Jupiter, Saturn’s, Mars And Moon Alignment on December 21, 2020

Astronomers use the word conjunction to describe meetings of planets and other objects on our sky’s dome. They use the term great conjunction to describe meetings of the two biggest worlds in our solar system, Jupiter and Saturn. The December 21, 2020, great conjunction of Jupiter and Saturn is already underway! It’s highly conspicuous in the west after sunset each evening now. On December 21, the pair will be only 0.1 degree apart. Some say the pair will look like an elongated star on that date. Will they? Or will they look like a double planet? What if you’re clouded out? How can you watch online? No matter how you see them, or which evening between now and December 21, Jupiter and Saturn will surely be an appealing and mind-expanding sight. Just don’t miss them! Though the two planets will appear spectacularly close together on the sky’s dome, Jupiter and Saturn are actually 456 million miles apart. They are lined up from our point of view, but Saturn is in fact nearly twice as far away as Jupiter. Jupiter-Saturn conjunctions happen every 20 years; the last one was in the year 2000. But these conjunctions aren’t all created equal. The 2020 great conjunction of Jupiter and Saturn will be the closest since 1623 and the closest observable since 1226! 2020’s extra-close Jupiter-Saturn conjunction won’t be matched again until the Jupiter-Saturn conjunction of March 15, 2080. At the 2000 great conjunction, 20 years ago, Jupiter and Saturn were near the sun in our sky and difficult to observe. We’re due for a more observable great conjunction. Jupiter and Saturn are up every evening now – not far from the sunset glare – easily visible and exceedingly noticeable as two bright objects near each other. Plus, in the days prior to the conjunction, the young moon is appearing in the evening sky, to point the way to the planets. Look below for a chart showing the pair near December’s young moon … and for a poster of an online viewing session.

Jupiter is brighter than any star. Saturn isn’t as bright as Jupiter, but it’s as bright as the brightest stars and shines with a distinctly golden color. Saturn is just to the east of Jupiter on the sky’s dome. The two are noticeable for their brightness and nearness to each other. While you’re looking at them, you might notice that – unlike the twinkling stars – Jupiter and Saturn both shine steadily. In November – during the period when the moon swept past Jupiter and Saturn (about November 16 to November 21) – the two gas giant planets were some 3 degrees apart. Over the time between November 21 and the day of the conjunction itself, December 21, Jupiter will travel about 6 degrees and Saturn 3 degrees on the sky’s dome. That movement will mean that Jupiter bridges the 3-degree gap between itself and Saturn. Saturn is the sixth planet outward from the sun. It’s the farthest and slowest-moving planet that we can easily see with the eye alone. Dazzling Jupiter, the fifth planet outward from the sun, is the second-slowest bright planet, after Saturn. It’s by virtue of their slow motions in front of the constellations of the zodiac that Jupiter and Saturn have the least frequent of bright-planet conjunctions. Saturn takes nearly 30 years to go around the sun, while Jupiter takes nearly 12 years. Thus, every 20 years, Jupiter catches up to Saturn as viewed from Earth. From the years 2000 to 2100 inclusive, as viewed from our planet Earth, these Jupiter/Saturn conjunctions (in ecliptic longitude) happen on these dates: Now, here’s why these great Jupiter/Saturn conjunctions happen every 20 years. Each year, Saturn completes about 12 degrees of its orbit around the sun, whereas Jupiter completes about 30 degrees. Therefore, in one year, Jupiter closes the gap between itself and Saturn by about 18 degrees (30 – 12 = 18 degrees). In a period of 20 years, then, Jupiter gains 360 degrees on Saturn (18 x 20 = 360 degrees), therefore lapping the ringed planet once every 20 years. So start watching Jupiter and Saturn now! And mark your calendar for the great conjunction of Jupiter and Saturn on December 21, 2020.
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