At least one good thing came out of 2020—a great conjunction appearing tonight in the south-west sky 45 minutes after sunset. At that time, Jupiter and Saturn will be only one 10th of one degree apart as viewed from the Earth. This is so close that, unless you have excellent eyesight indeed, they will appear to be right on top of each other, giving the impression of a very bright star (except that, unlike a star, it will be a steady, non-twinkling light source). In fact, you should be able to cover both Jupiter and Saturn at the same time with your baby finger extended at arm’s length. While it is not known for sure, it is speculated that the Star of Bethlehem, sometimes called the Star of the Magi, was an extremely close conjunction of Venus and Jupiter on June 17 in the year 2 BC.

Of course, Jupiter and Saturn will not really be close to each other this evening. The distance between the two of them is still hundreds of millions of miles. However, when two objects appear in the night sky very close to each other as seen from the Earth, whether it be two stars or two planets, astronomers call it a conjunction. When the two largest planets in our solar system, Jupiter and Saturn, are closely lined up, this is called a ‘great conjunction.’

Henry Throop, an astronomer in the Planetary Science Division at NASA Headquarters in Washington, D.C., explains the phenomenon well:

“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. 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.”

Jupiter, being far closer to the Sun than Saturn, moves considerably faster than Saturn and completes one revolution around the Sun in 12 Earth years, while Saturn takes 30 years. This means that approximately every 20 years, Jupiter and Saturn appear very close in the sky, with Jupiter racing past Saturn so quickly that, even looking through a small telescope, you can see the change from night-to-night.

If a great conjunction occurs once every generation, what makes this year’s spectacle so rare? It is because Jupiter and Saturn will be closer together, as seen from the Earth, than during any other great conjunction in almost 400 years.

The year was 1623, the year Shakespeare’s collected works were first published when a great conjunction appeared as close as that of 2020. It is possible that Galileo watched the 1623 great conjunction through his telescope since he first turned his scope to the night sky only 13 years earlier, on January 7, 1610. However, it is hard to say since the two planets were quite close to the Sun then, and so they may have already set by the time it was fully dark. 

1623’s great conjunction was also during the Little Ice Age, which was much colder than today and, also, according to some experts, cloudier as well (almost certainly due to a weak Sun, which for various reasons, was likely a contributor to the cold conditions). We don’t know for sure, but there’s no record of anyone observing the 1623 conjunction through a telescope. Regardless, my wife Laurie and I were in the Galileo Museum in Florence, Italy, 13 months ago. I took the following photograph of the telescope he used in 1610 for his first views of the four large moons (later dubbed the Galilean moons) revolving around Jupiter.

Galileo’s observation of the moons revolving around Jupiter night after night allowed him to conclude that, contrary to the fashionable thought of his day, the Earth was not the center of everything. Like today’s politically-correct but scientifically-flawed views concerning human-caused climate change, Galileo was stepping into very dangerous waters by bringing this up in public. Ultimately, his publicizing of the mistakes of his day’s ‘establishment’ led to him spending the rest of his life under house arrest, something that may very well be about to happen to climate change skeptics if politicians like U.S. Senator (D-RI) Sheldon Whitehouse get their way (see here). 

I should mention that the Galileo museum is magnificent, by the way, probably the most exciting place we visited while in Italy. You can check it out here. With Galileo’s finger preserved in a case there (I am not kidding) and the original book, Dialogue Concerning the Two Chief World Systems (the book that got him into so much trouble, below), right there too, it was somewhat eerie, almost as if Galileo was still alive and looking over our shoulders. The museum shows what an incredible Renaissance Man he was, an expert in so many fields.

Besides Galileo’s discovery of the Galilean moons, he also saw that Saturn had an elongation on its equator and famously said that the planet has ears. It was not until many years later that astronomers finally realized that billions of particles were making up an elaborate ring system. 

Impressive though ‘the closest great conjunction in 400 years’ is, this year’s event is even rarer since the last time a great conjunction was clearly visible in a fully dark sky was on March 4, 1226. This was during the Medieval Warm Period when it was about as warm as today and likely less cloudy since the Sun would have been more active (displacing cloud-forming galactic cosmic rays from deep space). 

Side note: Later, in 1226, Genghis Khan suddenly decided to end his military actions in west China and withdraw to Mongolia (Khan died in 1227 before reaching his homeland (Mongolia)). The paper mentions that his decision to end his military operations in China was “probably based on the astrological interpretations of an unusual configuration of the planets.” There were apparently five conjunctions 1226-1227.

Back to Monday’s event (and, yes, the nights just before and after Monday are almost as good), which you will need a horizon clear of trees and buildings to see as it is quite low in the sky. At their closest approach (from our point of view, that is, the planets are actually about 450 million miles apart), the two planets will apparently fit within a disc the size of a full moon. This means that through a small telescope (say, an eight-inch diameter Celestron C8, which I use), Jupiter and Saturn will appear as follows, at moderate magnification:

An illustration of how Jupiter and Saturn will appear on 21 December 2020 through a small telescope. Jupiter is lower left, and Saturn is in the upper right.  Credit: Stuart Atkinson.

To the naked eye, this conjunction will look like a very tight double star or planet (as planets don’t twinkle as do stars), or, for some of us older folks, a slightly elongated star. It is a good test of eyesight to see if you can resolve the two planets as two distinct light sources. To get some idea of how hard this would be, consider that the ‘star’ second to the end of the handle of the Big Dipper is actually a double-star—Mizar and Alcor (horse and rider)—with twice the apparent distance between them as are Jupiter and Saturn in tonight’s great conjunction.

If you can see Mizar and Alcor as two independent light sources (indeed, separating that double has been a test for eyesight in many cultures for millennia), then you are halfway towards being able to see tonight’s great conjunction as two distinct light sources. Let me know in the comments section below if you can resolve Mizar and Alcor to be a double star (through a scope, you can easily see Mizar is itself a double) and whether you can see Jupiter and Saturn as two separate light sources. You can learn more about the event here.

Let’s hope the December 21, 2020, planetary spectacular, the first of its kind in almost 800 years, is a good omen for a far better 2021.