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Comet Chasing in December

Comet chasing is the visual observation of telescopic comets. Jump to:[Observing synopses](#Observing Synopses) [Summary data](#Summary Data) Buy me a Coffee

This month there is one comet visible in binoculars, one visible in small telescopes, and two visible in 6-inch or larger telescopes (from a dark site). See the [Observing Synopses](#Observing Synopses) for which comets are visible from your latitude, the best dates and the time of night.

• C/2024 G3 (ATLAS) will reach perihelion on January 13 2025, when it will pass within 0.1 AU of the sun. This may be an interesting comet to follow in the coming months, as it is too soon to predict how bright it will get, and there is a strong possibility that it will disintegrate. It brightened by a magnitude at the end of October, heightening the uncertainty.
• C/2023 A3 (Tsuchinshan-ATLAS) was very impressive in October. Although forgotten by the media, it remains a very nice telescopic comet throughout the month. If this was as good as it would get, we would still be highlighting it. See below for observing details, as well as the video above.
• 333P/LINEAR is a periodic comet that passes within 1.1 AU of the Sun every 8.7 years, most recently on November 29th. It will pass withing 0.5 AU of the Earth in early December, when it will reach maximum brightness of 11.5.
• 29P/Schwassmann-Wachmann has frequent outbursts, typically resulting in a brightening of 0.5 - 1.0 magnitudes, which occur roughly every 59 days, typically taking 5-10 days to subside. Up to three subsequent outbursts may occur 5-10 days afterward, each typically smaller than the last, although on some occasions they can be even brighter than the first. These outbursts make 29P one of the most interesting comets to follow, both visually and scientifically. 29P/Schwassmann-Wachmann has a 14.8-year orbital period, and last passed perihelion in early March 2019. It varies in its distance from the Sun from 5.8 AU (at perihelion) to 6.3 AU (at aphelion), which is an unusually small variation for a comet, and remains quite far from the sun at all times. This means that it can be observed more or less continuously.
• C/2024 S1 (ATLAS) has disintegrated near the sun. Despite all the talk, there was little chance that this comet was ever going to be more than a SOHO comet. Beware that some software and web sites may still show it as active.
• C/2022 E2 (ATLAS) is past perihelion, which occurred in mid September 2024. It is predicted to reach maximum brightness of magnitude 12.8 in early December.
• C/2023 C2 (ATLAS) will reach perihelion in mid November. It was at maximum brightness in October.
• Comet 13P/Olbers passed perihelion in late June and is fading.
• Comet 12P/Pons-Brooks passed perihelion on April 21 and has become a southern-hemisphere object. It is fading.This well-documented comet, perhaps observed as early as the year 245, experienced a large unexpected outburst in late July 2023, catapulting from a magnitude of 16.6 to 11.6 in a single day. Afterward it displayed an unusual coma shape, characterized by a distinctive 'notch' and a pair of 'horns' that persisted for weeks. The same pattern repeated in the following months, except in recent outbursts the 'horns' have become longer and curved. Here is a link to a video with an animation of the July outburst aftermath C/2021 S3 (PANSTARRS) passed perihelion in mid February 2024.
• C/2014 UN271 (Bernardinelli-Bernstein) is an interesting Oort cloud comet that was reported as a result of the Dark Energy Survey. Soon after it was made public, images showed a cometary coma. Discovery credit goes to Pedro Bernardinelli and Gary Bernstein. This is a remarkable comet that was at 29 AU in 2014, with a perihelion of 10.9 AU in 2031. Its orbit extends out to 40,000 AU! It was unusually bright for its distance. HST observations that isolate the nucleus estimate the diameter to be between 120 and 140 km, making this possibly the largest comet nucleus yet measured. Because of its distant perihelion, this comet is not expected to become bright enough to be visually observed except in large instruments, and not until the end of the decade, but it is likely going to be the subject of intense scientific scrutiny.

Comets that have apparently di sintegrated: C /2024 S1 (ATLAS), C/2022 S3 (PANSTARRS), C/2021 P4 (ATLAS), C/2020 Q1 (Borisov), C/2020 P1 (NEOWISE), and C/2019 Y4 (ATLAS). Beware that various other sources often fail to keep track of whether or not a comet still exists!

Comet Visibility in the Eyepiece

When you read below that a particular aperture is required to see a comet you can have a reasonable degree of confidence that the comet can in fact be seen in the eyepiece.

This page uses code developed for SkyTools to predict the visibility of a comet. Predicting how much aperture is required to see a comet is a very complex task. The magnitude alone is a very poor predictor. SkyTools considers the magnitude, coma diameter, degree of concentration of the comet, based on recent observations.The predictions are made for a Country/Suburban site, which is approximately Bortle5, unless indicated otherwise. But always remember, comets are like cats. They both have tails and do what they want, and not always what we expect. This is one of the things that makes comet chasing interesting!

Observing Synopses for December

Explanation of Comet Synopses and charts (read this if you have questions)

C/2024 G3 (ATLAS): A southern hemisphere morning comet visible in binoculars
This comet begins the month in Lupus at magnitude 10.1. Look for a 1' coma, condensation is a diffuse spot in coma center; moderately condensed. It should brighten rapidly, moving into Scorpius by month's end. The best visibility is late in the month as seen from the southern hemisphere. FINDER CHART

C/2023 A3 (Tsuchinshan-ATLAS): An evening comet visible in small telescopes
This comet begins the month in Aquila at magnitude 9.1. Look for a 2.5' coma, condensation is a diffuse spot in coma center; moderately condensed. It should fade by about 1.4 magnitudes by month's end. FINDER CHART

333P/LINEAR: A northern hemisphere morning comet visible in a 6-inch (15 cm) telescope at a dark site, or a 18-inch (46 cm) under a country sky
This comet begins the month in Canes Venatici at magnitude 11.6. Look for a 2.5' coma, center much brighter than edges, though still diffuse. It should fade by about 1.2 magnitudes, moving into Cygnus by month's end. FINDER CHART

29P/Schwassmann-Wachmann: A morning comet visible in a 6-inch (15 cm) telescope at a dark site, or an 8-inch (20 cm) under a country sky
This comet begins the month in Leo at magnitude 12.1. Look for a 1.5' coma, condensation is a diffuse spot in coma center; moderately condensed. It should brighten slowly overall, but will likely fade after the recent outburst. FINDER CHART

C/2022 E2 (ATLAS): A northern hemisphere evening comet visible in an 8-inch (20 cm) telescope at a dark site, or a 24-inch telescope under a country sky
This comet begins the month in Camelopardalis at magnitude 12.8. Look for a 1' coma, center much brighter than edges, though still diffuse. It should fade slowly, moving into Cassiopeia by month's end. FINDER CHART

C/2024 M1 (ATLAS): A morning comet visible in a 16-inch (41 cm) telescope at a dark site
This comet begins the month in Orion at magnitude 14.8. Look for a 35" coma, condensation is like a star that cannot be focused; strongly condensed. It should fade by about 0.7 magnitudes, moving into Perseus by month's end. FINDER CHART

C/2023 C2 (ATLAS): An evening comet visible in a 24-inch telescope at a dark site
This comet begins the month in Sagittarius at magnitude 13.8. Look for a 30" coma, diffuse condensation at center. It should fade slowly, moving into Capricornus by month's end. FINDER CHART

Summary Data for This Month's Telescopic Comets

Comets brighter than 16th magnitude. This table is updated as necessary. The last column indicates the date of the last observation used to compute these values. The constellation listed is where the comet was on the first of the month.

*In solar conjunction and generally not visible*In solar conjunction and generally not visible

For information about specific comets see Gary W. Kronk's Cometography

Further reading: see Comet Chasing, Sky & Telescope, April 2005, pg. 83.

Make your own visual observing custom charts for your location and telescope/binoculars: Software for visual comet observing

Select comets that are appropriate for your imaging system, and plan when they are best imaged: Software for comet imaging