planets were lost in the light of the setting sun.
There also has been speculation that the conjunction of the planets formed the "Christmas Star" or Star of Bethlehem that the three wise men in the Nativity story in the Bible were thought to have seen that inspired them to ultimately travel to Bethlehem for the birth of Jesus.
"One possibility is that these two planets did join together in 7 B.C., about a year before the earliest possible time of the birth of Jesus, so that it could've been a conjunction of the two planets," Shanahan said. "If the wise men were, as we think, in fact astrologers, that could've been a thing they saw in 7 B.C. and said, 'Oh, there's a big event happening, let's go to Bethlehem and check it out."
Alignments between Jupiter and Saturn occur about every 20 years, but normally they are no closer than the width of two full moons. On Dec. 21, they will appear — emphasis on appear, as the planets are still millions of miles away from each other — just a tenth of a degree apart, which about the thickness of a dime held at arm's length, according to NASA.
The two planets will appear close in the sky for this whole month, which people can see by looking southwest just after sunset, NASA astronomer Henry Throop told TODAY in an email.
"For people who see it a few times during the period, it is a great chance to see the motions of the planets," Throop said. "You can imagine Jupiter and Saturn as runners on a track. Jupiter is moving faster, and looking from one night to the next, people will be able to see Jupiter approaching and passing Saturn in their paths around the sun."
Shanahan had some tips for the best way to view the rare planetary alignment on Dec. 21, which is the winter solstice, meaning it's the shortest day of the year in terms of sunlight in the Northern Hemisphere.
Make sure you have a view to the southwest unobstructed by trees or buildings for a little over an hour after sunset. Viewers will also have to hope for clear weather on that night as well.
You don't want to miss it, as Jupiter and Saturn will not appear this close in the sky again until 2080, according to NASA.