December 21, 2020 Jupiter passes Saturn for a once-in-a-generation Great Conjunction
Once a generation, Jupiter catches and passes Saturn. Both planets move slowly around the sun because of their distance from our central star. A Jupiter year is nearly 12 Earth years long while Saturn revolves around the sun in nearly 30 years.
See our detailed article here.
Jupiter takes nearly 20 years to move past Saturn, travel around the sun, and catch Saturn again. When Jupiter catches Saturn on December 21, 2020, they will be very close, only 0.1° apart! This is the closest the planets have appeared since the Great Conjunction of July 16, 1623!
When viewing the sky, the actual sizes of objects are difficult to determine because there is no depth perception. We measure objects by their apparent angular size. Apparent is how large they seem to our eyes. Angular size is measured in degrees, like the way a protractor measures angles. The moon appears to be about 1/2° in diameter. At the Great Conjunction, Jupiter and Saturn are 0.1° apart. This seems to be close, but they are easily distinguished from each other. The image above shows the angular size of the moon and 0.1° of angular size on the moon. The large circular feature is the largest lunar feature, the Imbrium Basin, easily visible without a binocular or telescope. So the planets are close together, but they do not appear as a “single star” at the conjunction.
The image at the top of this article shows a close conjunction of Venus and Jupiter. Notice the separation of the two planets. At the 2020 Great Conjunction, Jupiter and Saturn are closer than this August 27, 2016 conjunction.
In 1961, the Jupiter – Saturn conjunction occurred in the morning sky, about 2° below 56 Sagittarii. (Note the star’s location on the accompanying chart, nearly 5° west of the Capricornus – Sagittarius border.) The 2020 conjunction occurs about 6° farther eastward, just east of the constellations’ border.
The chart above shows the motions of the planets against the background stars. Two apparent motions occur to the Bright Outer Planets – Mars, Jupiter, and Saturn. As Jupiter and Saturn emerge from their solar conjunctions, early in 2020, they appear higher in the sky when weekly observations are made. They somewhat match the annual westward march of the stars. This is caused by the earth’s revolution around the sun. The stars are a calendar. Over several human lifetimes, the same star is in the same position at the same time and same date each year.
The second motion is a combination of the planets’ slow orbit around the sun, especially for Jupiter and Saturn, since they don’t appear to move far during a year and Earth. Jupiter and Saturn appear to move eastward (direct motion) compared to the starry background. During the next year they are among a faint starfield in eastern Sagittarius and western Capricornus. As our planet catches up and passes between them and the sun (opposition), they appear to move westward (retrograde motion) compared to the stars — retrograde motion. After Earth passes them, Jupiter and Saturn seem to resume their direct motion compared to the background that moves farther west and rise earlier as the seasons progress.
Jupiter finally catches Saturn in late December for this Great Conjunction. On the chart, notice that Mars passes Jupiter and Saturn during late March 2020.
1623 Jupiter – Saturn Great Conjunction
The Jupiter – Saturn conjunction of 1623 occurred in the wake of the invention of the telescope, so observing was in its infancy; yet, the sky was full of planetary activity. A partial lunar eclipse (April 15, 1623) was visible throughout the Americas and in Central Europe, where the moon was setting as the eclipse reached its 90% magnitude. Venus passed Jupiter and Saturn in late June and Mercury passed the planetary pair less than two weeks later, when the planets were about 22° east of the sun. With the inner planets in the vicinity of the impending Great Conjunction and Mars reaching opposition (July 4, 1623), surely sky watchers were observing the planets’ locations to test and revise their planetary motion equations.
By the time of the Great Conjunction on July 16, 1623, the planetary pair was less than 13° east of the sun. During bright twilight, the pair was near the horizon at mid-latitudes. Without optical help, the conjunction likely went unobserved, even for those with recently minted telescopes. (The telescope was documented to observe the sky by Galileo in 1610.) Even then, the observer needed some luck to find the conjunction. It’s very likely there is no record of the last time the planets appeared this close together.
In recent times, Great Conjunctions occurred February 18, 1961; followed by a triple conjunction of the two planets in 1980-81; and the last occurred May 30, 2000, although this was difficult to observe.
In the simplest description, a triple conjunction occurs when faster moving Jupiter overtakes slower moving Saturn before they reach opposition. Then as the planets retrograde, Jupiter again passes Saturn. After Jupiter begins its direct motion, it passes Saturn a third time. It should be noted that the two planets’ 2020 apparitions coincide with an apparition of Pluto. Jupiter has a triple conjunction with Pluto during this apparition. The conjunctions are listed in the highlights, but a detailed finder chart is not included here. I encourage those with the desire to see Pluto near Jupiter and have sufficient apertures to consult other sources that provide detailed guidance to find the distant, dim planet.
As 2019 closes, the Great Planets, Jupiter and Saturn, are near their solar conjunctions. Jupiter’s occurs December 27, 2019, followed 17 days later by Saturn. They begin a slow climb into the morning sky and toward their Great Conjunction that occurs December 21, 2020.
The monthly summaries that follow document the locations of Jupiter and Saturn through their conjunctions later in the year.
The year 2020 may be the time to purchase your first telescope. At their closest, Jupiter and Saturn appear in a low power telescope. Jupiter’s four largest moons are visible as well as Saturn and its largest moon, Titan.
Monthly Summaries of What to Watch
(Bookmark this page to return for monthly updates of the planets’ locations.)