Two Green Comets Make a Rare October Appearance
This month brings a rare celestial treat: two greenish comets are streaking across the night sky, offering skywatchers in the Northern Hemisphere a unique chance to observe them through fall. The comets, officially named C/2025 A6 (Lemmon) and C/2025 R2 (SWAN), are traversing the inner solar system as they loop around the Sun, with visibility lasting into late October.
What Makes These Comets Green?
The emerald hues seen in these comets come from carbon-bearing compounds released as ices sublimate when they approach the Sun. This gives the cometary coma and tails a distinctive green tint that stands out against the night sky, especially against a dark, clear backdrop. While their color is captivating, observers should not expect a neon glow; binoculars or a small telescope will reveal the cometary features more clearly in most conditions.
Comet Lemmon (C/2025 A6)
Discovered in January by the Mount Lemmon Survey in Arizona, Comet Lemmon is currently visible in the pre-dawn sky in the Northern Hemisphere. In the coming days, it will swing from near the Big Dipper region to a more western position as it moves along its orbit. The best viewing window in the morning is slightly northeast of the Big Dipper, just before sunrise. As the month progresses, Lemmon will shift to the evening sky and rise in the west, potentially becoming visible to the unaided eye around its close approach to Earth around October 21.
Astrophotographers have already captured Lemmon’s emerald glow and elongated tail, underscoring the comet’s visual appeal. Observers should seek a dark, low-light horizon and allow their eyes to adapt to the night sky for the best chance of spotting the comet with minimal equipment.
Comet SWAN (C/2025 R2)
Comet SWAN has a later peak in visibility and is best spotted roughly 90 minutes after sunset, looking toward the southwest. It is less likely to be seen with the naked eye and generally requires binoculars or a small telescope. Over the next week, SWAN will climb higher in the sky after sunset and brighten as it approaches Earth’s vicinity around October 20.
Font of potential for double-detection: under ideal conditions—clear weather, dark skies, and minimal light pollution—you might catch both Lemmon and SWAN in a single night near the end of October. Binoculars will reveal their tails and coma, while a small telescope can provide more detail on the structure of the comets.
How to Observe Both Comets
Tips for successful viewing include choosing nights with stable atmospheric conditions and limiting light pollution. Use a comfortable, low magnification on your binoculars or telescope to frame both comets against a shared background of stars. Tracking the comets over several nights can reveal subtle changes in brightness and geometry as they move along their orbits.
Discovery Stories and What They Mean
Comet Lemmon’s discovery by the Mount Lemmon Survey demonstrates the ongoing contributions of ground-based surveys to our understanding of near-Earth objects. Comet SWAN, discovered by an amateur astronomer reviewing NASA’s Solar and Heliospheric Observatory data, highlights how citizen scientists can play a pivotal role in identifying new visitors from the outer solar system. Together, these comets offer a rare month-long window into the dynamics of small bodies as they voyage through the inner solar system.
Whether you’re a seasoned stargazer or a curious beginner, October presents a meaningful opportunity to witness two greenish visitors that remind us how dynamic and accessible our night sky can be.
