A bright comet is on track to become visible with the naked eye later this autumn. Comet C/2025 A6 (Lemmon) is heading for a fairly close encounter with the Sun and the Earth in the second half of October 2025. It may become quite bright and a beautiful sight in darker skies. Get your telescopes, binoculars and cameras ready!
A bit about comet C/2025 A6 (Lemmon)
Comet C/2025 A6 (Lemmon) was discovered on January 3rd in images taken with the 1.5-m reflector of the Mt Lemmon Survey. It orbits the Sun in an elongated elliptical orbit with an orbital period of about 1000 years. It will reach perihelion at only 0.53 a.u. from the Sun on November 8th.
Orbit of comet C/2025 A6 (Lemmon) with the position of the comet and inner planets on 30 September 2025. Plot: NASA Solar System Dynamics Orbit Viewer.
How bright will comet C/2025 A6 (Lemmon) become
Comet C/2025 A6 (Lemmon) could put on quite a show in late October. It could possibly become as bright as magnitude 2 to 4 and potentially sport a fairly long tail.
Predicting a comet’s brightness is tricky business. As the famous comet hunter David Levy says, “comets are like cats: they have tails, and they do precisely what they want”. Comets brighten while approaching the Sun. As the Sun increasingly heats the cosmic icy dirtball the ices within it begin to sublimate. As the ices turn directly to gas, they blow off of the comet’s nucleus and form the comet’s coma and tail. The more gas and dirt (dust particles) the comet releases, the brighter it is. Comets usually brighten following an inverse 3rd power to 4th power law. That is if the comet’s distance to the Sun is reduced by a factor of 2x, the comet’s brightness increases 8-16x. More rapid brightening may be indicative of the comet undergoing fragmentation and disintegration.
Light curve of comet C/2025 A6 (Lemmon) using a simple fit through data in the COBS database. Peak magnitude is 2.
Comet C/2025 A6 (Lemmon) currently appears to be brightening quite rapidly, now brighter than 8th magnitude. It is on track to reach naked eye visibility threshold under dark skies (6th magnitude) in the first days of October. It will reach peak brightness in late October and early November as it reaches perihelion and perigee (closest points in its orbit to the Sun and to the Earth). It may become as bright as 2nd magnitude at that time, becoming an obvious naked eye object in the evening sky.
Comet C/2025 A6 (Lemmon) tail length
The comet and the Earth will pass in October through observing geometry that may make the comet’s tail quite long. The comet will pass relatively near the Earth, approaching to 0.60 a.u. on October 21st. At that time it will be within the Earth’s orbit and inside of it. The gas tail will flow above the Earth, we will see it from underneath.
Viewing geometry of the comet’s tail in mid to late October, the diagram valid on October 21st. The comet’s gas tail, drawn in light blue, points directly away from the Sun. The white arrows indicate the direction of view from the Earth.
If the comet becomes bright and forms a significant gas tail, this observing geometry may make it appear quite long. Tail lengths of 10-20° are possible photographically or perhaps even visually. The gas tail will point directly away from the Sun, pointing more or less directly upward in the sky.
Finding and observing comet C/2025 A6 (Lemmon) – in mid-latitudes of the northern hemisphere
At the time of writing of this article the comet is slightly brighter than 8th magnitude and brightening. Within reach of binoculars under dark skies, but below the naked eye threshold. It is located in the constellation Lynx and visible in the morning sky. It will track through Lynx, Ursa Major and Canes Venatici in late September the first half of October and become visible in the evening sky. It may reach 6th magnitude and the naked eye threshold (under dark skies) in the first week of October. By mid-October it may be
The comet will likely peak in late October: it will brighten through the end of October, but the Moon climbs into the evening sky and becomes increasingly bright starting on October 27th. The comet will be visible quite high in the evening sky, moving through Bootes and Serpens Caput. Here is where comet Lemmon is different. While many bright comets are buried deep in twilight and visible only low above the horizon, comet Lemmon will be almost 20° high above the W horizon just after dark. With the tail pointing straight upwards. It may be quite a sight!
Good luck observing comet C/2025 A6 (Lemmon)! And remember – comets are like cats: they have tails, and they do precisely what they want. Comet Lemmon may follow our expectations, or it may surprise us. The only way to know is to look up in the sky.
This article will be updated with new observations and predictions in the coming days and weeks.
