September ’24 – Rick’s picks

 All month: the planets’ repopulation of the evening sky continues its slow advance this month, while the pre-dawn sky now begins to lose its solar system lineup. Venus sets roughly an hour after sunset throughout September, but the flat evening ecliptic keeps it close to the horizon, blazing at mag -3.8 to -3.9 while its gibbous disk swells slightly from 11 to 12 arcseconds as the month goes by. Saturn moves firmly into an evening time slot, rising at 7:45 pm as September begins and two hours earlier as it ends, reaching opposition the night of Sept 7. During September’s first week, the ringed planet brightens from mag +0.6 to +0.5, then fades to mag +0.7 over the remainder of the month, while its disk varies only slightly from 19 arcseconds and the rings extend to 44 arcseconds at a nearly-flat tilt angle of 3.5 to 4.5° to our line of sight. Neptune rises roughly a half hour after Saturn and reaches its own opposition on Sept 20. Neptune’s distinctly blue disk, just 2½ arcseconds across, glows at mag +7.8 all month. By month’s end, Saturn and Neptune both set before sunrise, beginning the planets’ exodus from the morning sky. Uranus continues to lead off the late night lineup, rising at 10:30 pm on Sept 1 and two hours earlier on Sept 30, displaying a powder blue mag +5.7 disk almost 4 arcseconds wide. It will be just 5° SSW of The Pleiades (M45) all month, 1½° W of a half-degree pair of stars almost exactly the same brightness as the planet. Jupiter arrives for the overnight shift, rising shortly before midnight as September opens and just after 10 pm as it ends, brightening from mag -2.3 to -2.5 as its disk grows from 38 to 42 arcseconds. Bringing up the rear is Mars, which rises 20 minutes after midnight on Sept 1 and 20 minutes before midnight on Sept 30, brightening from mag +0.7 to +0.5 as its disk slowly expands from 6½ to 7½ arcseconds. It forms a big orange triangle with Betelgeuse (Alpha [] Orionis) 18° to its SSW and Aldebaran (Alpha [] Tauri) 27° to its WSW, with Jupiter midway between them. That leaves Mercury as the solar system’s sole morning riser, cresting the horizon at 4:50 am on Sept 1 but disappearing into the Sun’s glare by Sept 20, when it will rise at 5:45 am, just 50 minutes before sunup. It begins the month as a fat, 28% illuminated crescent 8 arcseconds wide and glowing at mag +0.5, and becomes brighter (mag -1.3), smaller (5 arcseconds), and gibbous (95% lit) by the time it becomes lost in the morning twilight. Mercury will reach superior conjunction on Sept 30 and return in mid-November as an evening target. Among the asteroids, 1 Ceres and 7 Iris continue to be within the reach of binoculars at mag 8-9 brightness – this month’s charts will show you where to find them.

Something to watch for: I usually don’t cover comets in Rick’s Picks, because (as David Levy once famously said) comets are like cats: they have tails and do whatever they want. I make an exception once in a while, however, and this month that’s comet C/2023 A3 (Tsuchinshan-ATLAS), which may reach mag +1 in late September or October – or so the soothsayers tell us. Keep an eye out for it 45 minutes before sunrise starting towards the end of the month, 5° to 10° above the horizon in the ESE sky. Just don’t be too surprised if it doesn’t show up.

Sept 1: take a look at Saturn tonight as its mag 8 moon Titan skims by, just a few arcseconds from its south limb. And if you’re lucky enough to be at a dark site during the next two weeks, take advantage of the steep morning ecliptic and look for the zodiacal light rising like a subtle elongated triangle into the eastern predawn sky.

Sept 2: it’s a challenge for big apertures as Saturn’s mag 10 moons Tethys and Dione and their shadows transit their planet tonight. Tethys makes the trek from 8:25 to 11:05 pm CDT and Dione follows from 9:15 to midnight. Look for their almost imperceptible dark shadows in Saturn’s equatorial and tropical bands, just below the rings.

Sept 4 and 5: new Moon arrived on Sept 2 at 8:56 pm CDT, after the Sun had set; on the 3rd, there were only 25 minutes between sunset and moonset, with the Moon less than 1° above the horizon 20 minutes after sundown. It’s still a significant challenge on the 4th: at 7:50 pm (30 minutes after sunset), the Moon will be a paper-thin crescent 1.9 days old, 3% illuminated, barely 2° above the horizon, and 14° left of the spot where the Sun went down. To make the hunt easier, you can use Venus as a finding aid. It will be a brilliant mag -3.8 beacon almost 5° above the horizon; the Moon will be 3° lower and 3° farther to the right. If you miss it on the 4th, try the 5th: at 7:48 pm (a half hour after sunset), find bright Venus 5° above the horizon and 18° left of the spot where the Sun went down, then look for the thin crescent Moon, 8% illuminated, 7° directly to the planet’s left.

Sept 7: Saturn reaches opposition at midnight tonight. The rings often appear noticeably brighter than usual within a couple days before to a couple days after opposition due to the Seeliger Effect: the chunks of ice that form the rings don’t cast shadows on their neighbors from our perspective when the illuminating sunlight is aligned with our line of sight, and backscattering of light directly back towards us adds to the brightening effect. If you’re up late, also take a look at Mars perched less than 1° S of open cluster M35. The pair rise together shortly after midnight, and cross the sky in tandem until morning twilight dissolves the view.

Sept 9: try for a daylight occultation tonight as mag 2.9 star Pi () Scorpii slips behind the dark limb of the 37% illuminated quarter Moon, just above the Moon’s south pole, at 6:40 pm CDT (a half hour before sunset). The star pops back out from the bright side, again just above the south pole, at 7:29 pm.

Sept 10: check the SW sky between 8 and 10 pm CDT tonight for a 6° pairing of the first quarter Moon and Antares (Alpha [] Scorpii). Can you see the color contrast between the grayish Moon and the reddish star?

Sept 16: watch as the Moon and Saturn seem to be drawn toward each other tonight. They start out at a respectful 5° separation at 8 pm CDT and slowly approach one another, closing to 4° by midnight, 2° by 3 am, and only a half degree when the pair set around 5:30 am. (The Moon occults Saturn a half hour later, but the occultation won’t be visible from our part of the planet).

Sept 17: the full Moon displays a very minimal partial eclipse from 9:13 to 10:16 pm CDT tonight, but Earth’s umbral shadow will extend only from the lunar north pole to the north edge of crater Plato, covering barely 8% of the Moon’s surface. Later tonight, famous variable star Algol (Beta [] Persei) reaches minimum brightness for two hours centered on 12:42 am (if the algorithm used by Sky & Telescope is correct) or 12:28 am (if the one used by RASC and AAVSO is on the money).

Sept 20: Algol again reaches minimum for two hours centered on 9:31 pm CDT (S&T) or 9:17 pm (RASC/AAVSO).

Sept 21: the Moon dances with The Pleiades (M45) and Uranus tonight. The trio form a 5° triangle low in the east at 10 pm CDT. Uranus maintains its distance from the cluster, but the Moon inches closer and closer, reaching 2° by midnight, 1° by 2 am, and a half degree by 4 am. By 6 am it sits within the dipper, surrounded by the cluster’s stars, as morning twilight intrudes like an angry chaperone and erases the intimate tableau.

Sept 22: the autumn equinox arrives at 7:44 am CDT, closing the door on another too-short summer.

Rick Gering / August 2024