After a quiet 2025 for comets, skywatchers may soon get a very special naked-eye spectacle in the shape of Comet Lemmon (C/2025 A6). Discovered in January, the green comet has brightened faster than predicted in recent weeks and now looks set to be visible from dark rural skies during October 2025. It could be at its brightest during the peak of the annual Orionid meteor shower.
Comet Lemmon was discovered on Jan. 3, 2025, by the Mount Lemmon Survey in Arizona. It was initially thought to be an asteroid.
It last visited the inner solar system 1,396 years ago — in the seventh century — with astronomers calculating that it will return in the year 3421, making it a once-in-a-lifetime comet.
By mid-October, it will be close to the Big Dipper in the northern night sky. It could shine as brightly as 5th magnitude, making it visible to the naked eye from dark rural skies.
Since Comet Lemmon will be visible in a part of the northern sky that's circumpolar, it will be visible in both morning and evening skies by mid-to-late October as seen from the Northern Hemisphere.
It will get closest to Earth — and, therefore, shine brightest — on Oct. 21, getting closest to the sun on Nov. 8. Comet Lemmon may be visible during that entire time.
Forecasts for Comet Lemmon’s brightness have recently changed dramatically. Early predictions placed it at around 10th magnitude in October, but according to Bob King at Sky & Telescope, recent observations suggest it could peak at magnitude 4.5 around Oct. 21 — visible to the naked eye from dark skies and easy in binoculars. However, comets behave erratically and it could become brighter or fainter than currently predicted.
During its closest pass to Earth, it will be at a distance of 56 million miles from Earth — the equivalent of about 60% of the distance between Earth and the sun.
If Comet Lemmon brightens, it's sure to be widely observed by skywatchers in the Northern Hemisphere. Its closest approach to Earth on Oct. 21 coincides not only with a new moon, but also with the peak of the Orionid meteor shower, when about 20 "shooting stars" per hour are expected. October is also a peak month for displays of the Northern Lights; if geomagnetic activity is high, some observers at northern latitudes may also see the comet amid auroras. It will be a fast-moving comet, so its position in the night sky will change dramatically from night to night.