Jupiter and Saturn will come within 01 degrees of each other, forming the first visible "double planet" in 800 years

What is a great conjunction? 

In 1610, Italian astronomer Galileo Galilei discovered both the four moons of Jupiter — Io, Europa, Ganymede, and Callisto — and the rings of Saturn. Not long after, in 1623, the two planets were spotted aligning for the first time.

The conjunction is sometimes nicknamed the "Christmas Star" or "Star of Bethlehem" for its connection to the Christian nativity story. In the story, the star guides the wise men to the home of Jesus, which many Christians believe was a miracle. Astronomers have attempted to link the star to several rare celestial events — including a comet, a supernova and a conjunction.

Aligning with the winter solstice on December 21, 2020, the two planets will be just 0.1 degrees apart — less than the diameter of a full moon, EarthSky said. The word "conjunction" is used by astronomers to describe the meeting of objects in our night sky, and the great conjunction occurs between the two largest planets in our solar system: Jupiter and Saturn.

The planets will be so close, they will appear, from some perspectives, to overlap completely, creating a rare "double planet" effect. So close, that a "pinkie finger at arm's length will easily cover both planets in the sky," NASA said.

However, while they may appear from Earth to be very, very close, in reality, they are still hundreds of millions of miles apart.

How to watch the great conjunction

During the last great conjunction in 2000, Jupiter and Saturn were so close to the sun that the event was difficult to observe. But skywatchers should have a clearer view of the celestial event this time around. The great conjunction will be shining bright shortly after sunset, low in the southwestern sky, as viewed from the Northern hemisphere, NASA said.

Through the entirety of December, skywatchers will easily be able to spot the two planets with the naked eye. You can look up each evening to watch them get closer and closer in the sky — they are so bright, they are even visible from most cities.

Jupiter currently appears brighter than any star in the sky. Saturn is slightly dimmer, but still just as bright as the brightest stars, with a recognizable golden glow.

Saturn will appear slightly above and to the left of Jupiter, and will even look as close to the planet as some of its own moons, visible with binoculars or a telescope. Unlike stars, which twinkle, both planets will hold consistent brightness, easy to find on clear nights.

"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. "From our vantage point, we'll be able to be to see Jupiter on the inside lane, approaching Saturn all month and finally overtaking it on December 21."

The event is observable from anywhere on Earth, provided the sky is clear. "The further north a viewer is, the less time they'll have to catch a glimpse of the conjunction before the planets sink below the horizon," Hartigan said.

The planets will appear extremely close for about a month, giving skywatchers plenty of time to witness the spectacular alignment throughout the holiday season. The event coincidentally aligns with the December solstice, marking the shortest day of the year in the Northern hemisphere.

"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."

To learn more about when and where to look up to see the conjunction, Throop will be live on NASA's website to answer questions on Thursday afternoon.

This will be the "greatest" great conjunction for the next 60 years, until 2080. Hartigan said that, following that conjunction, the duo won't make such a close approach until sometime after the year 2400.