Winter Solstice has come and gone, with an extraordinary astronomical event happening to make the day even more notable. To celebrate this, Google featured a cute new Doodle on its website for the Great Conjunction of 2020.
On the first day of winter in 2020, we were fortunate to be witness to the closest great conjunction we've seen in hundreds of years. A 'great conjunction' is when the planets Jupiter and Saturn line up, appearing close together in the sky when seen from Earth. Although these events occur roughly every 20 years, this year's conjunction was extra special.
The last time these two planets appeared closer in our sky was in 1623. Due to their proximity to the Sun (from our perspective), though, the only way that great conjunction would have been visible is through a telescope. The last time the average person would have seen Jupiter and Saturn closer in the sky was back in 1226!
The Great Conjunction of 2020 earned special recognition from Google, in the form of a cute new Doodle.
Look out at the night sky 👀 Seeing double? Don't worry, you're not alone!
For the 1st time in 800 yrs, Jupiter & Saturn will be so visibly close, they'll form a double planet! 🪐🔭
See this once-in-a-lifetime Great Conjunction with #GoogleDoodle → https://t.co/B2fd2qqUac pic.twitter.com/YgPBXmz7Bn
— Google Doodles (@GoogleDoodles) December 21, 2020
The Doodle shows Jupiter and Saturn sharing a friendly high-five as they pass (accompanied by a 'tip of the rings' by Saturn), with Earth looking on in the background.
According to Google: "The two largest planets in our solar system will nearly overlap to form a 'double planet,' an event that hasn't been easily visible since the Middle Ages — almost 800 years ago. Today's animated Doodle celebrates the Northern Hemisphere's first day of winter as well as this rare double planet sighting — or 'Great Conjunction' — which can be viewed from anywhere around the globe!"
"So what exactly is creating this celestial phenomenon? Based on their orbits, from our vantage point on Earth, Jupiter and Saturn will cross within .1 degrees of each other (a fraction of the width of the full moon), a once-in-a-lifetime rendezvous recreated in the Doodle artwork. But looks can be deceiving, as the two gas giants will actually remain a vast distance of approximately 450 million miles apart!"
"Make sure you look out low above the horizon tonight and take in this momentous meet-and-greet between Jupiter and Saturn – it's sure to be out of this world!"
What's really going on out there in space?
Jupiter and Saturn are not actually all that close together right now. Despite appearing to merge in our sky on the Winter Solstice, they are still roughly 730 million kilometres apart from one another at this time. Observing the two from our perspective here on Earth, however, the two happen to be in the exact same part of the sky, so they appear to be right on top of one another.
The positions of Jupiter and Saturn in the solar system relative to Earth on December 21, 2020. Credit: Celestia/Scott Sutherland
Look at the two bright planets through a good pair of binoculars or a backyard telescope, and you will still see their separation in the sky.
You can even spot the four largest of Jupiter's moons — Io, Europa, Ganymede and Callisto — and possibly even Saturn's largest moon, Titan!
This simulated telescopic view of Jupiter and Saturn, an hour after sunset on December 21, 2020, shows the two planets and their largest moons. Credit: Stellarium/Scott Sutherland
With these conjunctions happening every 19.6 years, one might ask: why don't we see Jupiter and Saturn overlapping every time?
The answer lies in how the orbits of the two planets are tilted, compared to Earth's orbit around the Sun. Jupiter's orbit is tilted by a little over 1.3 degrees compared to Earth's, while Saturn's orbit is tilted by nearly 2.5 degrees. Those are small angles, but when you're talking about planets millions of kilometres away, it's enough to ensure that they only line up this well at specific times.
Looking at the solar system edge-on, the inclinations of Jupiter and Saturn's orbits are displayed compared to Earth's orbital plane. As noted in the diagram, the scales have been exaggerated for emphasis. Credit: Scott Sutherland
As shown above, even though the planets line up when viewed from above, if a great conjunction occurs with Jupiter and Saturn towards either the left or right sides of the image, they will appear far apart in our sky. The closer they are to the centre of the image, the closer they will appear in the sky, with the closest conjunctions showing them completely overlapping.
Although the Great Conjunction of 2020 has passed now, that doesn't mean the show has ended. Keep watching Jupiter and Saturn to the southwest, just after sunset, as they slowly draw away from each other in the days ahead!