October ’24 – Rick’s picks

All month: the solar system becomes a little more balanced this month, as Mars, Jupiter, and Uranus continue to light up the predawn sky even as they move inexorably into their evening apparitions, where Saturn and Neptune have already firmly settled, and all five of them can be seen by midnight. Mercury and Venus are both evening stars throughout this month, but because of the relatively flat ecliptic on summer nights and autumn evenings, neither of them gets very high in the sunset sky. Venus opens the month at an elevation of only 7° a half hour after sunset (and just 11° as the month ends), while its gibbous disk grows from 12 to 14 arcseconds and it brightens slightly from mag -3.9 to -4.0. The bright planet sets 1:26 after the Sun on October 1, and 1:49 after it on the 31st. Mercury fares even worse, remaining hidden in the Sun’s glare most of the month, and sitting just 5° above the horizon at sunset on October 10 when it sets 30 minutes after the Sun. By month’s end, it’s still less than 2° above the horizon a half hour after sunset and sets just 9 minutes after that. Your best bet might be to use Venus as a landmark and look for Mercury’s 5 arcsecond disk glowing between mag -0.5 and -0.3 just above the horizon, on a line from Venus to the spot where the Sun went down. Among the outer planets, Saturn is the first to rise (at 5:37 pm CDT as the month begins and 3:35 pm as it ends), its flattened disk 19 arcseconds wide but only 16 arcseconds tall, shining at mag +0.7 to +0.8, with rings set at a narrow 5° angle to our line of sight and shrinking from 43 to 41 arcseconds as the month goes by. Neptune rises a half hour after Saturn, a deep blue mag +7.8 glow emanating from its 2.4 arcsecond disk. Uranus is the first planet to rise after sunset (at 8:30 pm on October 1 and 6:29 pm on October 31), its 4 arcsecond disk displaying a powder blue mag 5.6 luster as it approaches its November 17 opposition. Ninety minutes later, mighty Jupiter follows Uranus into the sky, its 42 arcsecond disk growing to 46 arcseconds over the course of the month, as it brightens from mag -2.5 to -2.7 on its way to its own opposition on December 7. Bringing up the rear is ruddy Mars, which rises at 11:38 pm as the month begins and 10:45 pm as it ends. Its disk begins to show more surface detail as it grows from 7.5 to 9 arcseconds during October, brightening from mag +0.5 to +0.1 and giving us tantalizing hints at how it will look at opposition in January 2025. Both Jupiter and Mars reach dizzying heights this month, illustrating the other side of the low evening ecliptic, with Mars at 65° or more by 6 am all month long, and Jupiter towering at 70° elevation by 5 am as the month opens and 4 am as it ends. Among the asteroids, all of our targets are fainter than mag 9.0 this month, so instead, we’ll chart the anticipated track of comet C/2023 A3 (Tsuchinshan-ATLAS), which is still expected to reach naked-eye visibility after it passes the Sun and becomes an evening target beginning around October 10. It reaches its close approach to Earth on October 12, but the Moon will make it difficult to spot until October 20.

October 1: if you’re lucky enough to be at a dark site during the next two weeks, watch for the zodiacal light rising as a tall subtle triangle of softly reflected sunlight a couple hours before sunrise, and try not to dwell too much on the fact that it’s only there because our solar system is so full of loose dust and debris. Today, early risers can try to spot comet C/2023 A3 (Tsuchinshan-ATLAS) at 5:46 am CDT (an hour before sunrise), 2° above the horizon and 9° right of due east, before it disappears from the predawn sky.

October 2: the shadow of Europa marches across the face of Jupiter from 10:48 pm CDT (53 minutes after the planet rises) until 1:18 am, followed by Europa itself from 1:21 am to 3:52 am.

October 4-5: new Moon arrived on Oct 2 at 1:49 pm CDT, too late to let anyone catch the crescent that night, and even on the 3rd it set only 17 minutes after the Sun. It’s still a challenge on the 4th: at 6:46 pm (20 minutes after sunset) it will be just 2.2 days old, 4% illuminated, 3° above the horizon, and 20° left of the spot where the Sun went down, with brilliant mag -3.9 Venus perched 9° to its upper left. The 5th will be a little easier: at 6:55 pm (a half hour after sunset), the crescent will be 3.2 days old, 9% illuminated, 5° above the horizon, and 28° left of the spot where the Sun set, while Venus will be just over 4° to its upper right.

October 7: you’ll need binoculars to see Antares (Alpha [] Scorpii) just 2° to the right of the slim crescent Moon tonight. The pair will be 12° above the horizon at 6:51 pm CDT (a half hour after sunset) and will set around 8:30 pm. Can you see mag 2.8 Tau [] Sco just below the Moon’s lower cusp?

October 14: watch in the minutes leading up to 6:29 pm CDT as the narrow dark rim of the almost-full Moon seems to step backwards and gobble up mag 4.9-star Chi () Aquarii. You’ll need at least binoculars, and probably a telescope, to spot the star in the bright twilight just 19 minutes after sunset. Meanwhile, mag +0.7 Saturn marches 3½° ahead of the Moon as the pair cross tonight’s sky.

October 15: astrophotographers experience group tachycardia around 7 pm CDT as comet C/2023 A3 (Tsuchinshan-ATLAS) passes just 1.2° S of globular cluster M5 and 2° NE of fading comet 13P/Olbers. Later tonight, there’s a double disappearance at Jupiter, but the timing is a little unclear. RASC says Ganymede will slip into Jupiter’s shadow at 11:32 pm CDT, followed almost immediately by Io at 11:33 pm, while Stellarium shows Ganymede disappearing at 11:19 and Io at 11:30. Ganymede pops back out of the shadow in almost the same position as where it entered, at 1:35 am (RASC) or 1:47 am (Stellarium). Io pops out from the opposite side of the planet at 2:52 am according to both sources.

October 17: tonight marks the largest full Moon of 2024, so be ready for the supermoon brigade to come out of the woodwork. Despite the inevitable hyperbole, the Moon tonight measures 33.85’ at its largest, compared to the 29.86’ diameter of 2024’s tiniest full Moon (in February) – a difference of 13.4%, which is less than the relative difference in size between a nickel and a quarter.

October 19-20: the almost-full Moon stalks The Pleiades (M45) on the 19th, following 4° behind the cluster as the pair cross the sky, with Jupiter following the Moon from a slightly more respectful distance. On the 20th, Jupiter closes in to just 5° below the Moon.

October 23: the shadow of Jupiter’s innermost moon, Io, crosses the planet from 10:36 pm to 12:46 am CDT, followed by Io itself from 11:36 pm to 1:46 am.

October 25: you’ll need at least binoculars, and maybe a telescope, to spot ruddy mag 1 Antares (Alpha [] Scorpii) 3° below and left of blazing mag -3.9 Venus around 8:23 pm CDT (a half hour after sunset), when the star is 7½° above the horizon and the planet is 2½° higher than that.

October 29: night owls and early risers can watch as Mars joins with Castor (Alpha [] Geminorum) and Pollux (Beta [ Gem) to form a perfectly straight and nearly evenly-spaced line crossing tonight’s sky. The trio rise at 11 pm CDT and are almost directly overhead, extending from 70° to 80° elevation, by 6 am.

Rick Gering / September 2024