This type of conjunction between the Solar System giants only occurs once every 20 years (19.6 to be precise). The last Great Conjunction one was in 2000, and the next one will take place on December 21, 2020. This year’s great conjunction will be the closest one to take place since 1623.
Astronomers use the word conjunction to describe the encounters between two astronomical objects or spacecraft with either the same right ascension or the same ecliptic longitude, usually as observed from Earth. Great conjunctions occur regularly, every 19.6 years, due to the combined effect of Jupiter’s approximately 11.86-year orbital period and Saturn’s 29.5-year orbital period.
Although this conjunction will take place at the end of the year, it can already be observed every night of the rest of 2020, where we get to see how the king of the planets—Jupiter—gets closer and closer to the lord of the rings—Saturn—, closing the distance between them.
In the previous grand conjunction, which took place 20 years ago, Jupiter and Saturn were too close to the Sun in the sky, so the observation conditions were not ideal.
However, on December 21, we will have better luck, both worlds being visible on the horizon shortly after nightfall.
Saturn, the sixth planet from the Sun, is the farthest and slowest that we can observe with the naked eye. Jupiter is the fifth and second slowest planet after Saturn.
For this reason, the radiant conjunctions between this pair are among the rarest.
It takes Saturn nearly 30 Earth years to complete one orbit of the Sun, while it takes Jupiter about 12.
To be more specific, each Earth year, Saturn completes 12 degrees of its orbit and Jupiter 30 degrees of its orbit around our Sun.
This means that Jupiter closes the gap between it and Saturn in one year at approximately 18 degrees (30-12 = 18). Over a period of 20 years, ergo, Jupiter beats Saturn by 360 degrees (18 × 20 = 360), entering into conjunction and passing, from our perspective, one on top of the other.
There will be quite a few great conjunctions in this century, the most important of which took place on May 28, 2000, and we are looking forward to those on December 21, 2020, October 31, 2040, April 7, 2060, March 15, 2080, and September 18, 2100.
During the coming month and August, Earth will pass between Jupiter and Saturn and the Sun, putting these planets at the opposite point from the star in our sky, something called “opposition” for astronomers.
Jupiter will go into opposition on July 14, and Saturn will do so on July 20.
During opposition, both planets will emerge in the east at dusk, rising higher in the sky at midnight, and disappearing at dawn.
After this, they will start to disappear on the horizon earlier and earlier, gradually appearing for less time in the night sky as we go through September, October, November, and December.
The great conjunction of Jupiter and Saturn on December 21, 2020, will mark the end of the year to observe these two worlds.