2025 January 14

Comet C/2024 G3 (ATLAS) survives perihelion

Nick James

As I write this on January 14, the comet has survived perihelion and it has developed a spectacular tail which is visible in the SOHO LASCO C3 coronagraph. Numerous observers have imaged the comet in daylight or very bright twilight. Getting accurate magnitude estimates from daylight images is very difficult but estimates from the SOHO images indicate that the comet was around -4 on the day of perihelion.

The comet is now pulling away from the Sun and emerging into the evening sky. At present, C/2024 G3 is very bright but it can only be seen against a very bright twilight sky. If you have clear, transparent skies and a good horizon in the direction of sunset it should be possible to photograph it but seeing the comet will be much harder, even in binoculars. You would need to know exactly where to look relative to the position of the setting Sun. Over the next week the comet will move higher in the sky so the background will get darker but the comet will fade rapidly too. The comet is also moving rapidly south in declination and so will become increasingly hard to see from northern latitudes.

The elongation will increase from around 6 degrees today (January 14) to 19 degrees in a week’s time (January 21). For southern hemisphere observers between 10 and 30 degrees south latitude the comet will be almost directly above the setting Sun so the height above the horizon at sunset will almost equal the elongation. For observers further north the line joining the Sun and the comet gets shallower with respect to the horizon so the comet is lower. For example on the evening of January 19 the comet will be 9 degrees up at the end of civil twilight (i.e. when the Sun is 6 degrees down) for observers at 30S but only 3 degrees up for an observer at 30N. This is shown in the plots at the end of this news item. Unfortunately, for observers north of 30N, the geometry is very poor so there is almost no chance of seeing it once it has faded beyond detection in bright twilight.

If it follows predictions the magnitude will drop from around -2.5 now to possibly magnitude +3 or fainter in a week’s time so seeing the comet is a balance between its brightness and the brightness of the sky. It is quite difficult to predict how the comet will behave after its near-death encounter with the Sun so it may be a lot brighter or fainter than this. We also don’t really know how the currently spectacular tail will perform. The only way we will find out is by observing it. That is what makes comets so much fun.

Southern observers are definitely favoured but opportunities like this don’t come along very often so observers at low northern latitudes should make an effort to find the comet after sunset as it moves into darker skies. If the tail is bright it may be possible to see that even if the comet’s head has already set.

Images will appear in the section’s archive here.

Nick.