December 21, 2020: Jupiter passes Saturn in a close conjunction, known as a great conjunction. This conjunction is visible from across the globe. Across the world the planets are visible in the south-southwest after sunset. This is a slow-moving event that has been unfolding throughout 2020. This article describes how to observe and photograph this world-wide astronomical spectacular.
by Jeffrey L. Hunt
The December 21 conjunction is a slow-moving event, and on the evening before and after the conjunction, the two planets appear very close together, only slightly farther apart than on the conjunction evening. Watch Jupiter close the gap to Saturn until the conjunction. Step outside after sunset to find them in the southwest sky. A binocular is helpful; the pair is visible to the unaided eye as Jupiter overtakes and passes Saturn. On conjunction evening the planets fit into the eyepiece of a spotting telescope or small telescope at low power.
The photo at the top of the page shows an enlargement of the Venus – Jupiter conjunction of August 27, 2016. The separation of the planets was slightly larger than the Great Conjunction of 2020. Both planets were close together, but seen as distinctly separate “stars.”
The conjunction has been unfolding since Jupiter and Saturn appeared after their solar conjunctions earlier this year. Then they were visible in the morning sky in the east. As the year progressed, Jupiter started to close in on Saturn.
November 29, 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.
By early May, still largely appearing in the sky during the morning, Jupiter closed to within 5° of Saturn.
As Earth approached and passed both planets, they began to retrograde. This motion is an illusion as our planet overtakes and passes the two planets.
Earth passed between the sun and Jupiter on July 14. This is known as opposition. Saturn’s opposition occurred six days later.
Jupiter’s retrograde ended September 12, while Saturn’s ended September 28. When Jupiter’s retrograde ended, the gap to Saturn re-opened to 8.1°.
Since early summer, the planets have been visible in the southern after sunset.
During mid-November, the planets are in the south-southwest after sunset. They are 3.7° apart on November 15. They appear in front of the stars in eastern Sagittarius.
It’s easy to watch the progress of the planets as compared to the starry background. A binocular is helpful to watch this progress, but not necessary.
The planets are moving eastward compared to a dim star named 56 Sagittarii (56 Sgr on the photos). With a binocular, the planets’ changing place is easily observed.
To the unaided eye, the shrinking gap between Jupiter and Saturn is easily noticed. On the closest nights, Jupiter and Saturn are visible as individual “stars.” They do not merge into a single point.
Here is a slideshow with images since the planets appeared in the morning sky earlier during 2020.
A tripod-mounted camera with exposures ranging up to 10 seconds can capture the planets and the background stars, as those included in this article. Exposures range from 2.5 to 10 seconds in the above photos.
A smartphone camera, if steadily held, can capture the planets during a typical exposure with no settings changes as the above image illustrates.
From the southern hemisphere, Jupiter and Saturn appear in the south-southwest, although Jupiter appears to the upper left of Saturn. Leading up to the great conjunction, observers can see Jupiter approach Saturn, except from the upper left, opposite from northern hemisphere observers.
The chart above, shows the sky from Earth’s southern hemisphere (Santiago, Chile, latitude 33°S) on December 21, 20220, along with the moon and Mars.
From the northern hemisphere, on December 1, 2020, the planets are 2.1° apart.
By December 7, the gap between the planets is 1.5°.
On December 14, the Jupiter- Saturn gap is 0.7°. The tip of your pinky finger held at arm’s length fits between the two planets in the sky.
On December 16, the crescent moon joins the scene and the planets are 0.5°. After this evening and until about December 25, your pinky finger covers both planets.
The separation of the two planets is about the diameter of the Imbrium Basin on the moon, a large impact crater that was flooded with lava than hardened into dark rock. The feature is easily visible to the unaided eye.
For about the next ten evenings, the planets are close together and are a can’t miss for anybody wanting to see this conjunction.
December 21, the Great Conjunction! The planets are closest.
A small telescope (even one used for birdwatching) reveals Jupiter’s four largest moons with and planet and Saturn in the same field of view.
The planets begin to slowly separate. They appear lower in the sky and disappear into the sun’s glare during early 2021.
Jupiter slowly moves away from Saturn as both revolve around the sun. Jupiter catches Saturn again October 31, 2040.
April 19, 2021: The first evening appearance of Venus for this apparition occurs this evening. Look for it low in the west-northwest about 20 minutes after sunset.
April 19, 2021: Venus begins to appear in the west after sunset. The moon lines up with Pollux and Castor, while Mars is above Bull’s horns in the western evening sky.
April 19, 2021: The bright morning planets, Jupiter and Saturn, are in the southeastern sky before sunrise. Capricornus is the starry background for this giant planet duo.
April 18, 2021: The crescent moon is high in the west after sunset among the stars of Gemini, below Pollux and Castor. Mars is above the Bull’s horns. Daylight is 13 hours, 30 minutes long.
April 18, 2021: The bright morning planets, Jupiter and Saturn, are in the southeastern sky before sunrise. Capricornus is the starry background for this giant planet duo. Daylight is 13 hours, 30 minutes long.