The Great Conjunction of 2020
"Conjunction Junction, What's Your Function" has been playing on loop through my head for a week leading up to last night's Great Conjunction of Jupiter and Saturn. In astronomical terms, a conjunction is the alignment of two planets, celestial objects or spacecraft as seen from a line of sight from Earth. When we use "Great Conjunction" we are specifically talking about the alignment of Saturn and Jupiter.
Jupiter orbits the Sun every 11.86 years and Saturn has a 29.36 year orbit around our Sun. This means that as viewed from Earth, Saturn and Jupiter align on average every 20 years. So, if this is a fairly common occurrence, why are astronomers so hyped up about the conjunction? The answer has to do with how close the two planets are in the sky. While conjunctions of these two planets occur every 20 years, this conjunction is exceptionally rare because the planets were only 6 arcseconds from each other. This is less than 1/10 of a degree and only 1/5 the diameter of the moon. This means standing on Earth if you held a dime up to the sky, the two planets would be about the thickness of a dime apart from each other.
Stephanie Erickson using Stellarium
The last time the two gas giants were this close was on July 16, 1623 (397 years ago). However, we have no record of this being observed through any astronomers telescope. During the years leading up to the conjunction, Galileo had identified four of Jupiter's moons and Saturn's rings. So, we know that Galileo and other astronomers were making observations of these planets during this time. The fact that there are no observations of the conjunction leads us to to believe that the conjunction was not observed. In fact, due to the line of sight from Earth being so close to the Sun, it is likely that few areas of the planet, like the equatorial region would have been a been given a brief glimpse of the conjunction.
We have to go back even further in time until March 5, 1226 (794 years ago) to have the planets less than 6 arcseconds apart. In 1226, the planets were even closer than they were last night. They were only 2 arcseconds apart. This is THREE TIMES (3X) closer! This distance equates to approximately 1/14 of the diameter of the moon. During this year, Mercury and Venus also aligned with Earth and Jupiter. This is also before the telescope was invented in the late 1500's. The Great Conjunction of 1226, however, would have been difficult but not impossible to see. The Sun was 20 degrees away from the planets and the alignment would have occurred in the morning hours as the Sun was rising.
On average, the two planets form a Great Conjunction every 300 years. If we factor in visibility of viewing the conjunction, the average increases. So, the conjunction of 2020, really is a rare site. It will be the first Great Conjunction viewed through a telescope where the planets are this close together and able to be viewed through a single frame.
There has been some speculation as to if the "Christmas Star" or "Star of Bethlehem" written in the Book of Matthew, was in fact a Great Conjunction of Saturn and Jupiter. The gospel of Matthew tells us that a bright star appeared in the eastern sky when Jesus was born. This star was seen by a group of wise men, the biblical "Magi," sometimes called the Three Kings. There is some evidence to support that the Star of Bethlehem was a Great Conjunction. In 7 BC, the planets aligned 3 times in a Triple Great Conjunction on May 29th, September 30, and December 5. This would have been enough time for Magi to have left their homeland in Persia and travel to Jerusalem and Bethlehem. However, without additional observations and notes during this time we have no way of knowing if this star referenced was a conjunction, a comet or a supernova.
My astronomy students made this viewing guide for observing the conjunction. https://docs.google.com/document/d/1SskEc_Fi4f6L47IuWxvKJggR0qLGn8dvC5oiiCFOowQ/copy
Here in the northeast, tonight is shaping up to being a good viewing for the conjunction. Last night we were in the clouds. While the planets won't be as close as they were last night, they will still be in close proximity and a really cool sight to behold in the night sky. So, look to the southwest horizon between 4 and 6pm this evening and located the brightest object you can find. Jupiter and Saturn will be about 10 degrees off the horizon and will set as the sun is setting into the horizon.