We are quickly approaching the “Great Conjunction” on December 21, 2020, which means Jupiter and Saturn will be about one-tenth of a degree apart. The Amateur Astronomy Association is setting up a range of observation stations around New York City for members to witness this exciting celestial event. We will also organize one or more livestreams in the days before, on, and/or after the closest point of conjunction. Please pay close attention to our website for details, updates, and changes.
Six Minutes of Separation
The last time Jupiter and Saturn appeared in such close vicinity to each other was on July 16, 1623, when they were separated by a mere five arc minutes. The giants meet up in our skies, albeit at a greater distance, on a twenty-year cycle.
If you have a telescope and want to assess how close the two planets will be in this month’s conjunction, look at Mizar and Alcor, the middle stars in the handle of the Big Dipper asterism. They are about 11.5 arc minutes apart. Then look at our Moon, which on average is thirty arc minutes in diameter when Full.
The duo at point of greatest conjunction would easily fit inside the First Quarter Moon, where it could be placed along the perimeter of the Sea of Tranquility, end to end.
The next comparable conjunction— with six-arc-minutes of separation— will occur on March 15, 2080. There will also be a particularly close approach in the year 7541.
The Jupiter Effect
It is interesting to see the conjunction taking place around this time of the year with the holidays upon us. It brings to mind discussions regarding the Star of Bethlehem and its possible origins. Of the four Gospels, only Matthew’s account suggests that the Magi knew, from seeing the star “at its rising,” that a new “king” had been born.
The prevailing theory is that the Magi, who were astrologers, saw a sign in the sky of super conjunctions of the planets and then waited for a final sign before heading to Bethlehem to greet the infant Jesus.
The British astronomer, David Hughes (b. 1941), called attention in his book to a spectacular group of conjunctions that took place in 7 BCE as a triple conjunction of Jupiter and Saturn in the constellation Pisces. Hughes suggested that the nativity occurred on September 15th of that year. This was the day that Jupiter and Saturn had their achronical rising, meaning they rose together in the east at the same time as the Sun set in the west.
Other suggestions say that the nativity occurred later, in 2 BCE. This is based on two remarkable planetary configurations involving Jupiter and Venus that took place in the constellation Leo in August of 3 BCE and on June 17, 2 BCE, with the latter being the most spectacular as Venus occulted Jupiter. The Magi might well have associated this impressive conjunction, which involved the king of the planets, with the birth of the “lion of Judah.”
So what exactly was the Star of Bethlehem? The Magi were a patient trio not easily excited. They read the signs in the conjunctions and then waited. They were alerted to the imminent nativity by the triple conjunction in 7 BCE, but they waited further for more signs.
Once again the planets gathered the following year in the constellation of Pisces. This time, the king of the planets was joined by Mars, the bringer of war. This might have suggested to them a new king to be born who would liberate the Jewish people. Finally, the following year in March of 5 BCE, a nova blazed out of the dawn sky. This was the celestial sign that told them to make their plans and prepare for the journey. Author Mark Kidger suggests that no single event has sufficiently explained “the star”, but this one probably gets as close as science can get us.
David Hughes, The Star of Bethlehem: An Astronomer’s Confirmation. Walker & Co., 1979.
Mark Kidger, Astronomical Enigmas: Life on Mars, the Star of Bethlehem, and Other Milky Way Mysteries. Johns Hopkins University Press, 2005.