How and when to see the Great Conjunction of Jupiter and Saturn in December 2020

 

The “great conjunction” of Jupiter and Saturn will occur on December 21, 2020 – the northern hemisphere’s winter solstice. At that time, the two planets will  be in the constellation Capricornus, low toward the southwest horizon, and separated by a mere 0.1°.  
This will be the closest Jupiter/Saturn conjunction since the year 1623 CE! Jupiter will be at magnitude -2.0, and significantly dimmer Saturn at magnitude +0.6. 

When and How to observe? 

• You can Start today! 
• The two planets are impressive sight easy to locate in the southwestern sky, after the sunset.  Using the little finger as a measuring instrument, you can note how they are getting closer with each passing day.The width of the tip of your little finger at arm's length is about one degree.  You can watch perfect Conjunction on the evening of December 21, 2020.
•  As the twilight falls, Jupiter and Saturn will appear low in the sky. Look in the direction of the south-west. They will be so bright that it is hard to miss. The planets will set around 1930 IST, and hence the best time to watch is between the local sunset and 1900 hrs IST.

What is "Conjunction" ?

The inclination of the orbits of the planets around the Sun is more or less the same. Therefore, the five visible planets, Mercury, Venus, Mars, Jupiter, Saturn and the Moon, appear to follow the same path that the Sun takes in the sky. As athletes run along the track, one whose speed is higher catch up the slower one from behind. Likewise, as the speed at which one planet appears to move in the sky are not same, once in a while, the fast-moving one will catch up with the relatively slow-moving planet in the sky. Such events are called 'conjunction'. 

What is "Great Conjunction" ?

A “great conjunction” is a conjunction of the planets Jupiter and Saturn. Great conjunctions occur regularly, about every 20 years, due to the combined effect of Jupiter's approximately 12-year orbital period and Saturn's approximately 30-year orbital period. As it travels around the Sun, Jupiter catches and moves past slower orbiting Saturn – in a somewhat regular 20-year pattern. 

Once a lifetime opportunity: The last time Jupiter and Saturn were this near was on July 16, 1623. The next time they will come again this close will take place only in 2080.

How rare is this "Great Conjunction"?

• On the 21 December solstice, the planets will look like one brilliant star as Jupiter’s and Saturn’s 12- and 29-year orbits bring them together. The last great conjunction was in May 2000, but its position in the sky meant it was difficult to see. The great conjunction of 1623 (as Galileo Galile's study) was also hard to spot because, the Perth Observatory explains, it appeared close enough to the sun that it would have been “lost in the sun’s glare”.
• A conjunction of Jupiter and Saturn occurs every 20 years. However, this time they will be exceptionally close. Last time when the great conjunction took place in 2000, the separation was 1.18 degree. When it again takes place in 2040 the separation will be 1.23 degree and in 2060, 1.15 degree. Which means the next two times, when the conjunction takes place, the planets will be eleven times farther apart than this year.

How Jupiter “Laps” Saturn Every 20 Years ?

Each Earth year, Saturn completes about 12º of its counterclockwise orbit, around the Sun along the ecliptic (360º ÷ its orbital period of 29.5 Earth years); whereas Jupiter, on its speedier “inside lane”, completes approximately 30º for each Earth year (360º ÷ its orbital period of 11.9 Earth years). 

Therefore, in one Earth year, Jupiter closes the gap between itself and Saturn by about 18º (30º – 12º = 18º). 

So, in a period of 20 Earth years, Jupiter has orbited around for another pass of Saturn along the ecliptic (360º ÷ 18º per yr. = 20 years), thereby lapping the ringed planet once every 2 decades.

What after this Conjunction?

• The last time Jupiter and Saturn were this near was on July 16, 1623. The next time they will come again this close will take place only in 2080.
• After  December 21, 2020 conjunction, the two planets will appear to trade positions, as Jupiter overtakes Saturn. Jupiter will progress to the east and draw farther and farther away from Saturn  throughout the decade of the 20s. After that, the two planets will approach each other through the  2030s, reaching conjunction again in November 2040. Beyond that, Jupiter-Saturn conjunctions will  occur in April 2060, March 2080, and September 2100.

When is next Conjunction ?

Jupiter and Saturn, which takes 11.86 years and 29.4 years, respectively to go around the Sun. Therefore, every 19.85 years they are aligned in a straight line with Earth, resulting in great conjunction. Likewise, other planets also produce periodic conjunction as they orbit Sun. For example, next year, on July 13, 2021, a conjunction of Venus and Mars will take place.

-PVGaniga