09/12/2020
Close approaches of the two planets are called great conjunctions because they are the rarest of all conjunctions between the planets that are visible to the naked eye. The last time such a conjunction was observable was in 1980. The most recent great conjunction, in 2000, was not observable since it took place while the two planets were a mere 3° away from the Sun.
The rarity of great conjunctions is due to the slow motion of Jupiter and Saturn across the sky. Among the planets that are visible to the naked eye, they are the two most distant from the Sun, taking 11.86 years and 29.5 years respectively to orbit it. As the two planets gradually move through the constellations at different speeds, they follow almost the same path across the sky, called the ecliptic. Periodically, Jupiter catches up with Saturn and overtakes it, resulting in a great conjunction, on average once every 19.6 years.
Not all of these great conjunctions are equally dramatic. Sometimes they happen when the planets are too close to the Sun to be observable, as happened in 2000. And at other times they may not pass any closer than five degrees (the width of ten full moons) apart.
21 December 2020: Jupiter and Saturn will make their closest approach since the year 1623. – In-The-Sky.org.
