Transient Events That Can Give Us a Good Enough Reason to Get Outside and Do a Little Observing
September 2025
All month: the early evening sky is still bereft of planets this month, other than unimpressive mag +1.6 Mars, now just 4” across, and it too will disappear into the sunset by month’s end. But help is on the way in the form of Saturn and Neptune: they rise almost simultaneously around 8:15 pm as September begins and 6:15 pm as it ends. The two planets are within 1.7° of one another at the beginning of the month and 3° at its close. Neptune spends the month at mag 7.8 with a 2½” disk, while Saturn’s 19” disk glows at mag +0.6. The rings extend to 44”, tilted just 2.6° to our line of sight as September begins and flattening even more to 1.5° by month’s end – they’ll be nearly edge-on with a 0.3° tilt in late November, then will begin to open again. Uranus isn’t far behind, rising at 10:45 pm on Sept 1 and 8:45 pm on Sept 30 with a mag 5.7 disk nearly 4” across. It spends the month 4° SE of The Pleiades (M45) – the two of them should just fit in a 5° binocular field. Saturn, Neptune, and Uranus all linger into morning twilight, although by month’s end Uranus is the only one of the three that remains up after sunrise. Mighty Jupiter rises just before 2 am as September opens and before 12:30 am as it ends, its disk growing slightly from 34” to 37” during the month as its brightness remains around mag -2.0. Venus continues its long morning run, rising shortly after 3:30 am on Sept 1 and before 4:45 am on Sept 30, its disk shrinking slightly from 12” to 11” while becoming more gibbous (from 84% to 91%) and maintaining its brightness at mag -3.9. Mercury is visible as a morning star only for the first couple days of the month, after which it slips into the solar glare on its way to superior conjunction on Sept 13. It will return as an evening target in October. Among the asteroids, minor planet 1 Ceres is an easy binocular catch, brightening from mag 8.0 to 7.6 during the month, as it heads towards opposition on October 3. This month’s charts will take you to it.
Sept 1: early risers get a visual treat this morning as Venus rises at 3:34 am CDT, with the stars of The Beehive Cluster (M44) just 1° above left of the brilliant mag -3.9 planet. You’ll need at least binoculars, and maybe a telescope, to see the stars of the cluster in the brightening sky. Fifteen degrees above Venus, Castor (mag 1.9) and Pollux (mag 1.2) make a large right triangle with Jupiter (mag -2.0). At 5:12 am, just over an hour before sunrise, tiny Mercury (mag -1.2) breaks the horizon, almost exactly as far behind Venus as Jupiter is ahead of it, giving us an evenly-spaced planetary parade spanning roughly 40° of sky.
Sept 3: it’s another episode in the series of Titan shadow transits across the face of Saturn late tonight, as the big moon’s shadow appears high on the ringed planet’s upper left limb around 12:25 am CDT, reaches the planet’s midpoint around 2 am, and slips off the upper right edge shortly after 4 am. Titan itself skims the edge of the disk, passing above Saturn’s north pole near the end of the event. You’ll need at least 6” of aperture (but preferably more) and at least 150x magnification to see this show, which can only be seen while Saturn’s rings are relatively flat from our perspective.
Sept 5: watch the slim dark edge of the almost-full Moon as it covers mag 2.9 star Delta (δ) Capricorni tonight at 12:58 am CDT.
Sept 9: if you’ve never caught Neptune in binoculars, tonight would be a good time to cut that notch in your astronomical belt. Start by putting Saturn in the field – it makes a right triangle with two mag 5 stars. Follow the long leg of the triangle from the star to Saturn and three more steps to a fainter mag 6 star, then slowly backtrack a half-degree to spot the dark blue disk of the most distant major planet.
Sept 12-14: if you have an unobstructed west horizon, turn your binoculars in that direction around 7:35 pm CDT – a half hour after sunset – on any of these three evenings and try to spot mag +1.0 Spica (Alpha [α] Vir), 5° above the horizon and 25° left of the point where the Sun went down, with reddish but featureless Mars appearing as a slightly fainter mag +1.6 target 2° higher than the whitish star.
Sept 19 (morning): if you ever get up early, today’s the day to do so. Around 4:15 am CDT a delicate, 5% illuminated crescent Moon will rise just a quarter-degree from mag -3.9 Venus and a half-degree from mag +1.4 Regulus (Alpha [α] Leonis) in what Sky & Telescope calls “one of the most striking conjunctions of the year.” The show continues until the growing twilight erases Regulus around 6 am.
Sept 19 (night): the shadow of Titan crosses the disk of Saturn again tonight, becoming visible high on the upper left edge by midnight, reaching Saturn’s meridian around 1:30 am CDT, and slowly crossing off the upper right limb beginning around 2:30 am. Titan itself will transit the disk quite close to its shadow tonight: it will be a difficult target to spot against the underlying Saturnian clouds, but easy to confuse with its shadow if you are able to see it.
Sept 19-Oct 3: if you’re lucky enough to be at a dark site during these two weeks, keep an eye out for the subtle triangle of the zodiacal light rising from the eastern horizon a couple hours before the Sun.
Sept 21: Saturn reaches opposition tonight at mag +0.6. Neptune follows it on Sept 23 at mag +7.8.
Sept 22: another summer slips into the shadows of memory as the equinox arrives today at 1:19 pm CDT.
Sept 24: new Moon arrived at 2:54 pm CDT on Sept 21, but even yesterday it set barely 30 minutes after the Sun. Tonight’s crescent is a more reasonable quest: at 7:15 pm CDT (a half hour after sunset), the Moon will be 9% illuminated, 4° above the horizon, and 29° left of the spot where the Sun went down. Once you have the crescent, see if you can spot the faint red disk of Mars (mag +1.6), 7° to its upper right.
Sept 27: look SW around 7:10 pm CDT (a half hour after sunset) for a pleasant color contrast as the thick crescent Moon cruises just 3° left of ruddy Antares (Alpha [α] Scorpii) until the pair set shortly after 9 pm.
Sept 29: we haven’t covered lunar sunrise rays for a while, but tonight you have a chance to spot the Maginus ray lighting up the crater’s floor from around 10:00 to 11:30 pm CDT. Maginus is the most southerly major crater sitting directly on the terminator during this time period, but you’ll want to use a Moon map to be sure you’re on the right one. As the Sun creeps over the lunar horizon, a gap in the east wall of Maginus, created by a later impact, allows a shaft of light to enter the crater before the Sun overtops the crater’s wall. While the surrounding area remains in darkness, you’ll see a narrow shaft of light slowly fan out as it spreads across the crater’s floor.
Rick Gering – September 2025
This month’s One Good Target, Rick’s guided tour of one area of the sky: A Remarkable Group of Doubles in Cygnus: Chaple’s Arc and The Fairy Ring