Two Green Comets Grace the Autumn Sky
Two rare emerald visitors are sweeping through the inner solar system this fall, offering skywatchers in the Northern Hemisphere a unique chance to observe two comets in the same month. C/2025 A6 (Lemmon) and C/2025 R2 (SWAN) are making their paths around the sun, delivering visible tails and a greenish glow that has drawn the attention of amateur and seasoned astronomers alike.
What Makes the Color Green?
Both comets display a distinctive green hue, a signature often tied to the presence of carbon-containing gases in the coma surrounding the nucleus. When solar radiation energizes these molecules, they emit a faint emerald light that can be spotted with modest optical aid under dark skies. The result is a pair of icy wanderers that not only captivate with motion but also with color that hints at their primordial makeup.
Visibility in the Northern Hemisphere
Starting now and continuing through mid-October, Comet Lemmon will be visible near the Big Dipper before dawn. For best results, face northeast and scan just below the bowl and handle of the dipper a bit before astronomical twilight begins. As the month progresses, Lemmon shifts into the evening sky and rises in the west, offering a potential naked-eye glimpse at its peak around October 21, though visibility remains highly dependent on local conditions such as light pollution and weather.
Comet SWAN, on the other hand, stays in the evening sky through the end of October. The optimal viewing window falls roughly 90 minutes after sunset, with a view toward the southwest. While SWAN is unlikely to be bright enough to see with the unaided eye, binoculars or a small telescope will reveal its slim, greenish tail as it climbs higher on the horizon and brightens toward its perihelion around October 20.
Two Comets in One Month: A Rare Alignment
Seeing two comets simultaneously in a single calendar month is unusual and adds extra excitement for skywatchers. If conditions align—clear skies, dark skies, and minimal light pollution—both Lemmon and SWAN could be visible on the same night toward the end of October, providing a memorable celestial double feature for those lucky enough to observe.
Discovery Stories
Comet Lemmon was discovered in January by the Mount Lemmon Survey, which uses telescopes tucked in the Santa Catalina Mountains of Arizona to track near-Earth objects. Comet SWAN’s discovery is even more recent: an amateur astronomer identified it in early September while reviewing images taken by NASA’s Solar and Heliospheric Observatory’s SWAN instrument, which has been studying the sun since its 1995 launch. These discoveries underscore how both professional surveys and dedicated citizen scientists contribute to our changing sky.
Tips for Observing
To maximize your chances of seeing Lemmon and SWAN, prioritize a night with clear, stable air and minimal light pollution. Allow your eyes to adapt to darkness for at least 15–20 minutes, bring a pair of binoculars (a 7×50 or 10×50 pair works well), and a small telescope if available. A simple star map can help you locate the Big Dipper and guide your gaze toward the northeast before dawn (for Lemmon) or the southwest after sunset (for SWAN). Keep an eye on local astronomy sites or apps for real-time updates on brightness and visibility forecasts as the objects move through their respective paths.
What These Comets Tell Us About the Solar System
Emerald comets like Lemmon and SWAN are more than pretty sky spectacles; they offer clues about the materials that built the early solar system. Their greenish halos reveal the presence of carbon-bearing gases, while their tails trace the interaction between solar radiation and volatile ices as they approach the Sun. Each apparition gives astronomers a chance to study composition, activity levels, and dust production, painting a richer picture of how comets evolve on their journeys through the inner solar system.
