Follow the moon at dusk from Sept. 4-5 through Sept. 18. Catch a slender 4 percent young crescent moon within 4° to the lower right of Venus on Sept. 4, and an 8 percent moon within 8° to the left of Venus and 8° to the lower right of Spica on Sept. 5.
Venus is very low, so start looking early, about a half-hour after sunset, from a place with an unobstructed view between the west and west-southwest. Binoculars will help you spot Spica as the sky darkens. The star will be 4° to the lower right of the 14 percent crescent moon on Sept. 6. On Sept. 9, the nearly 40 percent fat crescent moon will appear 6° to the lower right of Antares, heart of the Scorpion. On the next evening, the nearly first quarter (half-full) moon will appear 7° to that star’s upper left. On Sept. 11, this month’s southernmost moon passes due south 20 minutes after sunset, only 27° up as seen from the Coachella Valley. As the sky darkens, you’ll find the moon near the tip of the spout of the Teapot of Sagittarius.
A partial eclipse of the Harvest Moon on the evening of Sept. 17 is shallow. At best, not quite 9 percent of the moon’s diameter will be immersed in the umbra, or dark central core of Earth’s shadow, at maximum eclipse at 7:44 p.m. The moon’s encounter with the umbra lasts for just more than an hour, from 7:13 p.m. until 8:16 p.m. This full moon, the fourth one of this summer season, occurs 58 hours before the start of autumn, marked by the sun’s passage from north to south across the equator, on Sept. 22 at 5:44 a.m. Observe the sunrises and sunset this month, and note how the places along the horizon where they occur are progressing farther south. If you live in a neighborhood where some streets are oriented east-west, it’ll be easy to notice the changes! Along with these shifts of the sunrise and sunset points, the midday sun of Sept. 30 is 11° lower than on Sept. 1—and the days are getting shorter.
Saturn is at opposition on the night of Sept. 7. Find Saturn appearing as a bright 0.6-magnitude “star” low in the east-southeast at dusk, high in the south in middle of night, and low in the west-southwest at dawn. As our faster-moving Earth overtakes Saturn, telescopic viewers will get an improving view of the rings, which appear 4° from edgewise on Sept. 15, resembling a needle piercing a ball of yarn.
The best lunar occultations of stars and planets in the Western U.S. in September are morning events, requiring a telescope for viewing, because the moon is bright: Saturn on Sept. 17; and stars in the Pleiades cluster on Sept. 22. From Palm Springs on the morning of Sept. 17, Saturn disappears behind the moon’s narrow dark side at 4:12 a.m. and reappears along the bright sunlit edge at 5:06 a.m. On the morning of Sept. 22, the 2.9-magnitude star Alcyone, the brightest member of the Pleiades cluster and mother of the Seven Sisters, is covered by the moon’s leading sunlit edge at 3:08 a.m. and uncovered at the dark edge at 4:29 a.m.
After its encounter with the Pleiades, the waning moon passes widely north of Aldebaran, brilliant Jupiter, and Mars on the mornings of Sept. 23-25; on Sept. 24, the northernmost moon of this month reaches its highest point only 5° south of overhead nine minutes before sunrise in Palm Springs. It’s no coincidence that the moon is at last quarter phase, half full, that morning. That’s because the last quarter moon—90°, or one quarter-circle west of the sun in the zodiac—reviews the sun’s position three months ago, near the beginning of summer, when the midday sun was high. Similarly, the first quarter moon of Sept. 11 previews the sun’s position in the zodiac three months hence, near the start of winter, when the midday sun will be low.
After passing the high point of the zodiac, the moon passes closely south of Pollux, the brighter of the Twin stars of Gemini, on Sept. 26; 3° to the north of the Beehive star cluster on Sept. 27; and 3° to the lower left of Regulus, heart of Leo, on Sept. 29.
Watch for other morning encounters, especially worth following in days before and after, when the event involves two planets or a planet and a star. Use binoculars to see Mercury-Regulus 0.5° apart, very low in the east to east-northeast in twilight on Sept. 9. Mars, creeping eastward in Gemini on Sept. 11 and 14, passes 1° north of the third-magnitude stars Eta and Mu in the foot of Castor. On Sept. 22, Mars passes 1.7° south of third-magnitude Epsilon Gem. On Sept. 27, Mars forms an isosceles triangle with the “Twin” stars Pollux and Castor, 12° from each.
Jupiter is currently the brightest morning “star.” Next in brilliance is the “Dog Star” Sirius, crossing due south in mid-twilight by early in October. Note the three-star belt of Orion, extended southeastward, points toward Sirius. Extended in the opposite direction, the line of the belt stars points past Aldebaran toward the Pleiades.
While Jupiter slows in Taurus, the Mars-Jupiter gap increases from 9° on Sept. 1 to 23° on Sept. 30. In Saturn’s constellation of Aquarius, locate 3.8-magnitude Lambda nearly 10° southeast of 3.7-magnitude Zeta, the central star in the asterism of the Water Jar. Next, locate 4.2-magnitude Phi Aqr, 5.6° east-northeast of Lambda. On the night of Sept. 27, Saturn is equidistant from Phi and Lambda, 3.2° from each and just south of a line joining them. As Saturn continues retrograde for another seven weeks, watch for changes. Saturn will end retrograde in mid-November, 1.9° from Lambda and 4.9° from Phi.
Comet C/2023 A3 (Tsuchinshan-ATLAS), whether or not it performs up to early expectations, is almost here! On Sept. 27, it reaches perihelion, 0.3914 a.u. (36.4 million miles) from the sun, and will pass closest to Earth on Oct. 12, at a distance of 0.472 a.u. (43.9 million miles). It may be visible Sept. 22-Oct. 4, rising in morning twilight, 7°-8° south of east. Try with binoculars! The comet will be 14° to the lower right of the 5 percent waning crescent moon on Sept. 30, and 12° to the upper right of a very low 1 percent crescent on Oct. 1. A most intriguing possibility is for a forward-scattering brightness surge as it passes halfway between Earth and the sun, within 4° to the upper left of the midday sun on Oct. 9, and a favorable appearance climbing in western evening sky in the following days. We’ll have more in our October column. Also, watch for updates on the Sky Calendar Extra Content Page, www.abramsplanetarium.org/msta.
The Astronomical Society of the Desert will host a star party on Saturday, Sept. 7, at Sawmill Trailhead, a site in the Santa Rosa Mountains at elevation 4,000 feet. For dates and times of other star parties, and maps and directions, visit astrorx.org.
The Abrams Planetarium Sky Calendar is available by subscription from www.abramsplanetarium.org/skycalendar. For $12 per year, subscribers receive quarterly mailings, each containing three monthly issues.
Robert Victor originated the Abrams Planetarium monthly Sky Calendar in October 1968 and still helps produce an occasional issue. He enjoys being outdoors sharing the beauty of the night sky and other wonders of nature. Robert Miller, who provided the evening and morning twilight charts, 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.
