Good news about Comet C/2023 A3 (Tsuchinshan-ATLAS): After being lost in the glare on the far side of the sun for several weeks, the comet was again seen in mid-September by observers in Australia, who report that its brightness should provide an impressive show in October.
The comet will pass closest to Earth on Oct. 12, at a distance of 43.9 million miles. The comet is visible in the mornings, rising in twilight 7-8° south of east, just more than an hour before sunrise through Oct. 4; binoculars should give the best views. The comet will be 12° to the upper right of a rising, 1 percent crescent on Oct. 1.
It seems likely that there’ll be a surge in the comet’s brightness, caused by forward scattering of sunlight by cometary dust grains, as the comet passes halfway between Earth and the sun, while appearing within 4° to the upper left of the midday sun, on Oct. 9. Will Comet C/2023 A3 become visible in the daytime, as Comet C/2006 P1 (McNaught) was in Palm Springs on Jan. 14 and 15, 2007?
To try for a daytime naked-eye or binocular sighting, stand in the shade on the north side of a building at 12:33 p.m. in Palm Springs, and be very careful to block the sun entirely with the top of the building. Then look 4.3° to the upper right of the (hidden!) sun’s midday position on Oct. 8; then on the next day, Oct. 9, look 3.9° above and slightly to the left of the sun’s position; and look 7.2° to the upper left of the sun on Oct. 10. Let’s hope for a very dusty comet to make these observations possible!
You can also try looking during sunrise and sunset on Oct. 9, 3.6° to the upper left of the rising sun, and 4.5° to the upper right of the setting sun. Again, be absolutely sure the sun is completely covered! The conjunction of comet with the sun on Oct. 9 will be followed a few days later by a favorable emergence into the western evening sky, as the comet climbs higher and farther away from the sun nightly. The comet’s daily eastward shift against background stars is 5.6° on Oct. 11-12, and then 5° on Oct. 14-15, slowing to 4° on Oct. 17, and 3° on Oct. 20. It’s down to 2° per day on Oct. 24, and just 1.5° on Oct. 27.
Predictions of a comet’s brightness can be notoriously uncertain! A forecast by comet researcher Joseph Marcus, reported by Bob King on the website of Sky and Telescope, cautiously predicts magnitude -4.8 at peak brilliance on Oct. 9; magnitude +0.2 on Oct. 13; magnitude +2.5 on Oct. 17; and magnitude +3.6 on Oct. 20.
This September Evening Skies map closely depicts the sky one hour after sunset from Palm Springs on Oct. 13, the first date the comet is plotted on September’s star map. By Oct. 13, the comet will set after the end of evening twilight and be visible in a dark sky—except for the presence of a bright moon through Oct. 19. (The closest full moon of this year occurs on the night of Oct. 16-17.) The comet’s position against background stars is shown on the star map daily through Oct. 20, and then every 2 or 3 days until Oct. 28. Keep in mind that the stars in the western sky appear a little lower each night, so by Oct. 28, the sky at the end of evening twilight will be a bit more than an hour later than the sky shown on the September map. On Oct. 28, Antares will have set by the end of twilight, and Arcturus will be very low, while the comet and nearby Beta and Gamma in Ophiuchus will be lower than the map shows. Note there are labels on some of the stars of Virgo, Serpens and Ophiuchus. These stars will help locate the comet as it passes by. We may place comet updates and links on the Sky Calendar Extra Content Page atwww.abramsplanetarium.org/msta.
In other news: Keep checking the Northern Crown, Corona Borealis, for the once-in-a lifetime eruption of the recurrent Nova T Coronae Borealis. (The position of T Cor Bor is plotted on the same star map, September Evening Skies,but as of this writing, the star hadn’t erupted yet, and it’s still well below naked-eye and binocular visibility.) The brightest star in the Crown is 2.2-magnitude Alphecca, or Alpha Coronae Borealis. It’s easy to spot, nearly 20° east-northeast of Arcturus, and one-third of the way from Arcturus toward Vega. (Vega and Arcturus are 59° apart.)
In reasonably dark skies, seven stars in the bowl-shaped crown are usually seen. Here’s a list of the seven stars, in order from west to east, counterclockwise around the arc, with their magnitudes: Theta (magnitude 4.2), Beta (3.7), Alpha (Alphecca, 2.2), Gamma (3.8), Delta (4.6), Epsilon (4.1) and Iota (5.0). Note the three stars along the southern edge of the crown—Alpha, Gamma and Delta—lie in a nearly straight line. When the “Blaze Star” T Coronae Borealis becomes visible, it will extend that line another 2.2° past Delta, so there will be four stars—Alpha, Gamma, Delta and T—arranged in a nearly straight line. Check often, because T CrB won’t remain near peak brilliance very long before it quickly fades.
Corona Borealis will sink low in the west-northwest at nightfall by mid-November. If the eruption of T Coronae Borealis hasn’t occurred by then, switch to the morning sky, and you’ll find the Northern Crown low in the east-northeast as twilight begins, and climbing higher each morning. You’ll be able to continue to watch for the eruption of the nova through its conjunction 46° north of the sun on Nov. 17.
A quick summary of our usual topics, the visibility of the moon and planets: At dusk, look for brilliant Venus, of magnitude -4, low in the west-southwest to southwest, and Saturn, of magnitude +0.7, well up in the southeastern sky. Watch the moon pass a few degrees south (to the lower left) of Venus on Oct. 5; pass Antares on Oct. 7; and hopscotch past Saturn on Oct. 13-14. Venus goes 3° north of Antares on Oct. 25. A telescope shows Venus’ tiny disk now in gibbous phase; the planet will be much more interesting to follow in January through March as it draws closer to Earth and displays half and backlit crescent phases. As Earth pulls ahead of Saturn, we see the rings temporarily opening to 5.0° or more from edge-on from Oct. 19-Dec. 5, before they present edge-on to Earth and the sun in early spring of 2025.
As morning twilight begins to brighten, Jupiter, of magnitude -2.6 in Taurus, and Sirius, of magnitude -1.5 in Canis Major, dominate. On Oct. 9, Jupiter begins 10° of retrograde motion, entirely bracketed between the stars Aldebaran and Beta and Zeta Tauri, the tips of the horns of the Bull. Mars, at magnitude +0.5 to +0.1, is in Gemini nearly all month before crossing into Cancer. At month’s end, the red planet still ranks below Capella and emerging Arcturus in brilliance, but overtakes Procyon and ends about equal to Rigel. Mars passes 5.7° south of Pollux on Oct. 19, the first of a triple conjunction between them in 2024-25. Watch Mars nearly align with the Twin stars on Oct. 29 and 30: A line from Castor to Pollux, 4.5° long, extended about 7°, will locate Mars.
Follow the waning moon in the morning: Look for the old moon low in the east at dawn on Oct. 1. After full, watch the moon pass widely north of Aldebaran on Oct. 20; pass north of Jupiter and very close to Beta Tauri on Oct. 21; move through the field of Castor, Pollux and Mars on the mornings of Oct. 23 and 24; and pass closely north of Regulus on Oct. 26. On Oct. 30, look for the star Spica rising below a thin crescent moon beautifully illuminated by earthshine on its upper, non-sunlit side. On the following morning, Oct. 31, look again about 40 minutes before sunrise for an even thinner old moon rising to the lower right of Spica. The event is not illustrated on the Abrams Planetarium Sky Calendar for October, but I think if the sky is very clear and you have binoculars and an unobstructed view, you’ll be able to see it!
The Astronomical Society of the Desert will host a star party on Saturday, Oct. 12, at the Santa Rosa and San Jacinto Mountains National Monument Visitor Center; and on Saturday, Oct. 26, 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 to the two sites, 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.
