A long-period comet, at perihelion in September 2024
Last at perihelion in September 2024. Not expected to return in our lifetimes.
Last at perihelion in September 2024. Not expected to return in our lifetimes.
Tonight from Georgetown it is highest at 6:50 pm GYT, 54 degrees up in the west (halfway up). When the sky turns fully dark at 6:50 pm it stands 54 degrees up in the west. It sinks below 20 degrees at 9:13 pm. It is at least 10 degrees up for 3.1 hours of darkness.
At about magnitude 18.7 (forecast) it reads 18.8 through the air at that height. From Georgetown's sky, where the faintest star overhead is about magnitude 4.3, a comet has to read 3.3 for the naked eye, 6.8 for binoculars, 8.4 for a small telescope and 12.1 for a camera. So it is out of reach tonight.
From Lethem, under a dark interior sky: Last at perihelion in September 2024. Not expected to return in our lifetimes.
Face west (azimuth 286 degrees) at 6:50 pm GYT and look halfway up, 54 degrees above the horizon. A fist held at arm's length covers about 10 degrees.
A comet's gas tail points away from the Sun: at that minute the anti-Sun direction leans 20 degrees to the right of straight up. It is 74 degrees from the Sun in the sky and 93 degrees from the Moon, which is 3% lit.
Against the stars it moves about 11 arcseconds an hour, so it is in the same field of view all night and a little further along the next night.
Nobody has reported its brightness to COBS in the last 30 days, so every number here is a forecast.
No one has followed this comet through a return like this one, so the forecast is a catalogue estimate with a wide range. It looks like a first-time visitor from the outer Solar System, and those usually brighten more slowly than their early numbers suggest.
Catalogue estimates, shown as a cross-check and never as how bright it will look: JPL's law gives 18.8 tonight and the Minor Planet Center's gives 18.7. Catalogue laws missed this season's measured comets by 2 to 4.5 magnitudes.
A comet's light is spread over a fuzzy patch, so it is harder to see than a star of the same magnitude. That is why the gear rungs ask for a magnitude in hand.
On a fixed tripod keep each exposure short enough that the stars stay round, then stack many frames. For a long stack, align on the comet as well as on the stars: it drifts against them from frame to frame. A dark site adds more than any lens.
C/2023 A3 (Tsuchinshan-ATLAS) passed closest to the Sun (perihelion) on 27 September 2024, at 0.391 au, where 1 au is the Earth's distance from the Sun. Its orbit is so stretched that it will not return in our lifetimes. The orbit is tilted 139.1 degrees to the Earth's and its eccentricity is 1.0002 (0 is a circle, 1 an open curve).
Tonight it is 8.64 au from the Sun and 8.85 au from Earth, moving away from the Sun.
Orbit: NASA/JPL Horizons osculating elements, solution JPL#113, retrieved 8 October 2026. Positions on this page are computed from them in your browser and agree with JPL's own tables to half an arcminute.
The bright comet of October 2024, seen with the naked eye from Guyana low in the west after sunset. It is on a path that will not bring it back in our lifetimes.
C/2023 A3 (Tsuchinshan-ATLAS) is a long-period comet. The "C/" marks a comet that is not known to return within 200 years; the year and letters record when it was found.
Each evening at the minute the Sun was 12 degrees below the horizon. Positions were checked against JPL Horizons; magnitudes are medians of eye estimates reported to COBS, then a fit to them.
It was at its brightest in the second week of October, when it stood only a few degrees from the Sun: the SOHO spacecraft's coronagraph recorded it near magnitude -3 (ICQ), while no observer on the ground reported brighter than -1.5. From a dark site such as Lethem it reached the naked eye on 13 October.
| Evening | Time (GYT) | Height | Direction | Moon lit | Magnitude | In reach with |
|---|---|---|---|---|---|---|
| 11 Oct 2024 | 6:24 pm | below the horizon | west | 62% | -0.7 (observed) | nothing: lost in the Sun's glare |
| 12 Oct 2024 | 6:24 pm | 4 degrees | west | 73% | 0.6 (observed) | binoculars |
| 13 Oct 2024 | 6:23 pm | 9 degrees | west | 82% | 1.0 (observed) | binoculars |
| 14 Oct 2024 | 6:23 pm | 13 degrees | west | 91% | 2.0 (observed) | binoculars |
| 15 Oct 2024 | 6:23 pm | 17 degrees | west | 96% | 2.5 (observed) | binoculars |
| 16 Oct 2024 | 6:22 pm | 21 degrees | west | 100% | 3.0 (observed) | binoculars |
| 17 Oct 2024 | 6:22 pm | 24 degrees | west | 100% | 3.4 (observed) | binoculars |
| 20 Oct 2024 | 6:21 pm | 33 degrees | west | 84% | 4.4 (fit to observed) | binoculars |
| 24 Oct 2024 | 6:20 pm | 39 degrees | west | 44% | 5.1 (fit to observed) | binoculars |
Orbits: NASA/JPL Horizons (Giorgini, JD and the JPL Solar System Dynamics Group), data retrieved 2026-10-08.
Catalogue brightness laws: JPL Small-Body Database and the IAU Minor Planet Center.
Observed brightness: COBS Comet Observation Database (https://cobs.si) and its contributing observers, CC BY-NC-SA 4.0.
Data updated 8 October 2026. "Early forecast" means: A catalogue estimate. Early predictions are often wrong in both directions.
Gear rungs and ranges are estimates for a typical observer: your eyes, your sky and the night will differ.