A celestial spectacle of Jupiter and Saturn is set to unfold after 400 years

The largest and second-largest planets will come closer on December 21 in what is known as the 'great conjunction'; they will appear so close to each other that we would not be able to distinguish them apart with the naked eye

The 'great conjunction' of Jupiter and Saturn will occur on December 21, 2020

A great celestial show is set to lift the gloom of 2020, as the year comes to a close. On December 21, after almost 400 years — 397 to be precise — Jupiter and Saturn, the largest and the second-largest planets, respectively, are set to come closer. Called the ‘great conjunction’, the two planets will appear so close to each other that we would not be able to distinguish them apart with the naked eye.

Both the planets are hard to miss during this month as they will appear after sunset, in twilight. The best time to observe the planets would be around 7 pm on December 21. The planets will appear low in the sky, just above the horizon. Hence choose a location with an unobstructed view–preferably a rooftop of a tall building or an open ground– to the west of south-west direction.

Also read: Scientists use AI to discover two planets from space telescope data

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Before the ‘great conjunction’ on December 21, do not miss the spectacle of two planets along with crescent Moon on the evening of December 16.

What is a conjunction? 

Simply put, a conjunction is when two celestial bodies meet each other in the sky. Imagine an athlete running on a track. The fast-paced competitor will catch up with the one running before and overtake at some point. The orbits of the planets are almost aligned in a plane. Hence, in the sky, the path of the celestial bodies, the sun, the moon, and the five visible planets, Mercury, Venus, Mars, Jupiter and Saturn, are seen to traverse along a path called the ecliptic. The speed at which the planets move is not the same, hence, like the athletes, the fast-moving planet will catch up with the slower one from behind. Periodically, two celestial bodies will appear next to each other.

Also read: Exomoons may be home to alien life: Study

What is the ‘great conjunction’? 

When the two giant planets, Saturn and Jupiter, meet each other, it is called as the ‘great conjunction’. With 29.4 years orbital period, Saturn moves much more slowly across the sky. Saturn appears to stay in each rasi (zodiac), the 12th division of ecliptic, for about 2.5 years. In comparison, the sun appears in each rasi for about a month, and the moon, about 2.5 days. Jupiter takes only 11.86 years for one complete rotation. Hence, it seems to stay put in each rasi for about one year. Therefore, every 19.85 years they appear to pass each other in the night sky, resulting in a ‘great conjunction’.

Further, once they meet at a particular location in the night sky, they meet in the same part of the sky after 60 years; note that 60 is the least common multiple of 12 and 30, the approximate orbital period of Jupiter and Saturn, which is why in South India, the sixty-year cycle of names are used. According to this cycle, 2020-21 is Saarvari and 2021-22 will be Pilava.

What is unique about this year’s great conjunction? 

Although planets traverse nearly along the ecliptic path, the actual tracks are slightly tilted. Hence when two planets meet, the angular distance between them would differ from one to another. This time, during the ‘great conjunction’, the gap between Saturn and Jupiter will be just 0.1 degrees. Only those with good distant vision would be able to spot the Arundhati (Alcor) from Vashista (Mizar), which have an angular distance of 0.2 degrees. Therefore, on December 21, when the ‘great conjunction’ takes place, Jupiter would appear to ‘rub shoulders’ with Saturn. This is the closest since July 16, 1623. The last time when the great conjunction took place, on May 31, 2000, the separation was 1.18 degree. The next time when it will take place, on November 5, 2040, the gap would be 1.23 degree. Once again, such a close encounter, with a separation of just 0.1 degrees, will take place on March 15, 2080. Little wonder, then, that astronomy enthusiasts bill this event as ‘once in a life time opportunity’.

Will there be any harm?

No. The orbit of Jupiter is closer than the orbit of Saturn and hence they will not collide into each other. In fact, on that date, Saturn will be about twice as far from Earth as Jupiter will be. The distance between Jupiter and Saturn will be around 600 million kilometres. The event is safe to watch with naked eyes, though with a small binocular the view would be even more spectacular. No harm will come to Earth or humans, and this is a periodic event, as the full moon or eclipse.

The conjunction of other planets

Conjunction can take place between any two planets. For example, next year, on July 13, 2021, a conjunction of Venus and Mars will take place. Subsequently, on April 5, 2022, there will be a conjunction of Mars and Saturn, on April 30, the conjunction of Venus and Jupiter and on May 29, Mars and Jupiter.

Conjunction and Indian astronomy

Conjunctions go by different names in Indian astronomy. When it is a conjunction of one planet with another it is called as yuddha (encounter), a planet and the Moon is called samagama (union), with the Sun is called astamaya, and the conjunction between Sun and Moon is the familiar grahan (eclipse).

Watch them getting closer

Stretch your arm and open your palm: the width of the tip of your little finger at arm’s length is about one degree. Using this, commence measuring the distance between the planets from today. All during this month, after sunset, low in the sky, both the planets are visible in the south-west direction. You can notice the distance between them rapidly reducing every day, and after December 21, the gap increasing day by day.

Fun facts

In an exceptional occasion, such as on June 17, 7541, that is 5521 years later, Jupiter will perfectly align with Saturn. Then Jupiter will completely eclipse Saturn.

While the planets go around the Sun, as seen from Earth, they may take retrograde motion (Vakragathi). If the great conjunction takes places at such time, there may be as high as three great conjunctions in a year. Called ‘triple conjunction’, last time it occurred in 1981, and it will next take place next in 2223.

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