WHAT IS IT?
In Astronomy, a conjunction is when Earth and two other celestial objects are close to perfectly aligned, with one of the objects that isn't Earth in the middle. That is, you get Earth--Object A--Object B in a near perfectly straight line. In this particular case Earth, Jupiter, and Saturn are all lining up in that order. Viewed from Earth, a conjunction results in the two other objects appearing closely adjacent to each other in the sky (or overlapping if the conjunction is perfect--this one isn't.) Of course, the two celestial objects aren't actually close to each other in space; Jupiter and Saturn will still be on their respective orbits separated by roughly 400 million miles. They'll just *look* like they are next to each other because they will be lined up with our view from Earth.
WHEN IS IT HAPPENING?
Just after sunset on December 21st. But you can see Jupiter and Saturn very close to each other at night right now. You can watch them get closer and closer each night between now and the 21st. The precise time of conjunction--the exact moment that the two planets are closest to each other--is 1:37:31 PM EST. (Wait... that's during the daytime! Doesn't that mean we will miss it on the East coast? Shhhh.... it'll still be awesome even if we're a few hours late to the party.)
WHAT WILL IT LOOK LIKE?
To the unaided eye, it will look like two really bright stars right next to each other. In fact, if your vision isn't top-notch, it might even look like a single, blob-like bright star. The two planets will appear about 0.1 degree apart, which is about one fifth the diameter of the full moon, or the width of a penny held at arm's length. If you look through a good hobbyist-grade telescope you will be able to see Jupiter and Saturn *right* next to each other in the same field of view. You'll also be able to see Saturn's rings and Jupiter's 4 Galilean moons (you might not see all 4.) You might also see Titan, Saturn's largest moon. Did you know we (mankind) landed a spacecraft on the surface of Titan? How cool is that? Very.
HOW CAN I SEE IT?
You're going to need to look quite low on the horizon almost directly Southwest. Starting looking right after sunset as both planets will set below the horizon by 6:30 PM EST. The planets will be so close to the horizon by the time it gets dark that you are going to need a nearly unobstructed view towards the SW. This means you need to find a hilltop, tall building, or ocean-going boat to have a good view. Note: it is challenging to use a telescope on a boat.
IS IT RARE?
A Jupiter-Saturn conjunction happens about every 20 years. How close the two planets get during this conjunction varies depending on the slight variations in the inclines of the orbits of the three planets. The last time the planets were this close during a conjunction was 1623. The next great conjunction will be on Halloween night of 2040, though the planets won't come quite as close together for that one as they are in 2020.
WHY IS IT CALLED A "GREAT CONJUNCTION?"
Because it is great. But also, any conjunction involving Jupiter and Saturn is called a "great conjunction" because they are the two largest planets. That said a conjunction of Venus & Jupiter can look a little more impressive to the unaided eye. But given the rings and the moons involved a "great conjunction" might be the greatest conjunction visible with a hobbyist-grade telescope.
WHAT IS THE SIGNIFICANCE OF IT HAPPENING ON THE WINTER SOLSTICE?
None. Just coincidence.