Once in a lifetime: Earth, Jupiter & Saturn to align in Great Conjunction on 21 Dec, after nearly 400 yrs

Once in a lifetime opportunity

About 30 minutes after the local sunset, in the southwestern direction, low in the sky, two giant gas plants, Saturn and Jupiter, will appear so close that we will not be able to distinguish them apart. The last time Jupiter and Saturn were this near was on July 16th, 1623. The next time they will come this close again will be only in 2080.

What is the ‘great 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, the one whose speed is higher catches up with the slower one from behind. Likewise, as the speed at which planets appear to move in the sky are not the 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’. When the jumbo planets, Jupiter and Saturn, come close to each other, it is called the ‘Great Conjunction’.

How frequently does it happen?

Jupiter and Saturn take 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 the great conjunction. Likewise, other planets also produce a periodic conjunction as they orbit the Sun. For example, next year, on July 13, 2021, a conjunction of Venus and Mars will take place.

Why is it so special this year?

The orbits of Saturn and Jupiter around the Sun are not on the same plane. So when they catch up with each other, the distance between them differs from one time to another. This time the separation between them will be only 0.1 degrees. In comparison, the angular distance between Arundhati (Alcor) and Vashista (Mizar) is 0.2 degrees. Hence, only if you have exceptional eyesight, will you be able to distinguish them. Since Jupiter will be brighter than Saturn, they will also shine like one single shining object in the sky.

Can’t I wait for the next one?

A conjunction of Jupiter and Saturn occurs every 20 years. However, this time they will be exceptionally close. The last time the great conjunction took place in the year 2000, the separation was 1.18 degrees. When it takes place again in 2040, the separation will be 1.23 degrees; and in 2060, 1.15 degrees. Which means the next two times the conjunction takes place, the planets will be eleven times farther apart than this year.

When should you observe them?

Start today. The width of the tip of your little finger at arm’s length is about one degree. Since the beginning of the month, Jupiter has been edging closer to Saturn. The two planets are impressive sights, easy to locate in the southwestern sky after sunset. On December 10th, the angular distance between them was only 1.12 degrees. Using the little finger as a measuring instrument, you can note how they are getting closer with each passing day. Of course, the grand show is on the evening of December 21st, 2020.

Where should you look for them?

Go to the top of a tall building, or a place where you have an unobstructed view of the western horizon. As twilight falls, Jupiter and Saturn will appear low in the sky. Look in a south-westerly direction. They will be so bright that they are hard to miss. The planets will set around 1930 IST, so the best time to watch is between the local sunset and 1900 IST.

Is there any possibility of being harmed?

The planets will not bump into each other at all. In fact, on December 21st, though Jupiter and Saturn will appear close together, they will actually be about 600 million km apart. They are safe to watch with the naked eye. Binoculars will be spectacular. Such events are periodic, and there is nothing to fear. No harm will befall either humans or the Earth.

Indian Astronomy: In ancient Indian astronomy, the conjunction of planets was known by different names. The conjunction of a planet with the Sun, i.e. a heliacal setting, is called ‘astamaya’; the conjunction of a planet with the Moon is called ‘samagama’ (union), and the conjunction of any two planets is called ‘yuddha’ (encounter). The conjunction of the Sun and the Moon is called ‘grahan’.

Amazing facts: If you can time travel to 7541 AD, you will witness two impressive celestial shows. On extraordinarily rare occasions, the great conjunction is so close that Jupiter is right in front of Saturn. When this occurs, either it partially eclipses (transit) or completely covers (occultation) Saturn. February 16th, 7541 AD there will be a transit and on June 17th, 7541 AD there will be an eclipse of Saturn by Jupiter. In fact, in the next 10,000 years only one more transit of Jupiter over Saturn will take place: on February 25th, 8674 AD.

Next conjunctions: A conjunction between Venus and Mars will take place on July 13th, 2021. In 2022, there will be a conjunction of Mars and Saturn on April 5th, and Jupiter and Venus on April 30th, and a conjunction of Mars and Jupiter on May 29th. There will be a solar eclipse on June 10th, 2021, and December 4th, 2021. Both of them will not be visible in India. Nonetheless, the two lunar eclipses, in May and November 2021, will be visible from parts of India.

By T.V. Venkateswaran

This information received via India Science Wire.

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