Follow the Moon and Naked-Eye Planets in December 2019 and January 2020
By Robert C. Victor
Dawn and dusk twilight sky maps by Robert D. Miller. Sky Calendar illustrations courtesy of Abrams Planetarium, Michigan State University. Graphs of the planet rising and setting times for School Year 2019-2020 by Jeffrey L. Hunt.
Venus becomes ever more prominent in the evening, while Jupiter, and then Saturn, disappears into the southwest twilight glow in December. Mars, very slowly brightening, appears in the southeast morning sky, with bright Mercury below it in the first half of December. Mercury passes behind the Sun in early January, then emerges into the west-southwest evening sky by late that month. Jupiter emerges into SE morning sky by mid-January, followed by Saturn by early in February.
Modeling the solar system on-orbit charts or by using students to act out the motions of planets in the classroom can explain the reason why, when an outer planet such as Mars, Jupiter, or Saturn is behind the Sun, it is transitioning from evening sky to morning sky, and why, when either inner planet, Mercury or Venus, is behind the Sun, it is transitioning from morning to evening visibility. What must be known prior to the modeling is that each planet moves at a faster angular speed around the Sun than any planet farther out, and that the direction of rotation of the Earth on its axis is in the same sense as the revolution of the planets around the Sun, i.e. counterclockwise as seen from “above” or north of the solar system.
Search online for the cartoon, “Professor Zlata! You’re just in time to be the planet, Neptune!” (I think you – and your students -- may enjoy it!)
In December 2019, don’t miss (1) Venus-Saturn within 5° Dec. 7-14, and 1.8° apart on Dec. 10. (2) Spectacular Venus-Moon pairing on Sat. Dec. 28. For a preview of sky events during the 2019-20 school year, with monthly all-sky charts for dusk and dawn, see the article in August-September issue of California Classroom Science, Follow the Naked-eye Planets in 2019-20 School Year and Beyond, August 2019 through August 2020:
Also, visit the Sky Calendar extra content page: www.abramsplanetarium.org/msta/
Three planets span 18° in SW at dusk in early December. On Dec. 1, find Venus, mag. –3.9, with Jupiter, mag. –1.8, just 8° lower right, and Saturn, mag. +0.6, 11° to Venus’ upper left. Venus is now on the far side of its orbit, its light taking 12 min. to reach us, compared to Sun’s 8 min. Light reflected from Jupiter and Saturn takes 51 min. and 90 min. to travel to Earth on Dec. 1.
Note the first-magnitude star Aldebaran, eye of Taurus, low in E at dusk Dec. 1. Visualize: As Earth passes between Aldebaran and Sun, the star is at opposition and up all night. Revolution of Earth and other planets around Sun is counterclockwise, from the viewpoint of the observer above the N side of our solar system. So Earth on Dec. 1 is moving directly away from point 90° E of Sun and 90° W of Aldebaran; that’s 2° W of Lambda Aquarii, in the southern sky at nightfall. Inner planets move faster, so Venus, on the far side of its orbit and gaining on us, continues to move farther out from last August’s place on the far side of Sun, until reaching greatest elongation, 46° E of Sun, on March 24, 2020. Venus will overtake us ten weeks later, when it passes inferior conjunction, nearly between Earth and Sun, on June 3.
We’re leaving Jupiter and Saturn behind. Earth’s faster revolution around Sun will cause Jupiter to pass behind Sun on Dec. 27, and Saturn to do so on Jan. 13. Before these solar conjunctions, when will you last spot Jupiter low in SW at dusk? When will you last see Saturn?
In the morning sky on Dec. 2, Earth is heading toward Leo, 10° east of Regulus. Speedy Mercury, a few days past greatest elongation of Nov. 28, is heading toward superior conjunction on the far side of Sun, on Jan. 10. When will you last see Mercury before then? Mars begins Dec. at faint mag. +1.7. We’re gaining on the red planet, so watch it brighten until its close approach and opposition in Oct. 2020, when it will gleam at mag. –2.6.
January 2020: The year begins with Mars as our only morning planet. On Jan. 1, we find Mars at mag. +1.6 in SE, about 12° upper right of brighter first-magnitude Antares (whose name, from Greek, ant + Ares, means rival of or opponent to Mars). The two red objects appear no more than 10° apart Jan. 5-30, and no more than 5° Jan. 16-20. They’re closest, within 4.8°, as Mars passes N of the star on Jan. 18. Enjoy them in the same field of view of binoculars for several mornings! Watch for Jupiter emerging lower left of Mars in mid-January, and Saturn to lower left of Jupiter by early February. Antares now wins its ongoing brightness contest with Mars, but during March, Mars begins to outshine the star, and there’ll be a rare compact gathering of all three bright outer planets late that month. For a preview of sky events over the next several months, visit www.abramsplanetarium.org/msta/
How often does Mars pass Antares in our sky? If we were located at Sun, then the answer would be every 687 days, or about 22.6 months, the sidereal period of revolution of Mars around the Sun. But as seen from our moving Earth, conjunctions of Mars-Antares do not occur at equal intervals. After Jan. 18, 2020, the next will occur low in the morning sky on Dec. 27, 2021, when Mars will pass 4.5° N of the star. On the next two occasions, on Dec. 8, 2023, and Nov. 18, 2025, Mars and Antares will appear too close to Sun to be seen from Earth. After those, the next visible conjunction of Mars-Antares will occur very low in the evening sky on Oct. 29, 2027 (3.7° apart). Then the next, also in the evening, will be on Oct. 7, 2029 (3.3°) and Sept. 10, 2031 (2.5°). Then the Mars-Antares pairings shift back to mornings, on Feb. 25, 2033 (5.3°), and Jan. 27, 2035 (4.9°, similar to this year’s).
On evenings in January, Venus climbs ever higher in a dark sky. Watch it pass background stars in Capricornus and Aquarius on Jan. 6-8, Jan. 21-24, and Jan. 27. Mercury emerges lower right of Venus last week of the month (30° on Jan. 23, to 26° on Jan. 31), and will climb highest in evening twilight around Feb. 10, when it will be 24° from Venus.
In the second week of January, the “Twin” stars Castor and Pollux in Gemini are in opposition to the Sun and are visible all night: Low in ENE at dusk, high in the south in the middle of the night, and low in WNW at dawn.
Follow Moon and planets in December 2019 and January 2020. The earliest sunsets of the year in December provide a convenient opportunity for families even with young children to enjoy the night sky, and this year, the planets provide many wonders. In the southwest, the brightest planet, Venus, is easy to spot by half an hour after sunset. In early December, find bright Jupiter to Venus’ lower right, and Saturn to Venus’ upper left. In December’s first week, Jupiter and Saturn are 18° apart, with brilliant Venus between them, while the waxing Moon moves farther from the planets nightly. Each evening in early December, Venus moves farther from Jupiter and closer to Saturn. Students can make simple, nightly sketches of what they see to follow the progress of the planets. On Dec. 2, foreground Venus appears nearly midway between the distant giant planets. On December 3, the Moon reaches the First Quarter phase, when it appears half full and is a one-quarter circle or 90° of the Sun, compared to Venus’ 28° that evening. On the evenings of December 10 and 11, Venus will pass less than 2° lower left of Saturn, as shown here. Can you still spot Jupiter those evenings, before it sinks into even brighter twilight? Look for Jupiter 17° lower right of the Venus-Saturn pair on Dec. 10.
Meanwhile, the Moon, shifting its place eastward by an average of 13° per day against background stars, has moved into the constellation Taurus, the Bull, as shown here. On Tues. Dec. 10, note the first-magnitude star Aldebaran, eye of the Bull and follower of the Pleiades, to the lower left of the Moon, and on Wed. Dec. 11, find Aldebaran to the upper right of the Full Moon.
Students can continue to follow the Moon for several more evenings after Dec. 11, but it would require staying up later each night. Instead, you can have students shift their Moon watching time to one hour before sunrise. At that time on Dec. 11, the Moon and Aldebaran are within 3° and about to set in WNW, while Orion is setting in the west, and Sirius, the brightest star, is low in WSW. Mercury, just risen in ESE, is in the head of Scorpius on the morning of Dec. 11, while the Moon is just above the head of Taurus, opposite to Scorpius in the zodiac, the belt of constellations where the Sun, Moon, and planets are always found. Look for faint Mars, 18° upper right of Mercury on Dec. 11, and for a first magnitude star, Spica, nearly 21° upper right of Mars. Spica is in Virgo, another zodiac constellation. Note that Mercury, Mars, and Spica are in a nearly straight line. In the next two weeks, watch the waning Moon pass several bright stars and planets in the zodiacal band: Pollux and Castor of Gemini on Dec. 14; Regulus, the heart of Leo, on Dec. 17; Spica in Virgo on Dec. 20 and 21; Mars in Libra on Dec. 22 and 23; and Antares in Scorpius on Dec. 24. By then Mercury has moved closer to the Sun and may be too difficult to observe in bright twilight. Mercury will pass superior conjunction, on the far side of the Sun, on Jan. 10.
The New Moon is invisible as it passes the Sun late on Christmas Day. At dusk on Friday, Dec. 27, the young crescent can be seen low in SW to lower right of Venus. Using binoculars, can you spot Saturn within 7° to lower right of the Moon? The view on Saturday, Dec. 28 will be very special. At sunset, Venus will be within 2° above and slightly right of the Moon. Can you spot Venus before sunset? By 40 minutes after sunset, the view of Venus just upper right of the crescent Moon will be spectacular. Keep watch as Moon and Venus sink toward the horizon. They’ll be little more than one degree apart as they set.
On Dec. 27, Jupiter will be in conjunction on the far side of the Sun, and by mid-January it will emerge into the southeast morning sky, to lower left of Mars and Antares. The waning Moon comes around again in January’s morning sky, passing near Regulus on Jan. 13, Spica on Jan. 17. Mars and Antares on Jan. 20, and approaches Jupiter on Jan. 22.
New Moon occurs on Jan. 24. On Sat. Jan. 25, just half an hour after sunset, you’ll need very clear skies to see the young, very thin crescent Moon, about one percent full and just 3 or 4 degrees up in WSW. Can you spot Mercury within 3 degrees to Moon’s lower right? Binoculars will give the best view of the delicate crescent Moon with Mercury in the same field. Mercury has just emerged from its Jan. 10 superior conjunction on the far side of the Sun, and now starts its best evening appearance of the year. Binoculars will help that evening, but it will get easier, as Mercury gets higher and sets later each evening until the second week of February. On Jan. 27 and 28 at dusk, Venus appears 6° or 7° from the lunar crescent.
After Jan. 25, the waxing Moon climbs higher nightly. On Jan. 26, the 5-percent crescent appears 13° upper left of Mercury and 16° lower right of Venus. On Monday, Jan. 27 at dusk, the 9-percent crescent Moon appears 6° lower left of Venus. That day, find the Moon about 30° up in SW shortly before sunset and try to spot Venus in the daytime, by looking 6° to the upper right of the lunar crescent. Then use a telescope for a closeup of Venus, which displays a tiny gibbous disk 15 arcseconds (1/240 of a degree) across and 75 percent illuminated. In the coming months, Venus will be even more fascinating to watch as it comes around to the near side of its orbit and looms ever larger in apparent size and becomes backlighted by the Sun. By late in March, as Venus stands high in the western sky 46° from the Sun, it will be half-illuminated, and by late in April Venus will reach the greatest brilliance, while appearing as a crescent, about one-quarter full.
Saturn will be in conjunction with Sun on Jan. 13 and will follow Jupiter into the morning sky by early in February. Then all three bright outer planets will be visible in the morning sky, in a gathering that will become ever more compact until Mars passes the two giant planets late in March 2020.
Wishing you and your students an abundance of clear skies!
Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. Now retired, he often collaborates with John S. French on the Sky Calendar, and enjoys providing sky watching opportunities for a variety of groups, mostly in the California desert and in Michigan.
Robert D. Miller did graduate work in Planetarium Science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.
Dr. Jeffrey L. Hunt, a retired planetarium director now living in the Chicago area, has taught astronomy and sky watching to all ages. He studied astronomy education at Abrams Planetarium at Michigan State University. Jeff writes an astronomy blog at jeffreylhunt.wordpress.com and can be followed on Twitter at @jeff_hunt.