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A newly discovered comet is putting on an unexpected show in the October sky. Comet C/2025 A6 Lemmon, first spotted in January by the Mount Lemmon Observatory in Arizona, is brightening faster than astronomers expected and will pass within 90 million kilometers of Earth on October 21. Northern Hemisphere observers now have a chance to see it in binoculars either before sunrise or just after sunset.
When first observed, Lemmon was faint at magnitude 21.5, barely distinguishable from an asteroid. But as it drew closer to the sun, it became active. By late September, the comet reached magnitude 6.6 and displayed a three-degree-long ion tail. As of October 9, it had brightened further to magnitude 5.8, making it a visible target in binoculars under dark skies.
The brightening is due to sunlight heating the nucleus, causing ices to vaporize and release gas and dust. The coma now shows a cyan tint from cyanogen, a common comet molecule. Lemmon follows a long retrograde orbit of about 1,350 years, though a gravitational nudge from Jupiter will shorten this to 1,150 years after it leaves the inner solar system.
For skywatchers, the first half of October offers morning views. The comet sits in Ursa Major, about 15 degrees below the Big Dipper’s cup. Around October 10, it will be high enough to remain visible all night for locations north of 48 degrees latitude, including Scotland and Alaska.
From mid-October, it will appear in the west after sunset, low on the horizon, moving through Leo Minor, Ursa Major, and Canes Venatici at about four degrees per day.
Key observing dates include October 9, when it passes near the star Omega Ursae Majoris, and October 15, when it comes within two degrees of Messier 94, a spiral galaxy. On October 16, it will pass close to Cor Caroli, a bright star in Canes Venatici. The closest approach to Earth comes on October 21, at 0.60 astronomical units. Its perihelion, the closest point to the sun, follows on November 8 at 0.53 astronomical units.
By late November, Lemmon will appear near Mars, offering another viewing opportunity. Its tilted path, inclined 144 degrees to the plane of the planets, explains why it favors northern observers during this apparition.
Lemmon is not the only comet active this month. C/2025 R2 SWAN will pass 0.26 astronomical units from Earth on October 20 and is currently at magnitude 6. Together, the two comets could provide favorable binocular views around Halloween. Interstellar comet 3I/ATLAS is also moving through the inner solar system, though much fainter at magnitude 11.
Observers hoping to see Lemmon should use binoculars or a small telescope. The coma spans about one arcminute, with a faint tail pointing away from the sun. Under very dark skies, it may eventually become visible to the naked eye if it brightens to magnitude 3 later this month.
Skywatching apps such as Stellarium can help locate the comet. For photography, short exposures of 10 to 30 seconds can capture the coma and part of the tail, while stacking multiple images reveals more detail. Recent photos have shown changes in the tail linked to interactions with the solar wind.
Astronomers note that Lemmon is brighter than predicted, about 600 times more than its initial estimate. That makes it one of the more surprising comets in recent years, drawing comparisons with NEOWISE in 2020 and Tsuchinshan-ATLAS in 2024. As it fades after perihelion, attention will shift to C/2025 R3 PanSTARRS, due in early 2026. For now, Lemmon offers a rare autumn viewing opportunity for comet enthusiasts.
