Omitting the orbital technicalities by a great deal, a conjunction between Jupiter and Saturn (A Great Conjunction) happens steadily over history, and humans have been keeping track of them since at least the time of Kepler. From the perspective of the sun, Jupiter takes 11.9 earth years to complete one orbit, and Saturn 29.5 years. This means that they line up roughly every
(29.5*11.9)/(29.5-11.9) ≈ 19.9 years.
Conjunction has to be able to be observed on earth to count, and this complicates matter a bit further, causing the exact moment of each conjunction event to vary by up to a few months.
From the perspective outside the solar system, we see that, when the two outer giant planets seem close together on earth, earth itself, jupiter, and saturn, almost coincide on a straight line, as illustrated below.
In reality, this line almost always point near the sun, and hence the two exterior planets are rarely well-separated enough from the sun to allow for a good observation session on earth. For example, the last Jupiter-Saturn conjunction was May 28, 2000, but this happened behind the sun. Last time Jupiter and Saturn appeared this close was July 16, 1623 — this conjunction in the lifetime of Galileo also took place behind the sun, and hence was unlikely to have been studied. In short, the previous comparable event to the one tomorrow dates all the way back to March 4, 1226 .
For people with a telescope, the rings of Saturn and the moons of Jupiter can appear in your view at the same moment, creating a sensation that hundreds of millions of kilometres of separation giving way to celestial coincidence.
WHERE TO LOOK?
Below are some observation tips from NASA:
Find a spot with an unobstructed view of the sky, such as a field or park. Jupiter and Saturn are bright, so they can be seen even from most cities.
An hour after sunset, look to the southwestern sky. Jupiter will look like a bright star and be easily visible. Saturn will be slightly fainter and will appear slightly above and to the left of Jupiter until December 21, when Jupiter will overtake it and they will reverse positions in the sky.
The planets can be seen with the unaided eye, but if you have binoculars or a small telescope, you may be able to see Jupiter’s four large moons orbiting the giant planet.
And here are some preliminary results that I rendered / photographed in the night of December 20, 2020.
(Realize that back then nobody on earth was using the modern Gregorian calendar, so pinpointing the exact date is as much a historical venture as it is astrophysical.)
And happy Solstice to my readers around the world. You probably won’t hear from me until 2021, so all the best in the new year. The hopes and dreams of humanity won’t just be put off by nature like this, I am fully convinced.