The Great Conjunction

If you’re a regular skywatcher you might already know the two biggest planets in our solar system – Jupiter and Saturn – have occupied the same portion of our cosmic ceiling for most of the year. In fact, in the preceding months, the two have crept closer and closer toward each other.

On 21 December, we are in for a rare treat. On that date, Jupiter and Saturn will reach the closest point in their orbits around the sun from our perspective. This point is known as the Great Conjunction. In general, a conjunction is the point at which any two planets appear closest in the skies. We call this specific one great because it features the two largest planets and because it is the one that happens the least often, among the planets visible to the naked eye.

The Great Conjunction happens every 20 years, but this year’s version is super-duper special. Because of the exact natures of orbits around the sun, including the Earth’s, the conjunctions are never quite the same. Sometimes they are still fairly far apart in the sky; other times they might appear as if they could collide. 2020’s Great Conjunction will be the closest Jupiter and Saturn have appeared to us in 400 years! To make it even better, it’s been 800 years since a Great Conjunction of this level happened at night! The one in 1623 largely occurred during the day, disappointing nascent telescope operators.

NASA astronomer Henry Throop provided a wonderful way to visualize the physics behind the Great Conjunction: “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.”

The Great Conjunction illustrates well the idea of perspective. To us, the planets look very close right now, but, in reality, they are 456 million miles apart. Saturn is almost twice as far from the sun as Jupiter! Because of these distance differences, the giants have varying year lengths. It takes Saturn 30 Earth years to orbit the sun; on Jupiter, one lap equals an Earth annum times 13. So, essentially, Jupiter laps Saturn every 20 years thanks to its inside-lane advantage.

To illustrate how much closer the planets are this year than in the past, we need only peer back to the previous Great Conjunction, which happened in 2000. That year, Jupiter and Saturn were within 69 arcminutes of each other – twice the diameter of the moon. Relatively close, but nothing compared to this edition. On 21 December, the mammoths will be a mere 6.1 arcminutes apart. Hold a dime or your pinky at arm’s length and you can just slide them in between the planets.

When the Great Conjunction happens this time of year, astronomers dub it the “Christmas Star” because the two planets might seem to be just one bright body. In 1603, the great astronomer Johannes Kepler speculated the Star of Bethlehem from the Bible might actually have been a Great Conjunction. He calculated a “triple conjunction” occurred in 7 BC. A triple conjunction is when Jupiter and Saturn are in conjunction at or near their opposition from the sun. Because of the orbital physics in this situation, the planets actually appear in the same spot three different times within months, hence a triple conjunction. A lot of debate surrounds the exact and proper dating of events at the dawn of the common era, so it is possible Kepler’s 7 BC conjunction lined up with the events in the Bible.

Jupiter will be brighter than any star in the sky during the conjunction. Saturn is not quite as shiny, but it’s still a bright light. They shine at such a magnitude that even city-dwellers should have no problem catching the planets in the night sky. Here in Columbus, I have been able to clearly see both Jupiter and Saturn for many months unaided. The planets are so close during this conjunction, in fact, that a gazer with a telescope will be able to see them at the same time. Zoom in enough and you should see Saturn’s rings and Jupiter’s moons simultaneously!

So you’d like to take a gander at the Great Conjunction? Here’s how to see with your own eyes. An hour after sunset, go to a spot where you can see the horizon well. Turn to the southwestern part of the cosmos. Until the 21st, Saturn will be slightly above and to the left of Jupiter. After Jupiter laps Saturn, their positions will swap. Jupiter will be so bright it will be hard to miss. If you want a better peek, take binoculars and you might be able to see the four big moons of Jupiter!

If you are cursed with opaque cloud cover during the Great Conjuntion or cannot make it outside when it happens, several astronomical organizations will livestream the event. The upshot of watching this way is fantastic magnification you probably cannot produce on your own. In the Further Reading and Exploration section below, I will link a few opportunities to virtually view the conjunction.

The next conjunction in 2040 will not be as spectacular as this year’s version, but it will happen at the end of October, perhaps dubbed the Halloween Star. The next time a Great Conjunction will be as close as this year will be relatively soon on the cosmic scale in 2080. After that, if you want to witness one this close you better hope we have perfected everlasting life, as it won’t occur until 2417.

It should be quite a treat in the sky, even if you can’t see Uranus!