In the “Great Conjunction” of December 21, 2020, Jupiter and Saturn will be so close to each other that most of them will appear as a very bright star, which some are already calling a “Christmas star”. Photo credit: Getty Images
From Paul Dorian
You may have noticed two closely spaced bright objects in the sunset sky in the past few weeks that happen to be the giant gas planets of Jupiter and Saturn. These two planets actually come together to meet us even closer here on earth – as has not been seen in many, many centuries. Jupiter and Saturn currently appear about 2 degrees apart and will actually be only 0.1 degrees apart by the winter solstice on December 21st – the day of the "Great Conjunction" in 2020.
Jupiter and Saturn will appear extremely close to each other in the low southwest sky just after sunset on December 21st. ("Sky Map" credit to spaceweather.com)
Jupiter and Saturn conjunctions are the rarest conjunctions on bright planets due to their slow orbits around the sun. It takes Saturn almost 30 years to complete its circle around the Sun while it takes Jupiter almost 12 years. Almost every 20 years, Jupiter catches up with Saturn when viewed from Earth, causing what astronomers call the "great conjunction". Why every 20 years? Each year Saturn completes about 12 degrees of its orbit around the Sun while Jupiter completes about 30 degrees. In one year, Jupiter will close the gap to Saturn by about 18 degrees. Over a period of 20 years, Jupiter gains 360 degrees on Saturn (18 x 20 = 360 degrees), which means that Saturn is lapped once every 20 years.
This particular “great conjunction” between Jupiter and Saturn is extremely rare, as the two planets are close to each other and are easy to recognize. Often times the sun's glare makes its convergence difficult or even impossible to see from here on earth, but this year is very special as the conjunction takes place comfortably outside of the sun. Here are the dates of the "major conjunctions" from 2000 to 2100:
May 28, 2000 December 21, 2020 October 31, 2040 April 7, 2060 March 15, 2080 September 18, 2100
Jupiter and Saturn already appear close together in the sunset sky and are getting even closer to our position here on earth. The last time these two gas giants appeared this close together was in 1623 in Galileo's time, but the sunshine that year most likely obscured the pair of planets from celestial observers on Earth. In fact, the last time the two giant gas planets were so close together and so easily seen in the Middle Ages in 1226. While Jupiter and Saturn appear pretty close to each other in the sunset sky, they're actually about 450 million miles apart. They seem to be getting closer and closer every night for the next few weeks, and as an added bonus, the crescent moon will join the planets on December 16th and 17th for a beautiful sight in good visibility.
By the winter solstice on December 21st, Jupiter and Saturn appear only 0.1 degrees apart, which is only 1/5 of a full moon diameter. In fact, they will appear so close together that Saturn will come as close to Jupiter as some of its own moons. Depending on the local viewing conditions, the two planets may appear as a "double planet" for some or as a very bright star in the low southwest sky shortly after sunset for others. Some are already referring to this great connection between Jupiter and Saturn as "Poinsettia" because the timing is just a few days before Christmas. Note: Some biblical scholars believe that the "Star of Bethlehem" was a triple conjunction made up of the following:
Jupiter (known as the royal planet) Venus (the brightest planet in our solar system) The star Regulus (known as the royal star) in the constellation Leo.A look at Jupiter's position in the sky relative to some of its largest moons, which actually appear linear. In January 1610, the Italian astronomer Galileo Galilei discovered four of Jupiter's moons – today Io, Europa, Ganymede and Callisto. It is believed that there are now up to 79 moons of Jupiter, with Europe being roughly the size of Earth's moon.
Jupiter and Saturn should be visible in the same field of view on December 21st, whether with binoculars or a small "backyard" telescope, along with four of Jupiter's largest moons spread out in a straight line. After December 21st, Jupiter and Saturn will diverge fairly quickly in terms of appearance to us here on Earth, and that trend will continue for the next decade before converging again in the 2030s. The next “Great Conjunction” of the year 2040 will thus be established. However, this one won't be as brilliant as this one. In fact, the two planets won't appear this close again until 2080, and in this close encounter of the late 21st century, Jupiter will completely cover Saturn, which is an extremely rare event and won't happen again until 7541.
Make sure to mark your calendars specifically for December 21st to see this once in a lifetime heavenly event as this opportunity is a one day affair only. Just a day before and a day after, the planets appear noticeably further apart and nowhere near as noticeable as on December 21st – hopefully the skies will work together on the first day of astronomical winter. The best observation on December 21st is between 30 minutes and an hour after sunset in the low southwest sky until the time the two planets set (~ 8:23 p.m. ET).
Meteorologist Paul Dorian
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Video discussion on the topic of "The great conjunction of December 21, 2020".