By Lucia Brimer
On Dec. 21st there will be an event which is easy to see and which has not been seen for hundreds of years. Jupiter and Saturn will appear only 0.1 degree, or 6 arc minutes, apart in the sky, so close that they may almost look like a single, extremely bright star! If you have a small telescope or a pair of binoculars, you will be able to see them both in the same field of view. All the month of December, you may have noticed them getting closer and closer each night. Be sure to check them out about an hour after sunset, in the southwestern sky. The brightest object you will see is Jupiter. Saturn is not nearly as bright, but it does look like a very bright star. If you can’t get out on the 21st, they will still be very close on the 20th and 22nd.
Credits: NASA/JPL-Caltech from https://www.nasa.gov/feature/the-great-conjunction-of-jupiter-and-saturn
View in a small telescope on Dec. 21. Created via Stellarium
What is a conjunction? It is when 2 objects in the sky appear to be very close together, as seen from the Earth. Of course, this does not mean that Jupiter and Saturn are actually close, they remain in their orbits but they appear along the same line of sight. In reality, Saturn is much farther from us than Jupiter; Saturn is 9.5 AU or 886 million miles (1.4 billion km) from the Sun, while Jupiter is 5.2 AU or 484 million miles (778 million km) from the Sun. So, even though they look like they are right next to each other, they remain at least 402 million miles (662 million km) from each other. Because Jupiter orbits the Sun in 12 years, while Saturn orbits in about 30 years, they appear to pass each other approximately every 20 years. When Jupiter and Saturn pass each other, it is known as a Great Conjunction.
Alignment of the planets for the Great Conjunction of 2020. Distances are to scale; sizes of the Sun and planets are not. Courtesy: https://www.timeanddate.com/astronomy/planets/great-conjunction
Why is this one so special? Well, even though they pass each other every 20 years, the minimum apparent distance between them varies. The last time they were closer than 6 arc minutes was in July, 1623, when they were 5.2 arc minutes apart. But not many people would have observed that Conjunction, at least in the Northern Hemisphere, because the planets were too close to the Sun to see properly. They would have been very close to the horizon at Civil Twilight. The last observable conjunction where Jupiter and Saturn were closer than 0.1 degree was in March 1226, nearly 800 years ago! The separation at that event was only 2.1 arc minutes, about 1/3 the distance they will be on Dec. 21, 2020. The next Great Conjunction of less than 0.1 degree, by the way, will be in 2080, so some of you may get another chance to see this in your lifetime.
Also, the date of this Great Conjunction is significant. December 21st is the Winter Solstice, also known as the first day of winter. On this date, the Sun is at its lowest point in the sky, as seen from the Northern Hemisphere. This is also the shortest day of the year. Various ancient peoples would have great parties on this date to appease the Sun-god, in hopes that he (or she) would agree to come back and start warming the Earth again. The ancient Romans held a festival called Saturnalia to celebrate the rebirth of the year. Amongst other things, the festival involved decorating houses with greenery, lighting candles, holding processions and giving presents. Sound familiar? Many ancient traditions associated with pagan Solstice celebrations were incorporated into our modern Christmas celebrations, including Christmas trees and the Yule log. Even today, there are people who celebrate the Solstice. For example, in most forms of Wicca, this holiday is celebrated at the Winter Solstice as the rebirth of the Great horned hunter god, who is viewed as the newborn solstice Sun.
Credit: Stroma Cairns for AnOther Magazine https://www.anothermag.com/design-living/10457/a-step-by-step-guide-to-creating-a-traditional-yule-log
Speaking of Christmas traditions, in the Bible, Matthew chapter 2 states,
"After Jesus was born in Bethlehem in Judea, during the time of King Herod, Magi from the east came to Jerusalem and asked, "Where is The One who has been born king of the Jews? We saw His star in the east and have come to worship Him." …
"After they had heard the king, they went on their way, and the star they had seen in the east went ahead of them until it stopped over the place where the child was. When they saw the star, they were overjoyed.”
Astronomers for centuries have wondered what this Star of Bethlehem actually was, and if it could be determined whether it was a supernova, a comet, or something else. Well, most astronomers now agree that this famous Star was actually a Conjunction of planets. Much research reveals that Jesus probably was not born on Dec. 25 (that date was probably adopted to coincide with the Solstice Festivals the pagans were already celebrating). In the springtime, during lambing season, shepherds would have out with their flocks at night. And he was probably born in about 6 or 7 BC, to correspond with the death of King Herod.
Well, In the year 7 BC there was a triple Great Conjunction of Jupiter and Saturn. This happens occasionally (but rarely) because the planets go through a period of retrograde (backwards) motion due to the relative motions of the planets and the Earth. It is like being on a racetrack and passing someone. Even though both cars are moving forward, it looks like the other is going backwards compared to the car that is passing it.
Artwork of a possible Christmas Star in the night sky of the year 7 BC. Traces of the planetary conjunction of Jupiter and Saturn are seen in the night sky in the constellation Pisces. Credit: Science Photo Library
Astrologers, or wise men, from the orient may have noticed this set of 3 Great Conjunctions over a period of 8 months, and traveled to find the Christ child that they believed the events foretold. If a conjunction of Jupiter and Saturn was the Star of Bethlehem, then on Dec. 21,2020, we can see for ourselves this Christmas Star, and worship in whatever way best fits our beliefs.