2020: December 21: The Great Conjunction of Jupiter and Saturn

by Jeffrey L. Hunt

On December 21, 2020, in a close conjunction, Jupiter passes Saturn in the evening sky This is known as a Great Conjunction.  Look toward the southwest about one hour after sunset.  The bright “star” is Jupiter.  Dimmer Saturn is immediately to the Giant Planet’s upper right.

In the weeks leading up to the conjunction, Saturn is to the upper left of Jupiter. The Jovian Giant slowly dances toward Saturn. They can be found in the south-southwestern skiy after sunset. Jupiter appears as an overly bright “star.” Dimmer Saturn is to its upper left. Each night Jupiter appears slightly closer to Saturn. This seems to be a slow-motion event.

Read more about the  planets during December 2020.

On the evenings of December 20 and December 22, Jupiter is nearly as close to Saturn as on the conjunction evening.

On conjunction evening, the two planets do not “merge into a single star.” While they appear very close together, they are far enough apart to be seen as separate points of light.

To make a photographic record of the conjunction, see hints and examples here.

Once a generation, Jupiter catches and passes Saturn. This is known as a Great Conjunction.  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.  A Jupiter-Saturn conjunction is rare enough for observers to take notice of this unique pairing.

See the gallery of Jupiter and Saturn images of their planetary motion during 2020 here

Jupiter takes nearly 20 years to move past Saturn, travel around the sun, and pass Saturn again.  When Jupiter passes Saturn on December 21, 2020, they will be very close, only 0.1° apart!  This is the closest conjunction since the Great Conjunction of July 16, 1623!  The next Great Conjunction is October 31, 2040, when the two planets are 1.1° apart.  At the next conjunction the planets are low in the east-southeast before sunrise.

November 25, 2020 Update:  Patrick Hartigan from Rice University has generated a list of Great Conjunctions spanning 3000 years.  The dates may be off a day or two from the actual conjunction dates. His list includes the following close conjunctions:

• March, 1226, separation 2.1′, one-third the separation of 2020.
• August, 1563, separation 6.8′, slightly larger than 2020.
• July, 1623, separation 5.2′, slightly less than 2020, but not likely visible.

So how do we properly describe this?  Closest since 1623? Yes, although not likely observed.  Closest since 1563?  Yes.  This was easily visible in the morning sky.  Closest observable since 1226?  Yes, this was clearly visible as well. 

November 2020

As viewed from above the solar system, Jupiter passes Saturn in a heliocentric conjunction on November 2, 2020, 49 days before the great conjunction.  The two planets are in along a line that starts with the sun and connects both planets.  As viewed from Earth on this date, Jupiter and Saturn are 5 ° apart.  (See this article for further explanation.)

December 2020

To follow the planet throughout the year, download the daily notes that are linked at the top (current month) and bottom (cumulative index of notes) of this article.  Here’s what to look for during December 2020.

Look low in the southwest, one hour after sunset.  Bright Jupiter is easy to locate.  Dimmer Saturn is nearby, to Jupiter’s upper left.

On December 16, the crescent moon enters the scene. One hour after sunset, the crescent moon (2.3 days past its New phase, 7% illuminated) joins the planets. It is over 6° up in the southwest, about 5° below Jupiter. The Jupiter – Saturn gap is 0.5°.  This is about the apparent diameter of the moon.  Find a clear horizon toward the southwestern sky to find the scene.

Not until March 17, 2080, does the moon appear near a very close grouping of Jupiter and Saturn. The moon passes Jupiter and Saturn every month, but the two planets are near each other every 19.6 years. Other Great Conjunctions occur in 2040 and 2060, but the conjunction distance is over 1°.

Each night thereafter, Jupiter closes on Saturn, until conjunction evening when they are 0.1° apart.  This is close enough to see them in through at a telescope’s low power. Here is the detailed note for conjunction evening:

• December 21: Jupiter – Saturn Great Conjunction! One hour after sunset, Jupiter is about 12° up in the southwest, 0.1° to the lower left of Saturn. They are 30° east of the sun. Both fit into the eyepieces of modest telescopic powers. Jupiter’s Galilean Satellites are nicely lined up along the equatorial plane of the planet. Ganymede, Io, and Calisto are east of Jupiter, and Europa is west of the planet. Titan is nicely placed to the northwest of Saturn. After the conjunction, Jupiter moves eastward along the ecliptic, separating from Saturn. Each evening the planetary pair appears lower in the sky.

Through a telescope – with an eyepiece that is in the 50x-60x magnification range – Jupiter and Saturn are visible in the same field.  Saturn’s rings are easy to locate.  Jupiter’s four largest and brightest moons – Io, Europa, Callisto, and Ganymede – are evident as well.  On closer inspection, some of Jupiter’s cloud bands are visible.  Some telescopes invert the view compared to the diagram shown here.  Others flip the image left to right.  So the actual view through a telescope may look differently than what is shown here.

Read about the 1623 Jupiter – Saturn Great Conjunction here.

Monthly Summaries of What to Watch

(Bookmark this page to return for monthly updates of the planets’ locations.)

• December 2020
• November 2020
• October 2020
• September 2020
• August 2020
• July 2020
• June 2020
• May 2020
• April 2020
• March 2020
• February 2020
• January 2020