Our evening twilight chart for September, depicting the sky about 40 minutes after sunset, shows brilliant Venus remaining low, creeping from west to west-southwest and gaining a little altitude as the month progresses. Its close encounter within 2.5 degrees north of Spica on Sept. 18 is best seen with binoculars to catch the star low in bright twilight.
The brightest stars in the evening sky are golden Arcturus, descending in the west, and blue-white Vega, passing just north of overhead. Look for Altair and Deneb, completing the Summer Triangle with Vega. The triangle of Mars–Saturn–Antares expands as Mars seems to hold nearly stationary in the south-southwest as the month progresses, while Saturn and Antares slink off to the southwest.
The morning twilight sky is rich with stars as the Winter Hexagon, made up of stars from Orion, his Dogs, the Twins, the Charioteer with Mother Goat, and Taurus, the Bull. Tracing out the Hex starting with Sirius, the brightest star, going clockwise, we encounter Procyon, Pollux (and nearby Castor, not quite first magnitude, and therefore not bright enough to be plotted), Capella, Aldebaran, Rigel and back to Sirius. Inside the Hexagon lies Betelgeuse, Orion’s shoulder. The Summer Triangle’s Deneb sinks into the northwest. Regulus, Leo’s heart, emerges into the east-northeast early in the month and is well up in the east by month’s end. By Sept. 22, emerging Mercury approaches within 15 degrees below Regulus. Brightening rapidly, in its best-of-year morning appearance, Mercury reaches peak altitude just before month’s end. Can you spot the second-brightest star, Canopus, before month’s end? It’ll be easier in October, when the star reaches its high point in the south (only 3 degrees up!), four minutes earlier each day, in ever-darker morning skies. Choose your vantage point carefully, with no high mountains nearby to your south.
There are two new moons in September: on Sept. 1 at 2:03 a.m., and on the 30th at 5:11 p.m. That means that this month, we can observe a complete cycle of the moon from start to finish, starting as a thin crescent moon very low in the west at dusk on Sept. 2, and waxing through the first half of the month, until it becomes full on the 16th. Next, we can follow the waning moon in the morning sky through Sept. 29. There are many striking events, starting with a close pairing of Jupiter and a young crescent moon on Friday, Sept. 2. Early that evening, get to a place with an unobstructed view toward west by 25 minutes after sunset, and look for Jupiter and the crescent moon, about 5-6 degrees to the lower right of Venus. Binoculars will help! From the Coachella Valley, the moon occults, or covers, Jupiter that afternoon, from 2:50-3:44 p.m., but the event will be impossible to see in the daylight, as it occurs only 18 degrees from the sun.
Watch the moon pass planets, as shown on the Abrams Planetarium Sky Calendar. By Saturday. Sept. 3, the moon is easy to spot, about 6 degrees to the upper left of Venus. On Sunday, Sept. 4, look for Spica 5 degrees to the moon’s south (lower left). Binoculars will help. Wait for the sky to darken some, but don’t wait too long, or Spica and the moon will set! Don’t miss the moon sliding past the beautiful triangle of Saturn, Antares and Mars on the evenings of Sept. 8 and 9. (See above right.)
The full moon on Friday, Sept. 16, comes up within 15 minutes after sunset. It’s fun to watch moonrises, and on the next five nights, the moon rises about 40-50 minutes later each night.
Beginning Sept. 17, you can shift your viewing times to mornings, about an hour before sunrise, and catch the moon passing by bright zodiacal stars: Aldebaran in Taurus on Sept. 21 and 22; Pollux and Castor in Gemini on Sept. 24 and 25; and Regulus in Leo on Sept. 27 and 28. On the moon’s final morning, Sept. 29, the old crescent moon will appear low in the east, just 2 degrees below Mercury.
A tip for telescopic observation of the moon in daytime: When the moon is within two days before or after half full—this month, late in the afternoons of Sept 7-10, near first quarter phase, and on mornings of Sept 21-24, near last quarter phase—insert a single polarizing filter into a low-power eyepiece of your telescope. Next, while viewing the moon, rotate the eyepiece until the surrounding blue sky appears darkest, increasing contrast of the moon against the sky for wonderful views of lunar craters! (Threaded polarizing filters and threaded eyepieces can be obtained from Orion at telescope.com.) I often enjoy setting up my telescope at schools before the school day begins on mornings in autumn, on days when the moon is near last quarter phase and high in the sky.
Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs. 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.