During late December, the moon appears near brilliant Venus in the evening sky and Mars in the morning sky. Here are the highlights:
December 22: One hour before sunrise, Mars, 18° in altitude in the southeast, is over 8° to the lower left of the waning crescent moon that is 25.9 days old and 16% illuminated. The moon is above a line that connects Zubenelgenubi and Zubeneschamali, the pincers of the Scorpion – on ancient star maps. The lunar crescent is 3.5° to the upper left of Zubenelgenubi.
December 23: One hour before sunrise, Mars, over 18° up in the southeast, is 6° to the upper right of the crescent moon. The old moon is 26.9 days old and only 8% illuminated.
December 28: Nearly a week later than its appearance with Mars, the moon returns to the evening sky in the west. Forty-five minutes after sunset, Venus is 17° in altitude in the southwest. The moon is 2.4° below the planet. The moon is 2.8 days old and 8% illuminated.
A close conjunction of Venus and Jupiter in 2016. At this conjunction the planets are not as close as the December 21, 2020 Great Conjunction of Jupiter and Saturn.
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.
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 conjunction on July 16, 1623!
Moon image by NASA
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 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.
2020: The apparent motions of Jupiter and Saturn compared to the background stars.
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.)
During January 2020, Mars is joined by Jupiter in the morning. Saturn is at its solar conjunction and invisible to us because of the sun’s glare.
Jupiter and Saturn are in the same region of the sky. Jupiter is among the brightest “stars” in the night sky while Saturn is dimmer. Mars varies in its brightness. When near Earth later this year, it outshines Jupiter, but during January it is dimmer in the eastern sky, and easily overlooked.
Jupiter makes its first morning appearance late in the month, joining Mars as morning planets. Saturn passes its solar conjunction near mid-month and slowly crawls into the morning sky.
January 5: Mars is less than one-third of the way up in the sky about one hour before sunrise. The planet is not very bright compared to our expectations. It is near the stars of Scorpius. This morning it is to the upper right of the star Graffias. Compare Mars’ brightness and color to Antares, sometimes known as the “Rival of Mars.” During the next several mornings, watch Mars move past Graffias and far above Antares.
January 7: Jupiter is beginning to move into the morning sky, but it rises only about 30 minutes before sunrise. Look for it later in the month.
January 13: Saturn is at its solar conjunction. It is hidden in the sun’s glare. We won’t see it for several weeks.
January 20: This morning, the old moon is above Mars. Notice how far Mars has moved during the past several mornings. During bright twilight, about 30 minutes before sunrise, Jupiter is just above the southeast horizon. You’ll need a binocular to see it, as well as the crescent moon and Mars.
January 22: Forty-five minutes before sunrise, Jupiter is about 4° up in the southeast. The crescent moon (27.2d, 6%) is 7° to the upper right of Jupiter.
January 24: Saturn rises during bright twilight and its very difficult to see.
January 28: About 45 minutes before sunrise, Jupiter is low in the southeast. Use a binocular to locate Mars with Antares to its right.
Next Month, Saturn becomes visible as Jupiter and Saturn head toward their once-in-a-generation Great Conjunction on December 21, 2020.
The brilliant Evening Star Venus passes Saturn this evening. They appear in the southwest as the sky darkens. This evening, Venus is 1.8° to the lower left of Saturn. Watch Venus move away from Saturn during the month. Saturn disappears into the sun’s glare and reappears in the morning sky by the end of February 2020.
Venus continues its appearance in the evening sky. Watch it during the next several months.
The image above shows Brilliant Evening Star Venus, Jupiter and Saturn about an hour after sunset. Venus is nearly midway between Jupiter and Saturn, but they are not along the same arc in the sky: Venus – Saturn, 8.6°; Venus – Jupiter, 9.7°. Watch Venus continue to close in and pass Saturn on December 10. (See the link below.)
Jupiter is becoming more difficult to observe at this time interval after sunset. This evening, it is less than 5° in altitude.
Read more about Venus as an Evening Star at these links:
One night past their conjunction, Venus appears to the left of Jupiter this evening about 45 minutes after sunset. Venus continues to move away from Jupiter and toward Saturn. Venus passes the Ringed Wonder on December 10. Meanwhile, look for the crescent moon and Venus on November 28, Thanksgiving evening in the U.S. See the links below for more details.
Read more about Venus as an Evening Star at these links:
One day before their conjunction, Venus and Jupiter shine from the southwest this evening about 45 minutes after sunset. (Notice the reflections of the two planets in the water.) This evening the space between them is 1.5°. Venus appears to the lower left of Jupiter. Tomorrow evening, November 24, the planets appear closest (conjunction). On this evening they are 1.4° apart. Venus appears to the lower left of Jupiter.
Read more about Venus as an Evening Star at these links:
