If you happened to see Jupiter and Saturn pass by the crescent Moon this
week, you'll have noted something else about the two main planets in the Solar System.
They are really really close to each other now, and they will almost appear to collide
to form one super-bright point of light on December 21, 2020, the date of the December solstice.
For the first time since the Middle Ages, Jupiter and Saturn would appear like double planets."
In fact, they won't be near at all of course. Think of the distance from the Sun to the Earth. This is what astronomers call an astronomical unit (au), and this is how distances in the vastness of the Solar System are determined. Jupiter is about 5 AU from us. Saturn is AU 10.
A few weeks ago, these two planets aligned with the Solar System, but on December 21, they will appear aligned with us on Earth. Our line of sight is different because we orbit the Sun rapidly.
The orbital resonance of Jupiter and Saturn is such that they coincide every 19.6 years in a "great conjunction," but they are rarely as similar as on December 21, 2020, when the two planets are separated by less than the apparent diameter of a full Moon, from our point of view on Earth.