December 21, 2020: Find Jupiter and Saturn closely paired in the southwest. Observe them beginning about 45 minutes after sunset until about 90 minutes after sunset, before they are too low in the sky. Farther east, the half-full moon is to the lower right of bright, rusty Mars.
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 7:15 a.m. CST; Sunset, 4:23 p.m. CST. Check local sources for sunrise and sunset times for your location.
The Great Conjunction of Jupiter and Saturn is occurring this evening in the southwest after sunset. Look for bright Jupiter as night falls. Saturn is dimmer and adjacent to the brighter planet. The two planets are 0.1° apart. In fractions of a degree, the planets are 377 arcseconds apart.
The best viewing time starts about 45 minutes after sunset and lasts for about another 45 minutes (until about 90 minutes after sunset), before the planets are low in the sky. The conjunction sets 2 hours, 20 minutes after sunset.
There are suggestions here to photograph the planets.
This is the closest the planets have been in the sky together since 1623, although it was not likely visible because of their proximity to the sun after sunset.
Another conjunction occurred in 1563, although in tonight’s conjunction, the planets are closer together. In 1226, the planets were very close together, about one-third the Jupiter – Saturn distance this evening. In order of close proximity: 1226, 1623, 2020, 1563.
About 25 Great Conjunctions have occurred since the close conjunction of 1226.
Great Conjunctions occur every 19.6 years when slow-moving Jupiter passes even slower Saturn. A conjunction occurs when both planets have the same celestial longitude.
The next close conjunction is March 15, 2080, in the morning sky before sunrise. Two Great Conjunctions, November 4, 2040 (1.2°) and April 8, 2060 (1.1°) occur in between the pair of close conjunctions.
If you have a binocular, you may see up to four of Jupiter’s largest satellites. Through a spotting scope or telescope with low power, Jupiter and Saturn fit into the same eyepiece field. The image above shows the view as with a spotting scope. A telescope may turn the view upside down and flip it left to right, depending on the telescope’s properties.
The moon is approaching Mars in the southeast. This evening the half-full moon is over 20° to the lower right of bright, rusty Mars. The planet continues to move eastward.
Great Conjunction Countdown: This evening!
Read about Mars during December.
Detailed note: Forty-five minutes after sunset, Mars (m = −0.5) is nearly 48° up in the southeast. The half-full moon (7.3d, 50%), over 40° up in the south-southeast, is about 24° to the lower right of Mars. The moon reaches its First Quarter phase at 5:41 p.m. CST. This is the evening of the once-every-generation Great Conjunction of Jupiter (m = −2.0) and Saturn (m = 0.6). They are 30° east of the sun. The conjunction is about 14° in altitude above the southwest horizon. The separation of the two planets is 377 arcseconds. Both planets 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. The gap stays within 0.5° for five more evenings. Tomorrow they are still close, 0.1° (602 arcseconds), but slightly farther apart than this evening. The Jupiter – Saturn gaps after the conjunction: Dec. 22, 0.1°, Jupiter left of Saturn; Dec. 23, 0.2°, Jupiter is to the upper left of Saturn; Dec. 24, 0.3°; Dec. 25, 0.4°, Dec. 26, 0.6°, Dec. 27, 0.7°, Dec. 28, 0.8°, Dec. 29, 0.9°; Dec. 30, 1.0°.
For more about the Great Conjunction, read our feature article. This is the closest Jupiter – Saturn conjunction since 1623.
Read more about the planets during December.
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