How to see Comet Limon this October
It was early January 2025 when a faint point of light at the observatory of Mt. Lemmon was spotted in Arizona. Subsequent observations revealed that the object was a comet visiting the outer edge of the Solar System, 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 as the months passed, it brightened rapidly, and by fall, it was bright enough to be seen with binoculars. As it gets closer to Earth and the Sun, it gets brighter and may even be visible to the naked eye.
According to NASA, Comet Lemmon will make its closest approach to Earth around October 21, 2025, passing by at a distance of about 0.60 astronomical units, or au (1 au is the distance between the Earth and the Sun). The comet is then predicted to reach perigee—the point at which it is closest to the Sun—on November 8. Around the time the comet reaches perihelion, it is likely to 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 stretch around the perigee. When solar radiation heats the frozen ice and gases in the comet’s core, trails form on comets, causing them to sublimate—turn from solid to gas—with radiation and then release some of that material from the comet as a telltale streak. The stronger the radiation, the greater the size of this effect, so comet tails grow as they approach the stars and shrink as they move away.
Comet Lemon is notable for its greenish color. This is caused by the cloud of gas surrounding the comet’s nucleus, known as the ‘coma’, with a color produced by carbon diatoms (molecules of two carbon atoms) interacting with sunlight.
How to see a comet
From early October, Comet Limon can be found just below the handle in Ursa Major (or The Plow, as it is known in the UK), which forms part of the constellation Ursa Major. If you look at the northern sky, you can follow its path. In the Northern Hemisphere, it is expected to appear in the northwestern sky after sunset from mid-October onwards. It will be low above the horizon and is expected to emit a faint glow just after sunset at twilight.
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 be 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 see 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 trying to find a comet, use a star map app on your smartphone or planetarium software to help you pinpoint its location.
