For those who have not already seen it, the next two weeks present an opportunity to witness a unique event – a very close conjunction between Jupiter and Saturn.
“Conjunction” is the term astronomers used to describe two astronomical objects or spacecraft having either the same right ascension or the same ecliptic longitude, and thus when seen from Earth, appear to be close together.
With the planets, such events are not especially rare – in fact as they and the Earth circle the Sun, conjunctions between Jupiter and Saturn tend to occur once every 20 years. However, most of these only see Jupiter and Saturn close to around one degree of one another, or about one-fifth the diameter of the Moon as seen from Earth. But sometimes they appear to get much closer, creating what is referred to as a “great conjunction”. This year, the two planets will appear to be just 6 arc minutes apart as seen from Earth on December 21st, 2020; so “close” (remembering that their respective orbits around the Sun will still be separated by 883 million km), they will almost, but not quite, appear as a single point of light when seen with the naked eye.
These “great conjunctions” occur, on average, once every 300-400 years, although such is the nature of orbital mechanics, they can actually occasionally occur more frequently, or have longer time gaps between them. As it is, the last time Jupiter and Saturn appeared as close as the will be between December 20th and 22nd was in 1623, not long after Galileo had observed both planets – although he was unable to witness the event, as the rising Sun would have rendered them invisible in its glare.
What is most rare is a close conjunction that occurs in our night time sky. I think it’s fair to say that such an event typically may occur just once in any one person’s lifetime, and I think ‘once in my lifetime’ is a pretty good test of whether something merits being labelled as rare or special.
Astronomer David Weintraub
However, the two planets can appear to be much closer. In 1226, and in the skies over the Mongol Empire, when the planets appear to be just 2 arc minutes apart.
Tracing these great conjunctions back in time reveals that Jupiter and Saturn may well have played a role in the legend of the Star of Bethlehem. In 7 B.C. not one, but three great conjunctions occurred, with the two planets again being within 2 arc minutes of one another as seen from Earth.
The first occurred in May of that year, when Jupiter and Saturn appeared as a morning star over the middle east. As the Magi were practitioners of (among other things) astronomy and astrology – both at that time pretty much joined at the hip – such an event may well have caused them to start out on their long journey towards Judea, the second conjunction, in September of the year, encouraging them to continue. The third conjunction occurred in December, 7 B.C., the time at which they were said to have met with Herod the Great.
This year’s conjunction will be not long after sunset, with the two planets located low over the south-west horizon. With a reasonable telescope or good pair of binoculars, you’ll have an ideal opportunity to see both planets and their major moons in the same field of view. Should you do so, you’ll be looking at over 90% of the planetary mass of the entire solar system.
Beyond the 21st, the two planets will gradually move “apart” as noted, until by the 25th December, they’ll be separated in the night sky by roughly the diameter of a full Moon, and will continue to draw apart relative to Earth as they pass below the horizon.
And if you miss this close conjunction between the two, the next will be along in a relatively (and unusually) short period, occurring on March 15th 2080. The next time they’ll be as apparently close as they were in 7 B.C. will be on Christmas Day, 2874.