7:15 AM | *Jupiter and Saturn are converging for a rare “Great Conjunction” on the day of the winter solstice - December 21st*

The “Great Conjunction” of 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

You may have already noticed two close together bright objects in the sunset sky in recent weeks which happen to be the giant gas planets of Jupiter and Saturn. These two planets are actually converging for an even closer encounter in terms of their appearance to us here on Earth - the likes of which haven’t been seen in many, many centuries. Jupiter and Saturn currently appear about 2 degrees apart and they will actually look only 0.1 degrees apart by the time we get to the winter solstice on December 21st - the day of the “Great Conjunction” of 2020.

Jupiter and Saturn will appear extremely close together on December 21st just after sunset in the low southwestern sky. [“Sky map” credit to “spaceweather.com”]

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 lapping Saturn once every 20 years.

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. Often, the glare of the sun makes their convergence difficult or even impossible to see from here on Earth, but this year is very special because the conjunction happens comfortably away from the sun.  Here are the dates of the “great conjunctions” from 2000 to 2100:

• May 28, 2000

• December 21, 2020

• October 31, 2040

• April 7, 2060

• March 15, 2080

• September 18, 2100

Jupiter (brighter, lower right) and Saturn (dimmer, upper left) appeared at approximately two degrees apart on December 3rd when this photograph was taken at dusk over the trees at Glenshane Pass in Northern Ireland. [Credit spaceweather.com, Martin McKenna]

Jupiter and Saturn already appear close together in the sunset sky and they are about to get even closer from our vantage point here on Earth.  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. In fact, 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. They will appear to get closer and closer on each night during the next couple of weeks and - as an added bonus - the crescent Moon will join the planets on the 16th and 17th of December making for a beautiful sight assuming good viewing conditions. 

By the time we get to the winter solstice on 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. 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.

A look at Jupiter’s position in the sky relative to some of its largest moons which 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.

Jupiter and Saturn should be visible within the same field-of-view on 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 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 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 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 on 12/21 is 4:39 PM in Philly; two planets set by ~8:23 PM).

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com

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