Evenings in October 2021 feature a string of three bright planets; the moon waxing from a young thin crescent to full; striking pairings of moon with Venus, Saturn and Jupiter; and a Full Hunter’s Moon, with convenient early evening moonrise times on subsequent nights. Predawn skies showcase a huge collection of bright stars; a waning moon; and, in late October and early November, the year’s best morning appearance of Mercury.
Autumn 2021 provides sky watchers a fine display of evening planets, anchored in the southwest by brilliant Venus, which will be a growing attraction for the rest of 2021. In August, Earth overtook Saturn and Jupiter, and they appeared at opposition, low in eastern sky at dusk and visible all night. After opposition, these giant planets remain visible at dusk for about half a year, drifting from eastern toward western horizon as months pass. Saturn will disappear into the western twilight glow in January 2022, and Jupiter in February.
In mid-October at evening mid-twilight, about 40 minutes after sunset from the Coachella Valley, Venus, at magnitude -4.4 in the southwest, appears as the most brilliant “star.” Jupiter is prominent at magnitude -2.6 in the southeast to south-southeast, with Saturn at magnitude +0.5 and just more than 15 degrees to Jupiter’s upper right. The sun is then 9 degrees below the horizon, a few degrees south of west. In mid-October, our Spaceship Earth is racing directly away from a point west of Saturn and the head of Capricornus and east of the Teapot of Sagittarius.
While outdoors observing the sky, visualize this: An observer high above the northern side of our solar system would see Earth and the other planets moving counter-clockwise in their orbits around the sun. Compared to Earth, Jupiter and Saturn are slower-moving outer planets; since we overtook them in August, we are now leaving them behind. Venus, the planet next inside Earth’s orbit, moves faster than Earth, so it will catch up and overtake us, on Jan. 8, 2022. Thereafter, Venus will quickly emerge into the morning sky.
Venus, the most brilliant planet,is low in the southwest at dusk. At the same stage of twilight, it will be 3 degrees higher at month’s end. Its setting time improves from 2 to 2 1/2 hours after sunset. Enjoy following Venus’ motion against background stars, now 1.1 to 1.0 degrees daily. Watch it go 0.8 degrees south of Delta, middle star in head of Scorpius, on Oct. 9; and 1.4 degrees north of Antares on Oct. 16.
Jupiter and Saturn, in the southeast to south at dusk, both now have background stars close at hand, so the planets’ motions can easily be detected. They both end retrograde this month in the constellation Capricornus, the Sea-goat—Saturn on Oct. 10, and Jupiter on Oct. 17—before resuming eastward motion against the stars. Saturn’s turnaround on Oct. 10 is 7.3 degrees to the lower right of 4.1-magnitude Theta. Jupiter’s pause on Oct. 17 is 1.8 degrees from 2.8-magnitude Delta, or Deneb Algedi, tail of the Sea-goat. Jupiter will be equidistant from these stars, 1.7 degrees from each, on Nov. 6. In the fourth week of October, soon after ending their retrograde, Jupiter and Saturn are a bit more than 15 degrees apart, closer together than they’ll appear again until October 2039.
The inner planets can appear above the western horizon at dusk, or above the eastern horizon at dawn—Mercury, for, at most, a few weeks at a time, and Venus for several consecutive months; the current evening apparition of Venus began in late April 2021 and continues until early January 2022. Appearances of Mercury and Venus alternately in western evening or eastern morning skies are consequences of their locations interior to the Earth’s orbit and their faster motions around the sun. Each emerges from the far side of the sun into the evening sky, catching up to the Earth and dropping back out of evening sky. After an inner planet passes between Earth and the sun, it emerges into the eastern morning sky, pulling farther ahead of Earth and working its way around toward the far side of the sun, dropping out of the morning sky and beginning the cycle again. Mercury’s complete cycle, including evening and morning visibility, averages 116 days, or just less than four months. That’s how often Mercury passes the far side of the sun, or how often it overtakes Earth. Venus’ mean cycle of visibility is 584 days, or nearly 19.2 months.
Telescopic views: As Venus closes in on our home planet, its disk grows from 19” (arcseconds) to nearly 26” across in October, while its illuminated portion decreases from 62 percent to 48 percent. The best is yet to come! After Venus’ greatest elongation of 47 degrees on Oct. 29, the next 10 weeks, until inferior conjunction on Jan. 8, will be fascinating! The crescent Venus will more than double in apparent size but become very slender, peaking in brilliance at magnitude -4.9 in early December, midway through the 10 weeks. Jupiter features its cloud belts and four Galilean satellites, while Saturn displays rings tipped 19.4 degrees from edge-on for most of October, offering the best view of the rings we’ll have until April 2029. Using higher magnification, look for Saturn’s shadow cast upon the rings at the northeast limb of the planet, giving the scene a 3-D appearance. In mid-October, Jupiter’s disk appears 44” across, slightly exceeding the 39” extent of Saturn’s rings.
Follow the moon in evening sky: See a thin crescent moon at dusk on Oct. 7. Look very early, about 30 minutes after sunset, for the 4 percent crescent very low in the west-southwest, 25 degrees to the lower right of Venus. The moon’s age is 39 hours after new. The 10 percent moon is much easier to spot on the next evening, Oct. 8. Look 11 degrees to the lower right of Venus.
Dusk on Oct. 9 provides a spectacular view low in the southwest of an 18 percent crescent moon, with Venus just 3 to 4 degrees to its lower right. As the sky darkens a bit, look for the red star Antares about 5 degrees to the left of the moon. Dusk on Oct. 10 finds Antares and Venus 10 and 16 degrees to the lower right of the 27 percent lunar crescent.
On Oct. 12, the moon, nearly half full, appears about 6 degrees east of the second-magnitude star Sigma in Sagittarius, brightest star in the handle of the Teapot.
On the evenings of Oct. 12-20, Antares is within 5 degrees of brilliant Venus. Observing with naked-eye or binoculars, watch for daily changes, especially around Oct. 16, when they’ll appear closest, as Venus passes 1.4 degrees above the red star.
On Oct. 13-15, bright Jupiter by now is in the southeast to south-southeast at dusk, with Saturn 15.5 degrees to its west (right). On Oct. 13, the 60 percent gibbous moon appears 6 degrees to the lower right of Saturn. On Oct. 14, the 71 percent moon appears within 11 degrees to the lower left of Saturn and 7 degrees to the lower right of Jupiter. On Oct. 15, the 80 percent moon appears 9 degrees to the lower left of Jupiter.
On Oct. 19, the nearly full moon rises a few minutes before sunset. On Oct. 20, the moon, just past full, rises 20 minutes after sunset. Over the next three days, the waning moon rises 29, 31 and 35 minutes later from one night to the next, farther north each time.
At dusk in early autumn, the belt of zodiac constellations makes its lowest angle with the horizon. This year, the visible portion of the belt is marked by three bright planets, in order from west to east, Venus, Saturn and Jupiter. Also within that part of the zodiac is the first-magnitude star Antares.
The southernmost part of the zodiacal belt, in the constellation Sagittarius, the Archer, is in the south around sunset in early October. This causes the low angle that the zodiac makes with the horizon, the “tipped over” appearance of the waxing crescent moon in the west at dusk, and the low positions of Venus and Mercury at dusk, despite considerable elongations (angular distances from sun) of 45 degrees for Venus at the end of September to 47 degrees in late October, and 27 degrees for Mercury on Sept. 13.
A phenomenon resulting from the low angle of the zodiac to the horizon is the “Harvest Moon effect”—early evening moonrises for several days around the full moons of late summer and early autumn. If you’d enjoy watching a string of daily moonrises without staying up very late, a few evenings starting with autumn’s first full moon on Oct. 20 will offer you a chance!
Besides the moon and planets, bright stars visible at dusk include golden Arcturus, in the west and dropping nearly to the west-northwest horizon by mid-November; and the Summer Triangle of Vega, Altair and Deneb passing overhead into early Oct. Blue-white Vega is its brightest member. For the rest of 2021, a line from Vega to Altair, 34 degrees long and extended 31 degrees past Altair, locates Saturn. Through mid-November, Fomalhaut, mouth of the Southern Fish, is 23 degrees to the lower left of Jupiter.
Attractive gatherings of the moon, stars and planets are illustrated on the Sky Calendar. Subscribe for $12 per year for three monthly issues mailed quarterly, or view a sample copy, at www.abramsplanetarium.org/skycalendar.
In the morning sky, there are no planets until Mercury appears in late October, but there are lots of bright stars! Daylight saving time, in effect through Nov. 7, creates late sunrises and dark morning skies accessible without getting up extra early, at least by the numbers on the clock!
Sirius, the “Dog Star,” is the brightest star in the morning sky, approaching south. Trace out the huge Winter Hexagon of bright stars. Beginning with Sirius, its southernmost and brightest member, proceed clockwise through Procyon; the Twin stars Pollux and Castor, 4.5 degrees apart; Capella, the Mother Goat Star and northernmost member; Aldebaran, eye of Taurus the Bull and Follower of the Pleiades star cluster; Rigel, Orion’s foot; and back to Sirius. Betelgeuse, shoulder of Orion, lies inside the Hexagon.
In mid-October, Spaceship Earth is racing toward a point about 7 degrees south of Pollux in the morning sky. The sun is in Virgo, below the eastern horizon in morning twilight.
The moon makes a pass through the morning sky, waning from full to a thin crescent through Oct. 5, and again Oct. 20-Nov. 3. The last, 2 percent, old crescent moon will appear very low in east 45 minutes before sunrise on Oct. 5.
Robert Victor originated the Abrams Planetarium monthly Sky Calendar in October 1968, and still produces issues occasionally. Whether here in the desert, at the Grand Canyon of Arizona, or in Michigan, he enjoys being outdoors and sharing the wonders of the night sky. He’s hoping for the pandemic to end! Twilight sky maps are by Robert D. Miller, who did graduate work in planetarium science and later astronomy and computer science at Michigan State University. He remains active in research and public outreach in astronomy.