Stargazers are in for a surprise this Christmas. If you happened to see Jupiter and Saturn move by the crescent Moon this week, you'd have noted something else concerning the two main planets in the Solar System.
Jupiter and Saturn in Earth's night sky can become closer to each other than they have been for almost 800 years. For the first time since the Middle Ages, these two would appear like double planets."
The planets are now very similar to each other and they would almost seem to collide to form one super-bright point of light on December 21, 2020.
The day is, coincidentally, the day of the solstice in December. That would be the first conjunction between Jupiter and Saturn since the year 2000, and the nearest conjunction between Jupiter and Saturn since 1623, just 14 years since Galileo made his first telescope.
They won't be near at all, of course. Think of the distance from the Sun to the Earth. This is what scientists term an astronomical unit (au), and this is how distances in the vastness of the Solar System are calculated. Jupiter is around 5 AU from us. Saturn is AU 10.
IN SPACE - APRIL 25: In this artist's impression supplied by the ESO (European Southern Observatory) on April 25, 2007, the planetary system around the red dwarf, Gliese 581, is pictured showing what astronomers believe is the most earth like planet found outside our solar system to date. Using the ESO 3.6-m telescope in Chile, astronomers have uncovered the planet which could have water running on its surface. The planet orbits the faint star Gliese 581, which is 20.5 light-years away in the constellation Libra.
Unusual Planetary Alignment
The orbital resonance of Jupiter and Saturn is such that they coincide every 19.6 years in a "great conjunction." Still, they are not as similar as on December 21, 2020, when the two planets are divided by less than the full Moon's visible diameter, from our point of view on Earth.
According to EarthSky, the term conjunction is used by astronomers to define meetings between planets and other structures in the dome of our sky. To define the encounters of the two largest worlds in our solar system, the mighty Jupiter and the majestic ringed planet Saturn, they use the word great conjunction.
Some like the famous German astronomer Johannes Kepler, claim that a remarkable triple alignment of Jupiter, Saturn and Venus may have been the "star of Bethlehem" in the tale of the Magi or "three wise men."
What to Expect
Anywhere on Earth where the sky is bright, the unusual celestial phenomenon would be detectable.
When seen from the northern hemisphere, the planets will appear low in the western sky for around an hour after sunset, and while they will be nearest on December 21, 2020, you will look at them that week each evening.
While the vision will sink below the horizon, it will remain light enough to be seen at dusk.
How to Watch
You need an uninterrupted view to the southwest and aim to the southwest from where you are about 45 minutes after sunset.
For telescope viewers, each planet and many of its larger moons will be visible in the same field of view that evening on December 21, 2020. This great conjunction would not arise again until March 15, 2080.
Stargazers should never miss this conjunction, by the way. Patrick Hartigan, an astronomer from Rice University, said that alignments between these two planets are rather rare.
But don't wait to start watching these worlds by December. They are clear tonight and every night for the rest of 2020, next to each other, an enticing and mind-expanding sight!
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