Introduction: A Rare Visitor from the Outer Solar System
Comet Lemmon (C/2025 A6) has transformed from a faint glimmer to a bright object in the night sky, offering a rare opportunity to witness a long-period visitor from the outer solar system. Discovered early in 2025 at Mount Lemmon Observatory in Arizona, this comet completes a journey that spans roughly 1,350 years per orbit. As October unfolds, the prospects for viewing Lemmon grow brighter, making it one of the season’s most anticipated astronomical events.
What to Expect This October
According to NASA projections, Comet Lemmon will reach its closest approach to Earth around October 21, 2025, at about 0.60 astronomical units (AU). It will then swing closer to the Sun for perihelion on November 8. Around this perihelion, the comet is expected to peak in brightness, potentially becoming visible to the naked eye under dark, clear skies. Its tail will also stretch to its maximum length near this time, driven by solar radiation that heats the comet’s ices and gases.
Why the Greenish Glow?
The distinctive green tint of Comet Lemmon comes from the coma, the cloud of gas and dust surrounding the nucleus. Diatomic carbon (C2) molecules, excited by sunlight, produce this telltale green hue that observers may notice with binoculars or small telescopes when conditions are favorable.
Best Places and Times to Observe
For observers in the Northern Hemisphere, Comet Lemmon will be positioned low in the northwestern to western sky after sunset starting mid-October. It will ride near the handle of the Big Dipper (the Plough in the UK), offering a recognizable guidepost for skywatchers. Reachable with a clear horizon and little light pollution, the comet is visible first in twilight and becomes easier as darkness deepens.
In October, aim for the week surrounding its closest approach to Earth for the best viewing window. By November, the comet will cross the celestial equator and become visible from the Southern Hemisphere too, broadening the opportunity for observers worldwide.
How to See Lemmon: Gear, Tips, and Safety
The simplest way to catch Comet Lemmon is with binoculars or a small telescope. A dark-sky site significantly improves contrast against the twilight sky. If you’re in a city or a bright suburb, consider traveling to a location with lower light pollution or planning a viewing session after moonset for darker skies.
Tips to maximize your chances:
- Check local weather and auroral or cloud cover forecasts for unobstructed skies.
- Use a star map app or planetarium software to locate Lemmon’s position and track its slow drift across the sky.
- Start observing about a week before and after its Earth approach for the clearest, most rewarding view.
- Be patient; the comet can appear faint at first, gradually brightening as it approaches perihelion.
When Will It Be at Its Brightest?
Comet Lemmon’s brightness should peak near perihelion in early November. During this window, observers with dark skies could glimpse the comet with the naked eye, provided atmospheric conditions are favorable. Even if it remains binocular-ready, the spectacle—especially the length of its tail—will be a striking reminder of the dynamic nature of these ancient travelers.
Why Comets Like Lemmon Matter
Comets are often described as frozen fossils from the solar system’s birth 4.6 billion years ago. The materials they release as they warm reveal the composition of the early outer solar system, offering clues about the environment where planets formed. By studying Lemmon’s coma and tail through spectroscopy, scientists peer into primordial mixtures of water, organic molecules, and other volatiles that inhabited the outer reaches of our planetary neighborhood.
A Moment in Time: Seeing a 1,350-Year Traveler
As Comet Lemmon sails through the night sky this October, observers become witnesses to a celestial journey spanning more than a millennium. The comet’s next appearance won’t occur for roughly 1,000 years, making this a unique chance to connect with the solar system’s distant past and to marvel at the very slow, patient clock of celestial mechanics.
For skywatchers, this October offers a chance to witness a visitor that carries 4.6 billion years of memories. If skies are clear, take a moment to look up and share the wonder with others who are curious about our cosmic neighborhood.
Note: This story originated in WIRED Japan and has been translated from Japanese.
