Stargazers are in for a treat this December as celestial gas giants Saturn and Jupiter are having a festive get-together, the closest they’ve been for 800 years. The planets will reach their once-every-20-years conjunction on December 21, which is not only the winter solstice, but their closest alignment since 1226.
Throughout 2020, Saturn and Jupiter have been visible close together in the night sky, but this conjunction, known as a "great conjunction" because it involves the two largest bodies in our Solar System, will appear less than the diameter of a Full Moon apart, easily fitting into the field view of most small telescopes.
“Alignments between these two planets are rather rare, occurring once every 20 years or so, but this conjunction is exceptionally rare because of how close the planets will appear to one another,” said astronomer Patrick Hartigan from Rice University. “You’d have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky.”
During the week of December 16-25, the planets will appear one-fifth the diameter of the Moon, just 0.1 degrees apart, and on their closest approach on December 21, they will be hard to distinguish apart. This is a socially distanced get-together, of course, as the planets are still hundreds of millions of kilometers from each other, but will appear in the night sky as a bright single point of light.
“On the evening of closest approach on Dec 21 they will look like a double planet, separated by only 1/5th the diameter of the full moon,” said Professor Hartigan. “For most telescope viewers, each planet and several of their largest moons will be visible in the same field of view that evening.”
A "double planet" is a binary system where both objects are of planetary mass. Pluto and it's moon Charon is our Solar System's only known double planet system because Charon is almost half the size of the dwarf planet.
The gas giants align closely every 19.6 years due to Jupiter's 11.8-year orbit and Saturn's 29.5-year orbit bringing them together in our field of view from Earth. As Professor Hartigan mentioned, the last time they were this close was in the Middle Ages, and they won't be this close again until March 15, 2080. In fact, according to Hartigan, in the 3,000 years from 0 CE to 3000 CE, only seven great conjunctions were or will be closer than this one, and two of those were invisible due to being too close to the Sun.
During this conjunction in December, Jupiter will be the brighter of the two, but the Sun may make viewing difficult in some locations. The closer to the equator, the better the view. The further north you are, the briefer the window for viewing the alignment before the planets sink below the horizon. The best time to see them will be with a telescope pointing west about an hour after sunset, but the planets will be so bright they will be visible at twilight, and if the skies are clear they will be visible from anywhere on Earth. You can use sites like Stellarium to figure out where to look from where you are.
This year may not have panned out how anyone wanted but this celestial show is a fitting reminder as the year ends to go outside and look up and remember how fleeting this moment in time really is.