On December 21, 2020, just hours after the official start of winter, an exciting event is going to take place in the night sky—a long-awaited “Great Conjunction” between Jupiter and Saturn. This event is a rare close encounter of the solar system’s two largest planets! What makes this astronomical occurrence even more special is that it will show you one of the theories attributed to the “Star of Bethlehem.”
When And Where To View
On the evening of December 21st, about 45 minutes after sunset (local time), look low toward the southwest horizon. Jupiter will appear to Saturn’s lower left. Both planets will be close enough to see in the same field of view of a high-power eyepiece. By month’s end, the two planets, having separated to 1.2° apart, will be setting about 1 hour and 45 minutes after sunset, right at the end of evening twilight.
All through the summer and into the fall, these two gas giants of the solar system have been calling attention to themselves in the southern evening sky. Jupiter of course, always appears brilliant and is usually one of the brightest nighttime objects, but in recent months it has stood out even more than usual because of the presence of bright Saturn trailing just off to its left (east). Appearing about one-seventh as bright, Saturn has, in a way, served as Jupiter’s “lieutenant” this year.
A Rare Sight
Whenever Jupiter and Saturn are in conjunction, that is, when they have the same right ascension or celestial longitude, it is referred to as a “Great Conjunction,” primarily because unlike conjunctions with the other bright planets, these two don’t get together very often. On average, they are in conjunction once in about 20 years.
Most of the time, they are separated by more than a degree. But come December 21, they will be separated by just about one-tenth of a degree, or 6.1 arcminutes. To gauge how close that is, on the next clear night, check out Mizar, the middle star in the handle of the Big Dipper. A fainter star, Alcor, is positioned only 11.8 arc minutes away and the ability to perceive the separation of these two stars, was once considered a test of good vision.
And yet Jupiter and Saturn will approach to within about half that distance! This means, under high magnification in your telescope you’ll be able to see both planets—Saturn with its famous ring system and Jupiter with its cloud bands and Galilean satellites—simultaneously in the same field of view!
How Often Do These Two Planets Come This Close?
Some astronomy websites indicate it has been nearly 400 years, while others say it’s been almost 800 years. Indeed, the last time these two planets appeared so close was on July 16th, 1623, when they were only 5 arcminutes apart; that’s actually 397 years ago.
However, visibility of this rare alignment was only visible from the tropics around the equatorial regions. For those in temperate latitudes, such as New York, London, or Tokyo, the two planets were not visible because of their close proximity to the glare of the Sun and low altitude above the horizon. But assuming you were not among those living in northern South America, central Africa or Indonesia, who had a brief view of a similar sighting of these two planets in 1623, the last time most of the world’s population had a favorable view of these two planets coming so close to each other was on March 5th, 1226—800 years ago—when they were a trifle closer together than what we will see on December 21st.
The star of Bethlehem?
Some have suggested that these two planets might have been the legendary Star of Bethlehem. Actually, one of many theories for the Star of Bethlehem was a close conjunction between Jupiter and Saturn in 7 BC. However, in that year Jupiter and Saturn met not once but three times that year (in May, September, and December).
The first conjunction (in May) presumably started the Magi on their way to Bethlehem from the Far East. The middle conjunction (in September) strengthened their resolve in the purpose of their journey, while the third and final conjunction (in December) occurred just as they arrived in Judea to meet with King Herod, who sent them on to Bethlehem to “go and search diligently for the young child.”
But while single conjunctions of Jupiter and Saturn occur once every 20 years, triple conjunctions are far less frequent, occurring about once every 180 years on average; the last time was in 1981, but the next won’t happen until 2239. For the Magi, Jupiter, pacing back-and-forth with Saturn in 7 BC would have been looked upon as something unique.
But this year, Jupiter and Saturn will only have a single meeting, on December 21st. Whether from an astrological point of view that one single “celestial summit” might have a been significant enough sign in the sky for the Magi to begin their trek to Judea is unknown.
Back on June 5, 1978, Mars and Saturn were separated by a similar distance, and both planets were clearly separated as viewed the naked eye. However, those who are near-sighted might see Jupiter and Saturn appear as one merely by removing their eyeglasses.
Watch Them Grow Closer Each Night
It will be interesting to watch how the gap between these two planets will gradually close the coming nights. On December 1st, they were separated by 2.2°, but by the 15th that gap will be down to just 0.7°. Then they will get 0.1° closer each night thereafter until their long-awaited meeting on December 21st.
We will be treated to another 6-arcminute separation on March 15th, 2080.