Updated: Comet Tsuchinshan-ATLAS, or “Choo-chin-SHAHN,” is dazzling skywatchers in the Northern Hemisphere this week with its nightly display. About the eleventh, in the evening twilight, it started to emerge extremely low in the west. Since the 16th, it has been steadily rising every evening shortly after dusk, even as the Moon turns full on the 16th and 17th and starts to fade and shrink into the distance. By unfortunate coincidence, this year’s largest and brightest supermoon is occurring at this time.
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Revised: During the week of its evening splendor, comet Tsuchinshan-ATLAS (pronounced “Choo-chin-SHAHN”) is visible to skywatchers in the Northern Hemisphere. About the eleventh, in the evening twilight, it started to emerge extremely low in the west. Since the 16th, it has been steadily rising every evening shortly after dusk, even as the Moon turns full on the 16th and 17th and starts to fade and shrink into the distance. By unfortunate coincidence, this year’s largest and brightest supermoon is occurring at this time.
The US map of C/2023 A3Comet Tsuchinshan-ATLAS presently shows the object high in the west, best viewed just after twilight ends, roughly 90 minutes after sunset. These dates (which are not Universal Time) are North America’s civil dates. October 18–22 will see a decline in C/2023 A3 from roughly third magnitude to fourth on the 18th. On Sunday, October 20, a window of moonless darkness just following dusk opens.
Images of the Sky and Telescope
However, on the evening of the 15th, even with thin cloud streaks, the comet was big and visible in my suburban sky lit by the moon. Using 10×50 binoculars, at least 5° or 6° of tail was visible; it appeared exactly as in the photographs, but naturally, the visual perception was darker and had a wider dynamic range (both brilliant and dark areas were displayed simultaneously). The nearly brilliant innermost nucleus observed in the 10x50s on October 12th had spread out.
OCT. 11, FRIDAY
■ On September 27, Comet Tsuchinshan-ATLAS (C/2003 A3) passed 0.39 a.u. from the Sun as it swung through perihelion. It now rises low into the evening-twilight sky in the Northern Hemisphere, its head probably glowing at about magnitude –1.5, as bright as Sirius, as its solar roasting proceeds.
This evening, start your comet search, though it may not be simple just yet. Locate a spot that offers a clear view almost all the way to the western horizon. Once the sun has set, observe the sky there from around 6° above horizontal for 30 minutes. At arm’s length, that is just three finger-widths.
As an alternative, the comet’s head will be roughly 28°, or nearly three fist-widths, to the right of bright Venus and most likely slightly lower (the lower with respect to Venus, the more south you are).
Tonight, it will probably appear tiny, with a bright inner tail that curves sharply to the right. While twilight is still relatively bright, the comet sets.
It will be little easier and around 4° higher tomorrow. After that, every evening the view will get better as it rises, and the tail will swing around to point upward. Comet T-A will, however, dim as it moves away from the Sun, even if it will become easier to find.
OCTOBER 12, SATURDAY
■ As of right now, 30 minutes after sunset, Comet Tsuchinshan-ATLAS is higher, at roughly 10° above the west horizon (as seen from mid-northern latitudes). It might be as brilliant as magnitude -0.8. Once more, if you have one, bring a telescope or binoculars.
A few stars are starting to emerge sixty minutes after sunset, and the comet’s head is still 5° above the west point of the horizon. The head appears at the end of twilight, but depending on your level of light pollution, you may be able to see the long, upward-extending dust tail as night falls. How much of the tail can you see peeking above the western horizon once nightfall has reached its peak?
October 13, SUNDAY
■ And even now, sixty minutes after sunset, the head of the comet is still approximately nine degrees high in the dimming sky. It lies around halfway between Arcturus to the right or upper right and Venus to the left, or lower left. The head’s brightness might be equal to that of Arcturus, which is zero.
T-A should by now develop a thin, pale antitail pointing in the opposite direction of the main tail. When Earth travels across a comet’s orbital path and we see the thin, broad strip of dust debris, it can seem as an antitail.
The comet is still a few degrees above the horizon at the conclusion of twilight. However, air extinction significantly reduces the brightness of any celestial objects that low.
Throughout the remainder of the upcoming week, the comet will ascend higher and be visible for longer until fading and vanishing into the distance. By the evening of the 16th, it should have roughly a magnitude of 2.5, and by the 21st, a mag of 4.
As the waxing gibbous Moon gets brighter, the post-twilight sky gets worse. On the evenings of the 16th and 17th, the Moon is full. However, a window of darkness between twilight’s finish and moonrise will open on or around the 20th.
OCTOBER 14, MONDAY
■ How early in the twilight can you see Saturn in the distant background of the Moon at magnitude +0.7? In North America, they will appear roughly 5° apart in the evening. Take a look at the scene above.
Tuesday, October 15,
■ Comet T-A’s head will pass close to the 6th-magnitude globular cluster M5 this evening for the Americas, presenting “a not-to-miss opportunity for photographers,” according to Bob King. By now, the comet’s head should be roughly second magnitude.
■ Full Moons both tonight and tomorrow night. On the 17th, at 7:26 a.m. EDT, or almost halfway between these two evenings for the Americas, the Moon is exactly full.
When identifying comets, keep an eye out for Arcturus, which is located low in the west-northwest and roughly two and a half fists to the comet’s right or lower right. When twilight ends, Arcturus will be at the same altitude above the horizon as brilliant Capella, which is located in the northeast. When that occurs, head southeast and you will see Fomalhaut of first magnitude at almost the same height as well. This is especially true if you are in the region of latitude 43° north, which includes Boston, Buffalo, Milwaukee, Boise, and Eugene.
OCTOBER 17, THURSDAY
■ The Great Square of Pegasus, the symbol of fall, is currently high in the east-southeast after nightfall and is centered on one corner for the mid-northern latitudes of the globe.
OCTOBER 18, FRIDAY
During this season, the Big Dipper is located at a horizontal low point in the north-northwest as dusk falls, approximately at 10 p.m. How little is it? The lower you are the further south you are. Even its lowest stars, when viewed from 40° north (New York, Denver), sparkle at a height of almost ten degrees at their lowest. However, the full Dipper will disappear barely below the northern horizon at Miami (26° N).
OCTOBER 19, SATURDAY
■ The Moon, now gibbous in declining phases, trails the fragile Pleiades by a few degrees as they move across the sky this evening. To reduce the brightness of the Moon, use your fingertip.
Jupiter and Aldebaran trail the Moon further behind it.
October 20, Sunday
■ As of mid-October, Deneb (for skywatchers in the mid-northern latitudes) has supplanted Vega as the zenith star after twilight. As a result, Capricornus is now the zodiacal constellation that is posing low in the south, replacing Sagittarius.
■ With the declining gibbous moonlight, don’t expect to see many meteors during the Orionid meteor shower, which should be active in the hours before dawn on Sunday morning. At that time, the radiant in Orion’s elevated club will be high in the southeast to south.
■ If you are outside before sunup on Sunday, you can still see Pollux, Mars, and Procyon making a straight line, from higher left to lower right, even if the meteors don’t impress. The brightness of the three stars is quite similar, with magnitudes of +1.1, +0.3, and +0.4, respectively. Compared to Procyon, Pollux is three times closer to Mars.
Against the stars, Mars is rapidly traveling eastward. The line won’t be quite so straight in the morning.
