How to See Comet Lemmon This October
It was early January 2025, when a faint bright spot was observed at the Mount Lemmon Observatory in Arizona. Follow-up observations revealed that the object was a comet coming from the outer edge of the solar system and was named Comet Lemmon (C/2025 A6). Its “period” – the time it takes to complete its long orbit around the sun – is about 1,350 years.
At first, the comet was so faint that it could only be seen with specialized equipment. But the light brightened quickly as the months passed, and by the time fall arrived, it was bright enough to observe with binoculars. It will now become even brighter as it gets closer to Earth and the sun, and may even become visible to the naked eye.
According to NASA, Comet Lemmon will be closest to Earth around October 21, 2025, passing at a distance of approximately 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 where it is closest to the sun, on November 8. By the time the comet reaches perihelion, it will likely reach peak brightness and may be visible to the naked eye if conditions are favorable in a dark sky.
The comet’s tail will also have its longest extension around the point of perihelion. Tails form on comets when solar radiation heats the frozen ice and gases in the comet’s core, causing them to sublimate – change from solid to gas – with the radiation then causing some of this material to be emitted out of the comet in a telltale sequence. The stronger the radiation, the greater this effect, so the tails of comets grow as they get closer to the stars and shrink again as they move away.
Comet Lemmon is distinguished by its greenish tint. This is caused by the cloud of gas surrounding the comet’s nucleus, known as a “coma”, whose color is created by diatomic carbon (molecules of two carbon atoms) interacting with sunlight.
How to see the comet
In early October, Comet Lemmon lies slightly below the handle of the Big Dipper (or the Plow, as it is known in the UK), which is part of the Ursa Major constellation. If you look at the northern sky, you can trace its path. In the Northern Hemisphere, it should appear in the northwest to west sky after sunset, starting in mid-October. It will be low above the horizon and should give off a faint glow at dusk, just after sunset.
The best time to view 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 observe it with the naked eye. If you look at the sky just after sunset, while it’s still a little light, you might be able to see it. If you’re having trouble finding the comet, use a star map app on your smartphone or planetarium software to help you pinpoint its location.
