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The ‘Great’ Conjunction of Jupiter and Saturn

Posted by Steven Wilcox on
The ‘Great’ Conjunction of Jupiter and Saturn


Skywatchers are in for an end-of-year treat.
What has become known popularly as the “Christmas Star” is an especially vibrant planetary conjunction easily visible in the evening sky over the next two weeks as the bright planets Jupiter and Saturn come together, culminating on the night of Dec. 21.

What exactly is a conjunction?

A conjunction is simply when two objects seem to be close to one another in the night sky. In more of a technical sense, there are many ways to define it. One of these ways is to say it is the moment of minimum separation between two objects. From this, the conjunction of these two major planets will occur at 10:20 PST. 

From Earth, what will be able to see?

As Jupiter has been traveling closer and closer to Saturn since September, wherever you are, you will be able to see it. The two planets will be extremely visible and easy to see after the sun goes down. When looking for the two planets, remember that Jupiter is the brighter of the two. Also, if you are in the Northern Hemisphere, look southwest, and if you are in the Southern Hemisphere look towards the west. Right now, in the beginning of December, the two planets are only two degrees apart and will still be moving even closer together. 

On the 21st Jupiter and Saturn will only be 0.1 degrees away from each other.

As this is so close together, the two will appear as a single large and bright “star”. As this is the closest conjunction since July 16, 1623, it is truly something you don't want to miss. Following the 21st and into the nights of January 2021 the two planets will continue to be an impressive sight but will soon become lost within the sun's glare.

Why do conjunctions happen?

When looking at the solar system, the moon, sun and the five planets(Mercury, Venus, Mars, Jupiter, and Saturn) roughly move along a disk which astronomers call an ecliptic. As they are moving on the same general path along the Earth’s sky, these meetings or conjunctions are bound to occur.

“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,” said Henry Throop, astronomer in the Planetary Science Division at NASA Headquarters in Washington. “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.”

Why is the conjunction between Jupiter and Saturn so rare?

Compared to other planets, Jupiter and Saturn are much farther away. Because of this, these planets travel slower as the orbital speed of an object decreases with distance. As the Earth only takes a year to travel around the sun, Saturn takes 30 and Jupiter takes 12. From this, Saturn completes two-thirds of its 30-year orbit, while Jupiter completes a single 12-year orbit with the remaining 8 years which is two-thirds of the next orbit. Simplified, it takes 20 years for Jupiter to catch up with and pass Saturn. 

But don't be mistaken, this conjunction only happens every couple hundred years! Even though Jupiter, regularly passes Saturn, they don't always align with the likes of which we'll get to experience tonight.

So, although it's a rather cold of frosty time of year, we encourage you to get outside to enjoy this one-of-a-kind spectacle in the sky that creates our 2020 Christmas Star (socially distanced, of course). 

- Happy Holidays and Merry Christmas from PARKIT

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