Adam Goriparthi
In a celestial phenomenon termed ‘The Great Conjunction’, Jupiter and Saturn are set to align just in time for Christmas – appearing so close that they look like they have become ‘one’. The celestial giants will be particularly close together this month in a spectacle that will be… out of this world.
The astronomical community is excited; the meeting of Jupiter and Saturn is promised to be a most incredible event. The Great Conjunction just so happens to align with the Winter Solstice this year and will be easiest to see on 21st December, shortly after sunset. But what exactly is The Great Conjunction?
During the closest approach, both planets will appear to be a binary object in the sky
What is the Great Conjunction?
A conjunction is when two celestial objects line up in the sky. It is often defined as the moment of minimum separation between two objects when viewed from the Earth. A conjunction of Jupiter and Saturn, which only happens about once every 20 years, is called a Great Conjunction. In 2020, the Great Conjunction occurs on December 21st at around 18:20 GMT.
During the event, Jupiter and Saturn sit just 0.1° apart (1/5th of the width of the Moon). This rare event means they will fit into a single telescopic field. It is important to understand that, while the two giants appear separated by less than a single degree, Saturn is still more than 730 million kilometres away from Jupiter – so they’re really not near each other at all!
Since September 2020, Jupiter has been moving closer and closer to Saturn in the early evening sky. Between November and December, the planets moved from 5° to 2° apart and continued moving closer together. The closest meeting on 21 December at 18:22 GMT is when Jupiter will be just 0.1° south of Saturn (the closest separation between the two planets since 1623). This means both planets will be visible in the same telescopic field of view (though they will be distinguishable from each other with the naked eye). During the closest approach, both planets will appear to be a binary object in the sky.
Some historians and astronomers have theorized that the original Christmas star, might also have been a Great Conjunction
The solar system is the shape of a thin disk (ecliptic): the Earth, the Moon, and the planets orbit the Sun in roughly the same plane. However, Jupiter and Saturn are much further away than the other planets observable to the naked eye. This means they move more slowly – a celestial object’s orbital speed decreases with distance from the sun.
The Earth takes 1 year to complete one orbit of the Sun, but Jupiter takes 12 (11.86) years, and Saturn takes 30 (29.46) years. As a result of their long orbits, Jupiter and Saturn meet in the sky only once every 20 (19.86) years. So, 20 years is the time it takes Jupiter to overtake Saturn again as they circle the Sun, and is why a Great Conjunction happens every 20 years.
‘Christmas Star’
Astronomically, a Great Conjunction happening precisely on the Winter Solstice is a rare and astonishing phenomenon. Since it’s the longest night of the year, it essentially provides the best time to observe such an occurrence. Furthermore, as Jupiter and Saturn will meet just a few days before Christmas and form what will appear as a single bright object in the sky, the 2020 Great Conjunction is sometimes also called the ‘Christmas star’ or ‘Bethlehem star’.
These events are a nice connection between generations
According to Christianity, it was a bright light in the sky that led the Three Wise Men to the location of Jesus’s birth. In fact, some historians and astronomers have theorized that the original Christmas star, known as the Star of Bethlehem, might also have been a Great Conjunction (or even a rare triple conjunction consisting of Jupiter, Saturn, and Venus too).
Tonight:
— NASA (@NASA) December 21, 2020
? Find a spot with an unobstructed view of the sky
? Look southwest an hour after sunset
? Spot the "Great Conjunction" of Jupiter and Saturn!
Why the two planets appear so close together: https://t.co/mX8x8YIlye pic.twitter.com/BMV3uMlJ6p
Viewing the phenomenon
Some important viewing tips: make sure buildings, trees, and hills won’t block your view of these planets, which will be low in the southwest. Though they will almost merge into ‘one star’, a way to differentiate the gas giants is that Jupiter will appear brighter. This is because it is bigger and closer to Earth than Saturn – Saturn is about twice as far away.
After 2020, the next Great Conjunctions will occur in November 2040 and April 2060. However, as the minimum separation of Jupiter and Saturn will be 1.1° (11x further apart than in 2020), this phenomenon isn’t expected to be quite as significant until 2080. Still, these events are a nice connection between generations and make you ponder about all those who have observed these conjunctions in the past and all those who will witness them in the future.
So now is the only chance for many of us to see it. Despite writing this article just hours before the event and the sky not being clear at the time of writing, I hope some of you are able to witness this event and fully enjoy the winter solstice of a very strange and difficult year. Remember to remain socially distant and keep… space.
Adam Goriparthi
Featured image by Giuseppe Donatiello from Flickr. Image in the Public Domain. No changes made to this image.
Embedded in-article image tweet from @NASA. No changes were made to the tweet.
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