Celestial highlights for September 2021 include a string of bright evening planets, old and young thin crescent moons, striking pairings of the moon with planets and stars, and a Harvest Moon with conveniently early evening moonrises. Meanwhile, pre-dawns showcase a huge collection of bright stars.
And as of Sept. 22 … welcome to autumn!
September provides sky watchers with the opening act of an evening planetary display that will get increasingly spectacular through 2021. Earth has recently overtaken the two giant planets of our solar system, Saturn (Aug. 1) and Jupiter (Aug. 19). Those are the dates of this year’s oppositions, when we see each planet by facing directly away from the sun. A planet at opposition is near its closest to Earth, reaching maximum brightness, and visible all night, from dusk until dawn.
On Sept. 1, at evening mid-twilight, Jupiter is prominent at magnitude -2.9, low in the east-southeast, with Saturn at magnitude +0.3 in the southeast, 17.5 degrees to Jupiter’s upper right. The sun is then 9 degrees below the west-northwestern horizon. On that date, our Spaceship Earth is racing directly away from a point 5 degrees above the star Antares in the south-southwest. This red supergiant star marks the heart of Scorpius, the Scorpion. An observer high above the northern side of our solar system would see the 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.
The two planets interior to Earth’s orbit are now visible in the western sky at dusk: Venus is prominent at magnitude -4 in the west-southwest. You’ll need binoculars to spot zero-magnitude Mercury, very low in bright twilight and within 16 degrees to Venus’ lower right. Both inner planets move faster than Earth, so they will catch up and overtake us, with Mercury passing inferior conjunction (between Earth and the sun) on Oct. 9, and Venus doing so on Jan. 8.
During September and October, Mars is lost in the glare of the sun.
Look within 5 degrees to the upper left of Venus on Sept. 1 as the sky darkens, and you’ll find Spica. Each day, Venus shifts position against the background stars by slightly more than one degree. On Sept. 5, Venus will pass 1.6 degrees to the upper right of Spica. The gap between them will widen to 4 degrees by Sept. 8, with Spica to the lower right of Venus.
A very thin crescent moon may be visible on Sept. 7. Using binoculars, look about 30 minutes after sunset for the 2 percent crescent, very low in the west, 28 degrees to the lower right of Venus. Next, find Mercury 14 degrees to the moon’s upper left. The moon’s age is a tender 26 hours after new. Mercury is highest in twilight around this date.
The 6 percent moon is much easier to spot on the next evening, Sept. 8. Look 15 degrees to the lower right of Venus, then find Mercury 5 degrees to the lower left of the moon, and Spica 4 degrees to the lower right of Venus.
Dusk on Thursday, Sept. 9, provides a spectacular view of a 12 percent crescent moon, just 3-4 degrees to the upper right of Venus. Find Spica 5 degrees below the moon, and Mercury 10 degrees to the lower right of Spica, and 14 degrees to the lower right of the moon.
Dusk on Sept. 10 finds Venus within 12 degrees to the lower right of the moon, and Mercury still 15 degrees to the lower right of Venus. On Sept. 11, locate Spica about midway between Venus and Mercury.
On Sept. 12, the fat 42 percent lunar crescent appears about 3 degrees above Antares. On Sept. 13, the 53 percent moon is just more than 90 degrees east of the sun, rather low in the southern sky around sunset. That same evening, Mercury reaches greatest elongation, nearly 27 degrees east of the sun.
On Sept. 16, the 83 percent waxing gibbous moon is 4-5 degrees to the lower right of Saturn, while bright Jupiter passes 1.4 degrees north of the third-magnitude star Delta Capricorni, or Deneb Algedi, tail of the Sea-goat.
On Sept. 17, the moon, 91 percent full in the southeast at dusk, is 5-6 degrees to the lower right of Jupiter. On Sept. 18, the 96 percent moon appears 11 degrees to the lower left of Jupiter.
On Sept. 20, the fourth full moon of summer 2021 appears very low, about 8 degrees south of east at evening mid-twilight. Try for a difficult pairing, very low in the west-southwest in bright twilight on Sept. 20 and 21: Mercury passing within 1.5 degrees south (to the lower left) of Spica. Using binoculars, try for the pair 19-20 degrees to the lower right of Venus.
The Northern Hemisphere’s autumn begins here on Sept. 22 at 12:21 p.m. as the sun, moving southward, crosses directly over the equator.
At dusk in September, the belt of zodiac constellations makes its lowest angle with the horizon. This year, the visible portion of the belt is marked by four bright planets, in order from west to east: Mercury, Venus, Saturn and Jupiter. Also within that part of the zodiac are the first-magnitude stars Spica and Antares. The southernmost part of the zodiacal belt, in the constellation Sagittarius, the Archer, is in the south around sunset in September. This causes the low angle that the zodiac makes with the horizon, the “tipped over” appearance of the waxing crescent moon in the west, and the low-at-dusk positions of Venus and Mercury. Another phenomenon resulting from the low angle of the zodiac to the horizon is the annual “Harvest Moon effect”—early evening moonrises for several days after the moon becomes full. During Sept. 19-23, the moon rises less than half an hour later each evening, and noticeably farther north each time. If you enjoy watching a string of daily moonrises without staying up very late, here’s your chance!
Besides the moon and planets, bright stars visible at dusk include golden Arcturus, nearly halfway from the horizon in the west as September opens, and dropping lower as the month progresses; Spica, some 33 degrees to the lower left of Arcturus; and the Summer Triangle of Vega, Altair and Deneb, reaching overhead. For the rest of 2021, a line from Vega to Altair, 34 degrees long and extended 31 degrees past Altair, locates Saturn. In September, watch for Fomalhaut, mouth of the Southern Fish, rising 22-23 degrees to the lower left of Jupiter.
These gatherings 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.
For the rest of 2021, Venus, Saturn and Jupiter form an attractive lineup in the southwestern evening sky, anchored by brightening Venus. By late October, telescopes will show Venus as a tiny “half-moon.”
The morning sky: There no planets—but lots of bright stars!
After Jupiter sets in the west-southwest before dawn, Sirius, the “Dog Star,” is the brightest star. In September’s morning mid-twilight, Sirius is found in the southeast to 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.
On Sept. 1, as Spaceship Earth races away from Antares in the evening sky, it’s heading toward a point about 5 degrees above Aldebaranin the morning sky. The sun is in Leo, below the east-northeastern horizon. As we follow our orbit around the sun during September, the stars will shift their positions westward. On Sept. 2 and 3, the waning crescent moon appears near Pollux and Castor. On the morning of Sept. 5, let the 3 percent, old crescent moon, low in the east-northeast, be your guide to Regulus, heart of Leo, just emerging from solar conjunction. Look for the star 6-7 degrees to the lower right of the thin, lunar crescent.
The moon makes its next pass through the morning sky, waning from full to a thin crescent, Sept. 20-Oct. 5. Watch the waning gibbous moon pause midway between the Pleiades and Aldebaran on Sept. 26. See a fat crescent moon pass 3 degrees south of Pollux on Sept. 30.
You’ll notice that the old crescent, with its cusps pointing upward, is oriented like a bowl on a table, signifying that the sun is almost directly below it. Also, the moon, near last quarter phase on Sept. 28 and 29, is very high in the sky around sunrise, in Taurus and Gemini, the northernmost constellations of the zodiac.
Robert Victor originated the Abrams Planetarium’s monthly Sky Calendar in October 1968, and still produces issues occasionally. Whether in the Coachella Valley, the Grand Canyon or in Michigan, he enjoys being outdoors and sharing the wonders of the night sky. Robert Miller 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.