As October both begins and ends, the moon will be sweeping through an evening lineup of four planets.
On October evenings, bright Jupiter is in the south-southwest to southwest at dusk, with Saturn to its left in the south to south-southwest; both remain outstanding for telescopic viewing, Jupiter with its cloud belts and four bright moons, and Saturn with its rings now tipped 25 degrees from edgewise. These giant planets appear 26 degrees apart on the sky’s dome on Oct. 1, narrowing to 22 degrees apart by Oct. 31. Follow their eastward motions against background stars, until the seasonal westward drift of the constellations drags both slow-moving planets to the southwest horizon before year’s end. Note reddish twinkling Antares, heart of Scorpius, 10 to 14 degrees to the lower right of Jupiter during October. Watch Jupiter pass 2.1 degrees north of a third-magnitude star on Oct. 22.
Look early in the evening twilight to catch Venus; binoculars are indispensable for nearby Mercury. From the Coachella Valley, Venus sets only 37 minutes after sunset on Oct. 1, improving to 64 minutes by month’s end. Mercury, near magnitude 0 almost all month, sets a maximum of 60 minutes after the sun Oct. 18-24; reaches greatest elongation, 25 degrees to the upper left of the sun on the 19th; and appears highest in twilight for a few days around then. But this is the year’s poorest evening apparition of Mercury, because the zodiac belt where the planets are found makes its shallowest angle with the evening horizon when the southernmost zodiac constellation Sagittarius is in southern sky. Using binoculars, find Mercury to the upper left of Venus during the first three weeks—by 7 degrees Oct. 2-3; 8 degrees Oct. 8-16; and back to 7 degrees Oct. 20-21. On Oct. 23, Mercury is 6 degrees to the upper left of Venus. Next, Mercury passes left of Venus, by 5.2 degrees on Oct. 25, and 4.6 degrees on Oct. 26. Then Mercury appears to Venus’ lower left, by 4 degrees on the 27th, and 3.4 degrees on the 28th. On Oct. 30, Mercury passes within 2.6 degrees south of Venus, but has begun its rapid fade.
Follow the moon at dusk through Oct. 13, and again Oct. 27-Nov. 12. On Oct. 3, find bright Jupiter within 3 degrees to the lower right of the moon. On Oct. 4, the fat (44 percent) crescent moon is 15 degrees to the upper left of Jupiter and 10 degrees to the lower right of Saturn. On Oct. 5, the moon, 54 percent full, is just past its first-quarter phase, when it’s half full and 90 degrees—a quarter of the way around the sky—from the sun.
On Oct. 13, watch the full moon rise some 20-25 minutes after sunset, just north of due east. As you looked daily in evening twilight through Oct. 13, the moon took two weeks to travel through a half-dozen zodiac constellations, from western horizon to eastern horizon, passing four planets along the way. In the first five evenings its next time around, Oct. 29-Nov. 2, the moon will pass the same four planets.
Other bright stellar markers not in the zodiac are Fomalhaut, mouth of the Southern Fish, climbing in the southeast, and golden Arcturus, the “Bear-watcher” star, sinking in the west to west-northwest.
By Oct. 16, moonrise is late enough to allow at least a brief interval of dark skies unaffected by moonlight. This “window” of darkness lasts longer each evening as the moon rises later. If you’re in a dark place, enjoy the Milky Way passing through the Summer Triangle of Vega, Altair and Deneb overhead, and the Andromeda Galaxy above the curved chain of stars starting at one corner of the Great Square of Pegasus in the east.
Follow the moon at dawn from when it is full, low in the west on Oct. 13, through last quarter (half full and 90 degrees west of the sun) on Oct. 21, to a thin crescent, low and south of east, on Oct. 26. The brightest star in October’s morning twilight is Sirius, in the southern sky, as dawn brightens. Confirm by noting that the three-star belt of Orion, with stars bluish Rigel marking his foot and reddish Betelgeuse his shoulder, points directly to Sirius. Extend the belt in the opposite direction, and you’ll pass near Aldebaran, eye of Taurus the Bull, and farther to the Pleiades or Seven Sisters star cluster, a wonderful target for binoculars.
Four days after it’s full and low in the west on Oct. 13, the moon is within 6 degrees to the lower right of Aldebaran on Oct. 17. For the next three mornings, Oct. 18-20, the waning gibbous moon moves through the huge Winter Hexagon. Next, on Oct. 21, the last-quarter moon, half full, is 7 degrees southeast of Pollux, and in line with the “Twin” stars, Castor and Pollux of Gemini. Our Spaceship Earth is heading in the direction of the half-lit moon in our orbit around the sun. If the moon stood still, it would take us only about 3 1/2 hours to reach it.
Our view of the peak of the Orionid meteor shower, in the predawn hours of Oct. 22, is affected by moonlight. On Oct. 23, the crescent moon will pass 4 degrees north of Regulus, heart of Leo, the Lion. In just three more mornings, on Oct. 26, look for the last easy old crescent moon, 4 percent full, just south of east, with Mars 5 degrees to its lower right. The dim red planet is now at magnitude +1.8, as faint as it ever gets. In just less than a year, in October 2020, the Earth will pass between Mars and the sun, and it will appear at opposition, and shine at magnitude -2.7, some 60 times brighter than now. At the end of October, Spica will be emerging out of the morning twilight glow, 7 degrees below Mars.
Notice the star Arcturus rising on the morning twilight chart. Arcturus is equally visible low in the west-northwest at dusk on Oct. 29 as it is in the east-northeast at dawn on Oct. 30. At this time of year, Arcturus leads the procession of stars and constellations through the night, and brings up the rear.
Abrams Planetarium publishes a monthly Sky Calendar with an evening sky map. The calendar illustrates many of the present and future events described above. Subscriptions are $12 per year at www.abramsplanetarium.org/skycalendar for three printed issues mailed quarterly. For a preview of evening and morning planet gatherings through August 2020, visit the Sky Calendar Extra Content Page at www.abramsplanetarium.org/msta.
Would you enjoy telescopic views of Jupiter, Saturn and an assortment of deep sky objects? Check the website of the Astronomical Society of the Desert at www.astrorx.org and come to our free evening star parties offered monthly at two locations.Our primary, more accessible venue is the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument, on Highway 74, within four miles south of Highway 111 in Palm Desert. Our next sessions there will be on Saturday, Oct. 5, from 7 to 10 p.m., and on Saturday, Nov. 2, from 6 to 9 p.m. Sawmill Trailhead, our high-altitude site (elevation 4,000 feet), will next host a session on Saturday, Oct. 26, starting at dusk.
Wishing you clear skies!
Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing sky watching opportunities for a variety of groups in the Coachella Valley. 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.