December 21, 2020: Jupiter passes Saturn in a close conjunction, a once-in-a-generation Great Conjunction. This Jupiter – Saturn conjunction is the closest Great Conjunction since 1623.
Read more about the August 2020 planets.
by Jeffrey L. Hunt
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.
Click through the gallery of Jupiter and Saturn images.
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See our detailed article about the Great Conjunction here. (updated May 28, 2020)
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 conjunction since the Great Conjunction of July 16, 1623! The next Great Conjunction is October 31, 2040, when the two planets are 1.1° when they are low in the east-southeast before sunrise.
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.
Each night thereafter, Jupiter closes more 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.
The half-full moon is higher in the south-southeast. Mars is to the left of the moon, over halfway up in the sky in the southeast. Four bright solar system objects are in the sky – moon, Jupiter, Mars, and Saturn.
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.
Detailed Motion of Jupiter and Saturn
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.
Read about the 1623 Jupiter – Saturn Great Conjunction here.
Monthly Summaries of What to Watch
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