Dec 21: Jupiter & Saturn’s Great Conjunction in 397 years

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The year 2020 will see one of the rarest and biggest astronomical phenomena on the night of the winter solstice, Jupiter and Saturn’s ‘great conjunction’ in almost 4 decades on December 21.

Jupiter Saturn in december 2019
Jupiter Saturn on 21st December 2019 (Image Source: solarsystemscope.com)

When the two planets appear incredibly close together in the sky because they line up with Earth with their respective orbits is called a conjunction. Close approach of these two planets is called Great Conjunction as they are rarest of all the conjunction that is visible to the naked eye. Although the conjunction between Jupiter and Saturn happens nearly every 20 years. But not all these conjunctions are equally dramatic as sometimes it happens close to the sun. In the year 2000, this conjunction was a mere 3° away from the Sun that’s why it was not visible to earth because of the sun’s glare. This year’s conjunction is special because of how close to each other they will appear.

Jupiter Saturn conjunction 0.1 apart
Jupiter – Saturn Conjunction

Jupiter and Saturn have been traveling across the sky together all year and on December 21st, 2020 the two giant planets will appear just a tenth of a degree apart (0.1 degree apart) – that’s about the thickness of a dime (a US 10-cent coin) held at arm’s length! according to the National Aeronautics and Space Administration (Nasa) skywatching statement. This means the two planets and their moons will be visible in the same field of view through binoculars or a small telescope. In fact, Saturn will appear as close to Jupiter as some of Jupiter’s moons.

This approach of the two largest planets of the solar system, which will be the closest on 21st Dec 2020 since the year 1623 that is almost in 400 years. But in the year 1623 great conjunction wasn’t visible due to the sun’s glare. According to Virginia Tech astronomer Nahum Arav, the last time the event was clearly visible from the Earth was in the year 1226 this means that the last people to witness this rare astronomical phenomenon is our ancestors that is almost 800 years before. The next time Jupiter and Saturn conjunction this close will be happening again in the year 2080.

solar system top view
Jupiter and Saturn are about 643 million kilometres apart – Solar system top view – Earth – Jupiter – Saturn (Image Source: solarsystemscope.com)

As a result of their long distances from the Sun (Jupiter: 763.52 million km and Saturn: 1.4916 billion km) It takes Jupiter 12 years to orbit the Sun and Saturn takes nearly 29 years, this means in every 19.8 years Jupiter overtakes Saturn. This is why the conjunction takes place in every 20 years. As Jupiter and Saturn will form a bright single star-like occurrence a few days before Christmas, sometimes this conjunction also called Christmas star also known as Star of Bethlehem.

 DE430 planetary ephemeris computed by the Jet Propulsion Laboratory (JPL).
Source: DE430 planetary ephemeris computed by the Jet Propulsion Laboratory (JPL).

The chart above is based on the DE430 planetary ephemeris computed by the Jet Propulsion Laboratory (JPL). Comet positions are computed from orbital elements published by the Minor Planet Center (MPC). A graph of the angular separation between Jupiter and Saturn around the time of closest approach is shown above. In the above 2 months, deg vs 1-week graph you can see on the 21st Dec gap between the two planets comes to 0.1 degree. And as days go the gap increases in degree.

How to watch conjunction?

Jupiter and Saturn will appear as a point of light (star) that does not twinkle on Dec 21 when looked at by the naked eye. It will appear brighter than every other star.

Jupiter Saturn Conjunction view from India at 626 PM IST
Jupiter Saturn Conjunction view from India at 626 PM IST (Image Source: timeanddate.com)
  • Jupiter and Saturn alignment will be visible in the west – southwestern direction.
  • Both the planets will lie in the constellation Capricornus low towards the southwest horizon.
  • Jupiter will be at magnitude -2.0 and dimmer Saturn will be at magnitude +0.6.
  • The conjunction will be seen as a single bright ball or dot in the western sky by the naked eye.
India view Jupiter Saturn conjunction
India view Jupiter Saturn conjunction (Image Source: solarsystemscope.com)

It’s a big coincidence that both great conjunction and winter solstice are occurring on the same day. On 21st December because of winter solstice day will be the shortest in the northern hemisphere and the longest night of the year. On 21st Dec Sunset will be at 6:05 PM IST in India. We will be able to see the conjunction of these two planets followed by Sunset. The closest approach of these two planets will be at 18:20 UTC (Universal Time) which is in India it will be at 11:50 PM. Unfortunately both the planets will set at 8:20 PM in India.

Jupiter Saturn conjunction telescopic view
Jupiter Saturn conjunction simulated telescopic view from India (Image Source: stellarium-web.org)

During the great conjunction, Jupiter and Saturn will be close enough to easily fit within the same field of view of a telescope eyepiece. The planet’s angular separations of a mere 6 arcminutes (0.1°) are only one-fifth of the diameter of the moon. The above image simulation is created by stellarium-web.org. With the use of the telescope, one can easily view both planet’s moons in a single field of view which is very rare to capture.

This cosmic phenomenon can be easily captured by cellphones, especially which have night mode will give a good quality view of the conjunction. With a DSLR camera, you can take very clear pictures including the nearest moons of Jupiter and Saturn with proper settings.

Next conjunctions will be happening on November 2, 2040, and April 7, 2060, on both these occasions the minimum separation will be 1.1° which means they will be 11 times apart from the December 21st great conjunction. Over a period of one thousand years from 1600 to 2599, there are only six great conjunctions where the minimum separation between Jupiter and Saturn will be less than 0.2° 1623, 1683, 2020, 2080, 2417, and 2477.

Over the next Ten Thousand years Jupiter will transist or occult Saturn 3 times that is on February 16, 7541 (a Transit); June 17, 7541 (an occultation); and February 25, 8674 (a transit). If Jupiter partially obscures Saturn, the event is known as a transit, and If Jupiter completely covers Saturn, it is called an occultation. The way technology is evolving, maybe in the next 5000 years, humans may live on Saturn, Mars, or some other planets.

Don’t forget to look out in the western direction just after the sunset and try to capture pictures of this conjunction as it is once in a lifetime opportunity to observe this rare astronomical phenomenon on December 21. If you want to make separations you can capture and see the difference in the positions of these two planets by taking pictures every day.

kotian
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