If you look to the south anytime after dark this season you’ll see a very bright object in the night sky. It’s Jupiter. Look 5° to its left (so to the south-east) and you’ll see the much dimmer sight of Saturn. These giant planets have been dominating the post-sunset sky for much of 2020, and after reaching their super-bright oppositions (July 14, 2020 for Jupiter and July 20, 2020 for Saturn) they are now past their best.
Both of the Solar System’s giant planets will be gone from the night sky by Christmas, but before they do, they will stage one last dazzling finale in the form of the “Great Conjunction 2020” on Monday, December 21, 2020.
On the the exact date of the winter solstice, Jupiter and Saturn appear closer together in the night sky that and at any point since July 16, 1623.
Back then, the Dutch were busy establishing the New Netherland colony in modern-day New York state and the first English settlement was founded in New Hampshire. It was about 15 years after the telescope was invented, so perhaps someone in Europe got a close-up.
So why am I telling you all of this today? Apart from giving you a chance to mark the date in your calendars, today is a landmark day that will culminate in December’s dazzling sight. Today sees Jupiter and Saturn in a rare position in the Solar System that will set-up the “Great Conjunction 2020.”
Here’s everything you need to know about the “Great Conjunction 2020” and why today is a critical point in making it happen.
What is a conjunction?
A conjunction is simply the sight of two objects appearing to be very close together in the night sky. Almost each month you can see the Moon in conjunction with a planet as it appears to pass close by. Planetary conjunctions are rarer.
So conjunctions are illusions, line-of-sight phenomenons that aren’t actually occurring. After all, the Moon orbits Earth from an average of 238,900 miles out and only appears to be close to the planets, which are obviously so much further away (for example, Mars is presently about 44 million miles from Earth).
When is the ‘Great Conjunction 2020?’
This “Great Conjunction 2020” will occur on Monday, December 21, 2020 when Jupiter and Saturn appear in our post-sunset night sky just 0.06º apart. That’s about the same as the Venus-Jupiter conjunction of August 27, 2016, so this is effectively the best planetary conjunction for over four years.
During the event, Jupiter and Saturn will appear to shine in the southwest during twilight as one bright point of light.
Although that specific date of December 21, 2020 will see them appear super-close, in practice you should take advantage of any clear skies you get for a few evenings either side.
What causes the ‘Great Conjunction 2020?’
The varying orbits of the planets. While Saturn takes 29 Earth-years to make one full orbit of the Sun, Jupiter takes 12 Earth-years. The maths is simple; every 19 years they will occupy the same part of the night sky as seen from Earth, which is zipping around the Sun far quicker.
Before all that happens the two planets are in a special position on November 2, 2020.
Why is November 2, 2020 so important?
November 2, 2020 sees Jupiter and Saturn locked in what astronomers call a heliocentric conjunction—the point when the two planets are aligned with the Sun. You can draw a straight line through Saturn, Jupiter and the Sun. Go over to The Planets Today to have a look at a simulated real-time view of the Solar System from above. Astronomers also call this a syzergy.
Even before November 2, 2020 the “Great Conjunction 2020” was already underway from our point of view on a rapidly orbiting Earth. Back in mid-September, after a few weeks of retrograde motion, Jupiter started heading east again to begin closing in on Saturn.
It’s now getting closer each night and you can watch the Solar System’s biggest planet catch-up on slower-moving Saturn over the next few months.
How to find Jupiter and Saturn in the night sky
Look southwest to find Jupiter—it’s shining at a magnitude of -2.2 and will be easy to spot (though don’t confuse it with reddish Mars, which is exactly the same brightness, in the eastern sky). Dimmer Saturn, to Jupiter’s left or further east, is a little more difficult. The two planets are tonight about 5º apart. There’s an easy way to make sure you’re looking at Saturn; stretch your arm out, hold up your three middle fingers together, and put your right-most finger on Jupiter. At the other end will be Saturn.
The closest pairing Jupiter and Saturn since 1623, there isn’t another conjunction of the pair until October 31, 2040 when the two will past slightly further apart in the pre-dawn night sky.
That makes the once-in-a-generation “Great Conjunction 2020” coming up on December 21, 2020 worth your attention. Mark your calendars!
Wishing you clear skies and wide eyes.