Credit: NASA via Unsplash

The great conjunction of Jupiter and Saturn

By Joas Sterk

While 2020 was a bad year for humanity, it was a fantastic year for astronomy.

While the Earth’s most recent orbit around the sun has been dominated by the many challenges humanity has faced, we were nevertheless given plenty of opportunities to gaze up to the sky and be awestruck. 2020 was a great year for astronomy! Whether it be the Penumbral Lunar Eclipses throughout the year, the comet NEOWISE lighting up the sky, or Venus shining at its brightest in April, there was certainly much to see in the skies. And, near the end of the year, fanatic sky watchers finally got their highlight: Jupiter and Saturn came together for a spectacular show. Throughout December, the two planets came closer and closer to each other and culminated on the night of December 21, together with the winter solstice. Many stargazers had been eagerly awaiting this spectacle in the sky, and for a good reason, as it is a rare occurrence.

In the past, conjunctions of planets were often seen as signs of big events to come, such as the fall of an entire civilization or a natural disaster. Historical records show that humanity has had an interest in conjunctions for a long time, dating back to the eighth century. Geoffrey Chaucer, for instance, described the great conjunction of 1385 in his poem Troilus and Criseyde, and wrote: “Saturn and Jove, in Cancer joined, were that such a rain from heaven poured down.” Johannes Kepler, a German astronomer perceived great conjunction in the year 1603, and, coincidentally, even got to witness a supernova the next year. Subsequently, Kepler worked out that great conjunction must have also occurred in 7BC, and, in current times, this spectacle is often referred to as the Christmas Star.

The conjunction of Jupiter and Saturn happens once every 20 years and is therefore called a “great conjunction”. In astronomy, a conjunction is the alignment of two astronomical objects in the sky such as planets, moons, or asteroids, that appear to be very close to each other when observed from Earth. A conjunction between the moon and a planet happens almost every month; planetary conjunctions, on the contrary, are a rarer occurrence. The cause for this alignment of planets lies within their differing orbits. All planets in our solar system orbit the sun, and each of these orbits is elliptical and are slightly inclined regarding each other. According to NASA, it takes approximately 12 Earth-years for Jupiter to make a full orbit around the sun, and over 29 Earth-years for Saturn. The difference in the period of the two orbits enables the two planets to pass each other every twenty years. Even though during the conjunction, the planets appear to be very close to each other, this is merely an illusion – fact is, they are still very far apart. For instance, a conjunction between the moon and Mars may give the appearance that the two bodies are close to each other, but the average distance to the moon is 238,855 miles, whereas Mars is approximately 44m miles from Earth.

Even though the two planets pass each other in the solar system several times in a lifetime, not all the conjunctions can be seen equally well (or at all). The planets pass each other with various distances between them and can be closer or further away from the sun. In addition to this, the conjunction can happen either during the day or the night. All these variations in position and time are the cause of the variable visibility of the conjunction. The last time Jupiter and Saturn passed this close to each other was nearly 400 years ago, in 1623, while Galileo was still alive. However, the two planets were close to the sun when seen from Earth, which probably made it very difficult for Galileo Galilei, Johannes Kepler and all the other people living at that time to witness the event. It has been a staggering 800 years since the last time the merge happened at nighttime when it was clearly visible. According to NASA, this alignment occurred at just a tenth of a degree apart. Nevertheless, the actual distance between the two planets still extended over hundreds of millions of miles.

If you missed the alignment this time, then you for sure will need a large dose of patience. The next time Jupiter and Saturn will appear this close together again will be on 15 March 2080, in the early morning. But this time, the planets will be very difficult to see as they will be down low on the horizon at dawn. The next time the conjunction will be as clearly visible as this one was will be on 24 August 2414. And who knows which planet we will be able to observe it from then? However, there’s no need to worry; in the coming year, the skies will nevertheless be filled with many spectacular shows for all the avid skywatchers, such as meteor showers, a super moon, and even a conjunction of Venus and Mars.


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