It was early January 2025 when a faint light spot was observed at the Mt. Lemmon Observatory in Arizona. Follow-up observations revealed that the object was a comet visiting from the outer edge of the solar system, and it was named Comet Lemmon (C/2025 A6). Its “period”—the time it takes to complete its lengthy orbit of the sun—is about 1,350 years.
At first, the comet was so faint that it could only be seen with specialized equipment. But it has brightened rapidly as the months have passed, and by the time fall arrived, it was bright enough to be viewed with binoculars. It will now get brighter still, as it gets closer to both the Earth and the sun, and may even become visible with the naked eye.
According to NASA, Comet Lemmon will be closest to Earth around October 21, 2025, passing at a distance of about 0.60 astronomical units, or au (1 au is the distance between Earth and the sun). The comet is then predicted to reach perihelion—the point at which it is closest to the sun—on November 8. Around the time the comet reaches perihelion, it will likely reach its maximum brightness, and may be visible to the naked eye if conditions are favorable in dark skies.
The comet’s tail will also be at its longest extension around the point of perihelion. Tails form on comets when solar radiation heats up frozen ice and gases contained in the comet’s nucleus, causing them to sublimate—transform from solid into a gas—with the radiation then causing some of that material to be emitted out from the comet in a telltale streak. The stronger the radiation, the greater the size of this effect, and so comets’ tails grow as they get closer to stars and shrink again when they move away.
Comet Lemmon is notable for its greenish tint. This is caused by the gas cloud that surrounds the comet’s nucleus, known as its “coma,” with the color being created by diatomic carbon (molecules of two carbon atoms) interacting with sunlight.
How to View the Comet
As of early October, Comet Lemmon can be found slightly below the handle in the Big Dipper (or the Plough, as it’s known in the UK), which forms part of the Ursa Major constellation. If you look up at the northern sky, you will be able to trace its path. In the northern hemisphere, it is predicted to appear in the northwestern-to-western sky after sunset from mid-October onward. It will be at a low altitude above the horizon and is expected to emit a faint glow in the twilight just after the sun has set.
The best time to observe the comet is about a week before and after its closest approach. In November, the comet will cross the celestial equator and become visible from the southern hemisphere.
Binoculars or small telescopes are best for observing it, but if you choose a location with little light pollution, you may be able to catch it with the naked eye. If you look up at the sky just after sunset when it is still a little bright, you may be able to see it. It you are struggling to find the comet, use a star map application on your smartphone or planetarium software to help you pinpoint its location.