2020 has certainly been the year of once in a lifetime events! This December 21st will be your chance to witness another. A planetary conjunction is when two planets appear next to each other in the sky. This year, Jupiter and Saturn will come together in the sky to form what is being called “The Great Conjunction of 2020”. Conjunctions are not that uncommon. In fact, Jupiter and Saturn are in conjunction every 20 years. This is because both Jupiter and Saturn orbit the Sun but Jupiter moves faster than Saturn as it is closer. Jupiter is 5.2 astronomical units (AU) from the Sun and Saturn is 9.5 AU from the Sun. This means that every 20 years Jupiter catches up and overtakes Saturn. To imagine this another way, imagine two circular race tracks where one is inside the other. The car on the outside track has further to travel than the car on the inside and therefore gets overtaken by the inside car.
What is so special about this conjunction is how close the planets will appear to be to each other. The separation on the evening of December 21st will be 6.1 arcminutes. To put that into context, the Moon is around 30 arcminutes across so the planets will be separated by less than one fifth of the diameter of the Moon. They will be so close they will almost appear as one star and it will be possible to see both planets in the same eyepiece. This is very rare and the last time they were this close was 1623.
Conjunctions are a line of sight effect. You can see how this works in the diagram below. As we look out across the Solar System, the line of sight between the two planets make a very narrow angle. Despite how close they look they are still very far away from each other. on December 21st the two planets are still 733 million km apart.
Over the years, many historians and astronomers have attempted to look for scientific and historic evidence of the Star of Bethlehem. The story told in the gospels is that the Magi followed a star to Jerusalem in search of a newly born King of the Jews. Historical evidence from other events in the story, such as the death of King Herod and the census in Nazareth in 8BC, point to the birth of Jesus around 7-5 BC. There was a conjunction of Jupiter and Saturn three times in 7BC. This occurred in the constellation of Pisces which has been associated with Israel. The Magi would have been astrologers so would have looked for signs in the sky. A conjunction such as this may well have prompted them to travel to Bethlehem. We will never know for sure but it’s possible that this conjunction could be similar what is now referred to as the Star of Bethlehem.
I would encourage you to have a look for the conjunction on a few nights before and after the main event on the 21st. The weather may not be good on the 21st but the planets will still be very close in the days before and after the main conjunction. Look southwest and you will see the conjunction close to the horizon. At first glance they will probably look like one star. Jupiter will be the brightest of the two. You should plan to start watching around 4.45 pm. If you have binoculars or a small telescope take a look and see if you can them.