On the near horizon just after sunset, a rare celestial occurrence awaits the earthlings on December 21, 2020. On this day, Jupiter and Saturn will come together in a “great conjunction” — the closest they’ve appeared together in nearly 800 years on the day of the winter solstice. It was last closely observed in 1623, when the solar system’s two giant planets, Jupiter and Saturn, traveled together across the sky and Jupiter caught up to and passed Saturn, in an astronomical event known as a “Great Conjunction.”

Astronomers use the word conjunction to describe meetings of planets and other objects within the sky, as seen from Earth. They use the term great conjunction to describe meetings of Jupiter and Saturn, which are the two biggest worlds in our solar system. Though the two planets will appear spectacularly close together on the sky’s dome now, Jupiter and Saturn are actually 456 million miles (734,000 million km) apart. Saturn is 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.

From the years 2000 to 2100 inclusive, as viewed from our planet Earth, these Jupiter/Saturn conjunctions (in ecliptic longitude) happen on these dates:

May 28, 2000

December 21, 2020

October 31, 2040

April 7, 2060

March 15, 2080

September 18, 2100

Why this year’s Conjunction is special?

This time it is even more special as the angular separation between the two planets in the sky will be a mere 0.1° or 6′ – 6 minutes of arc – the closest they have been since 16 July, 1623 (nearly 400 years). However, the smallest separation then occurred during the daytime. The smallest nighttime separation occurred last time in 1226, nearly 800 years ago.

Pic courtesy: ARIES, Nainital

On December 25, Christmas Day, will be the last evening when Jupiter and Saturn will appear nearer to each other than the apparent diameter of the Moon.