*Jupiter and Saturn continue to converge for a rare “Great Conjunction” on Monday, December 21st*

The “Great Conjunction” of Monday, December 21, 2020 will feature Jupiter and Saturn so close together that they will appear to most as one very bright star which some are already dubbing the “Christmas-star”. Credit: Getty Images

Overview

If you step outside tonight shortly after sunset and look to the southwest sky you should see Jupiter and Saturn quite close together.  The two giant gas planets have actually been converging in recent weeks in terms of their appearance to us here on Earth and they will appear closest together on Monday, December 21st - the day of the winter solstice and the day of what is being called the “Great Conjunction”.  In fact, it has been many, many centuries since the two giant gas planets have appeared so close together and have been so easy to see. 

Details

Jupiter and Saturn conjunctions are the rarest of bright-planet conjunctions due to their slow orbits around the sun. Saturn takes nearly 30 years to complete its circle around the sun whereas Jupiter takes nearly 12 years. So just about every 20 years, Jupiter catches up to Saturn as viewed from Earth causing what is referred to by astronomers as a “great conjunction”. Why every 20 years? Each year, Saturn completes about 12 degrees of its orbit around the sun, whereas Jupiter completes approximately 30 degrees. So in one year, Jupiter closes the gap to Saturn by about 18 degrees. In a period of 20 years, Jupiter gains 360 degrees on Saturn (18 x 20 = 360 degrees), meaning Jupiter catches up to Saturn once every 20 years.

Here are the dates of the “great conjunctions” in this century:

• May 28, 2000

• December 21, 2020

• October 31, 2040

• April 7, 2060

• March 15, 2080

• September 18, 2100

On the day of the “Great Conjunction”, Monday, December 21st, Jupiter and Saturn will appear extremely close together just after sunset in the low southwest sky. [“Sky map” credit to “spaceweather.com”]

This particular Jupiter/Saturn “great conjunction” is extremely rare because of how close the two planets will look to one another and how easy it will be to see by us here on Earth. Often, the glare of the sun makes their convergence difficult or even impossible to see, but this year is very special because the conjunction happens comfortably away from the sun.  The last time these two gas giants have appeared this close together was in the year 1623 during Galileo’s times, but the sun’s glare that year very likely obscured the pair of planets to sky watchers on Earth. The last time the two giant gas planets appeared so close together AND were so easy to see was during medieval times in the year 1226. While Jupiter and Saturn appear quite close together to us in the sunset sky, they are actually about 450 million miles apart from each other.

A look at Jupiter’s position in the sky relative to some of its largest moons that actually appear in a linear fashion. In January 1610, Italian astronomer Galileo Galilei discovered four of Jupiter's moons — now called Io, Europa, Ganymede and Callisto. There are now thought to be as many as 79 moons of Jupiter with Europa being about the same size as Earth’s moon.

When the month of December began, Jupiter and Saturn appeared about 2.1 degrees apart. By the time we get to the winter solstice on Monday, December 21, Jupiter and Saturn will appear only 0.1 degrees apart, which is just 1/5 of a full moon diameter. In fact, they will appear so close together that Saturn will actually look to us as close to Jupiter as some of its own moons.  The two planets may appear as a “double planet” to some depending on local viewing conditions or as one very bright star to others in the low southwest sky shortly after sunset. Some are already referring to this great conjunction of Jupiter and Saturn as the “Christmas Star” because of the timing just a few days before Christmas Day. Note- some Biblical scholars believe the “Star of Bethlehem” was a triple conjunction made up of the following:

• Jupiter (known as the king planet)

• Venus (the brightest planet in our solar system)

• The star Regulus (known as the kingly star) in the constellation Leo.

Jupiter and Saturn should be visible within the same field-of-view on Monday, December 21st whether using binoculars or a small “backyard” telescope along with four of Jupiter’s largest moons spread out in a straight line. Beyond December 21, Jupiter and Saturn will move away from each other rather quickly in terms of appearance to us here on Earth and this trend will continue for the next ten years before they start to converge again during the 2030s.  This will set up the next “Great Conjunction” of the year 2040; however, that one will not be as brilliant as this one. In fact, the two planets won’t appear this close again until the year 2080 and in that late 21st century close encounter, Jupiter will completely cover Saturn which is an extremely rare event and won’t happen again until the year 7541.

Final Comments

Make sure to mark your calendars specifically for Monday, December 21st to view this once-in-a-lifetime celestial event as this opportunity will be just a one-day affair. Just one day before and one day after the “Great Conjunction”, the planets will appear noticeably farther apart from each other and nowhere near as striking as on December 21st - hopefully skies will cooperate on the first day of astronomical winter. The best viewing on Monday, December 21st will be from about 30 minutes to an hour after sunset in the low southwestern sky until the time the two planets set (sunset time in Philly on 12/21 is 4:39 PM and in DC it is at 4:50 PM; the two planets will set by about 8:23 PM).

For info on how best to photograph Monday’s “Great Conjunction” visit this NASA web site.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com

Follow us on Facebook, Twitter, YouTube

Video discussion (recorded on December 10th):